Chemistry

CHEMISTRY. - I. DEFINITIONS AND DIVISIONS. - The word c. may derive from the Egyptian or Coptic terms "km.t" or "Kèmi" having the meanings of "black earth" or "Egypt": c. would thus be the science of Egypt, that is, of the region where it would have been first cultivated. The Arabs added to it the Arabic article "al", whence the name alchemy, by which this science was designated until the early times of the modern era, when it returned to being called simply c.

C. is the science that studies the specific properties and the substantial transformations of all material bodies as such: nuclear transformations are an exception, as they belong to physics because they are studied with the methods of that science.

It is divided into two branches. The specific properties and transformations of substances, without reference to particular substances or particular reactions, are studied in a general manner in physical c. or theoretical c.; besides the means proper to c., it avails itself of the means of study of mathematics and physics. The properties and the transformation processes that concern individual substances or each group of substances are studied in general descriptive systematic c.; which is divided, for didactic and practical reasons, into inorganic or mineral c. and organic c. or c. of the combinations of carbon. And when the various substances or particular groups of them are studied with more restricted purposes or in particular natural or artificial associations, c. gives rise to the numerous branches of its applications: astro-c., geo-c., bio-c.; industrial, agricultural, analytical, pharmaceutical, bromatological, metallurgical, photographic c., of dyes, of textiles, etc.

II. LAWS OF CHEMICAL COMBINATIONS; ATOMIC-MOLECULAR HYPOTHESIS

Nature has provided man with a great variety of substances. Some of them can also be found in the pure state, such as diamond, quartz and other crystalline substances; almost all other bodies are instead more or less homogeneous mixtures or more or less organized associations of various chemical species. First the men of prehistoric ages, then the peoples of greater culture, with greater effort in the Middle Ages the alchemists and in the modern age the chemists, made numerous attempts to modify bodies, especially through the action of heat. Thus changes of state were obtained (evaporations, condensations, fusions) and other predominantly physical transformations (mixings, dissolutions, crystallizations, dryings); sometimes chemical species were separated in a state of relative purity (sodium chloride by evaporation of sea water, sulfur by sublimation of the crude mineral, pure water by distillation of natural water, alcohol by distillation of wine). At other times technical operations based on chemical reactions were carried out, such as the preparation of cast iron by heating iron minerals with coal, of copper by the analogous operation performed on copper minerals, of lime by prolonged heating of limestone (prehistoric times), of glass by fusion of siliceous sand with plant ashes and with lime (ancient Chinese and Egyptian civilizations).

Chemical transformations received a correct interpretation when their quantitative aspect began to be considered, that is, when account began to be taken of the weight ratios according to which bodies combine and, when gaseous substances participate in or are formed by them, also of the variations in volume and of the ratios between the volumes of gases.

It was thus observed that certain substances, tested with physical and chemical means, no longer transformed, or else gave products always of greater weight (combinations, syntheses), but never products of lesser weight than that of the starting materials (decompositions, analyses); certain substances, that is, reveal the possible capacity to enter into combination, but not to give simpler products; thus the chemical elements were discovered, those simple substances, that is, not capable of further decomposition or simplification, from whose combination all other pure substances are formed and from these all bodies.

Combinations between elements do not occur in just any manner, but obey laws, some of which, the stoichiometric ones, were discovered at the dawn of chemical science:

Principle of the conservation of masses or weights (Lavoisier, 1743-94). The sum of the masses (or weights) of the substances that result from a chemical reaction is equal to the sum of the masses (or weights) of the substances that reacted. This principle is also called that of the conservation of matter; but improperly, since of matter the scientist detects only the physical and chemical properties, not the essence or complete definition, which belongs to philosophy. It is also expressed in the following ways of materialistic flavor: "nothing is created and nothing is destroyed, but everything is transformed"; or "the quantity of matter contained in the universe is constant", but with less exactness; in fact the concepts of creation and destruction are not the object of positive experience; nor are there sufficient elements to establish a law that refers to the whole universe, since of this an extremely small fraction is known.

Principle of definite proportions (L. G. Proust, French chemist, 1755-1826). Pure bodies present constant composition.

Law of multiple proportions (G. Dalton, English chemist, 1766-1844). In a series of pure substances that possess two elements in common, the quantities of one of them that are found combined with the same quantity of the other stand to one another in simple ratios.

Law on combinations between gaseous substances (G. Gay-Lussac, French chemist, 1778-1850). Gases or vapors combine with one another in simple ratios of volume if measured under equal conditions of temperature and pressure.

It was later found that these laws are not exactly observed in every case, and for reasons different and proper to each law. The principle of the conservation of masses is limited by possible transformations of a fraction of the mass into energy (nuclear reactions, radioactive transformations); the laws of definite and multiple proportions are limited by the possibility of the existence of chemical combinations (e.g., natural iron sulfides of the pyrrhotite and pyrite type) in which, within certain limits, two elements are found combined in non-simple ratios; the law on gaseous combinations is limited by the properties of real gases, which deviate from those of perfect gases.

The knowledge and application of these laws opened the way to the atomic hypothesis formulated by G. Dalton in 1804: 1) there exist as many different species of atoms as there are elements that constitute matter; 2) all the atoms of one and the same element have the same weight. This hypothesis was not yet sufficient to calculate the relative weights of atoms from the weight ratios according to which the elements combine with one another. To arrive at this, the intuition of Avogadro (1776-1856) was needed, who in various writings, from 1811 to 1838, proposed the celebrated hypothesis that bears his name: "equal volumes of gas under the same conditions of temperature and pressure contain an equal number of molecules". His writings, however, remained almost unknown, and therefore fruitless for the purposes of the progress of science, until they were studied and brought to prominence by Cannizzaro (1826-1910), who succeeded in drawing the attention of scientists to them at the congress of Karlsruhe in 1860. His work can be summarized in the following "Cannizzaro rule", which allows the atomic weight of an element to be determined when one knows how to determine the molecular weight of the combinations in which it participates: "by atomic weight of an element one must understand the smallest quantity of it contained in the molecular weight of its combinations".

Once the percentage elemental composition and the molecular weight were determined, and the atomic weights were known, the number of atoms of each kind making up the molecule became known — that is, the chemical formulas were known, which sum up all this knowledge in an extremely simple and complete way. A more thorough study of the properties of substances, and in particular the observation that certain atomic groupings passed unchanged from one substance to another in reactions (the theory of radicals), led to the establishment of structural formulas, which sum up in an equally simple, though less certain, way not only the composition but also some of the chemical properties of the substances represented.

III. PERIODIC CLASSIFICATION OF THE ELEMENTS

As in the 18th century for the descriptive natural sciences, so in the first half of the last century there were the first attempts at classifying the chemical elements; but only the Russian chemist Mendeleev succeeded in 1869 in proposing a classification which, with a few modifications, definitively entered science. Although it was constructed on empirical criteria, it was later seen, at the beginning of our century, that this classification had a rational basis, because the properties of the elements, and therefore their position in the classification, were a direct consequence of the electronic structure of the atoms of which the elements are formed.

Assuming as known the notions of physics contained in the entry MATTER (v.), a concise illustration of Mendeleev's table will be given here.

It must be noted that the classification is based first of all on knowledge of the atomic weight of the elements and then on their physical and chemical properties and those of the compounds derived from them: important among all these properties are the characteristic valences of each element.

By valence is meant the number of hydrogen atoms with which an atom of the element combines or which it replaces in its various combinations: the same element can display different valence depending on the combinations in which it participates. As an example, consider three of the simplest combinations of nitrogen. In ammonia, where one atom of nitrogen combines with three of hydrogen, nitrogen acts as trivalent; in nitric oxide, where it combines with one atom of oxygen and therefore replaces two of hydrogen, it acts as bivalent; in nitric acid, where it replaces 5 atoms of hydrogen (of three molecules of water), nitrogen acts as pentavalent:

N underset{H}{overset{H}{C}} N = O HO - N underset{O}{overset{O}{C}}

In order to bring out the periodic variations with increasing atomic weight, the following will be taken into account: the maximum valence that the element reveals in its combinations with oxygen and, for metalloids only, the maximum valence with respect to hydrogen (metals either do not form combinations with hydrogen or form combinations that are difficult to prepare and unstable).

This said, let the symbols of the chemical elements be arranged on a horizontal line in increasing order of atomic weight, beginning with H, He, Li, etc. (v. table below). Up to fluorine one finds elements all with properties clearly different from one another; but on reaching neon one observes that it is always zerovalent: that is, it combines neither with hydrogen nor with oxygen nor with any other element, exactly like helium; it will therefore be written in a column below this one. Then come sodium, magnesium, etc., with properties undoubtedly similar respectively to lithium, beryllium, etc.; and so one continues up to scandium on the third row. From titanium onward the analogies are no longer so marked (Mn, for example, is a metal that does not combine with hydrogen, whereas chlorine is a gas that forms a stable combination with hydrogen, hydrochloric acid), yet the formal analogies remain, in particular the maximum valence with respect to oxygen: this is taken into account by placing it in the relevant column but shifted to the left. After manganese there is no zerovalent element but rather the metals Fe, Ni, and Co, which cannot be written either under Ar or under K; they are therefore written in succession on the same row.

Because of undoubted analogies between Zn and Mg, As and P, Se and S, Br and Cl, increasingly pronounced going from left to right, copper too is placed in the same column as Li, Na and K, although the analogies between it and these are barely detectable: this is taken into account by writing it shifted to the right. And with these precautions one continues up to lanthanum. Starting from cerium up to lutetium the periodicity undergoes a great pause: in fact, for the rare-earth metals, whose properties are very similar to one another, there is no place in the table and they are written separately. The periodicity begins again with hafnium and then continues without other notable peculiarities up to uranium.

Because of the valence analogy with the alkali metals Li, Na, etc., hydrogen can be removed from the first row and placed above lithium or above fluorine. The table is thus constructed: it should only be noted that, in order to preserve evident analogies, Ni and Co, K and Ar, I and Te had to be inverted, making an exception to the progression of atomic weights in favor of the properties.

All the elements have thus been classified: into 6 periods, of which the first two, from He to F and from Ne to Cl, are small, of 8 elements each; and the three following ones are large, of 18 elements each, plus the 14 rare-earth elements for the fifth; the sixth is interrupted with uranium and remains with only 6 terms; and 9 groups, from the 1st to the 8th, plus the group of zerovalent elements; the 8th group carries 3 elements in every large period; the remaining groups, from the 1st to the 7th, are subdivided, only in the large periods, into two subgroups each.

At present it is preferred to present the periodic system of the elements as in the table at cols. 1541-42, in which: a) the numbering of the periods changes, with H and He forming part of the new 1st period, while the old 1st period, which begins with Li and no longer with Ne, is designated as the 2nd period; the subsequent periods undergo a similar shift in numbering. This is justified by the convenience of making the periodic classification correspond to the electronic structure of the atoms; b) the group of noble gases stands to the right of the table, at the end of each period rather than at the beginning of the next; c) the maximum valence of the individual elements with respect to oxygen is brought out by indicating the formula of the typical oxides; d) the subdivision into subgroups is brought out more clearly; e) the rare earths are listed separately, being indicated in the table simply by the expression "rare earths"; f) for the individual elements the following are brought out: the atomic number, the symbol, the atomic weight, and the number, not always certain, of isotopes (v. MATTER) so far found in nature.

Element 43 is poorly known: it has successively taken the names Masurium (Ma) and Technetium (Tc); element 61 has successively taken the names Illinium (Il), Florentium (Fl) and Promethium (Pm). Lutetium (71) is also called Cassiopeium (Cp). Radon (86) is also called Emanation (Em) or Niton (Nt). Element 87, poorly known,

CHIMISTRY - Formation of the table representing Mendeleev's periodic system.

(progr. Enc. Catt.)

CHEMISTRY - Periodic classification of the chemical elements.

(progr. Enc. Cutt.)

is designated by the name "Actinium k" (Ack); there is not yet (1949) sufficient information to classify the transuranic elements. The names Americium (95) and Curium (96) are perhaps not definitive.

The classification of the elements is truly natural and has been of great utility to chemists and physicists. It has made it possible to foresee not only the existence of certain elements that had not yet been discovered in Mendeleev's time but also to establish their properties with a precision that aroused wonder; it has made it understood that the atomic number has an importance superior to that of the atomic weight and that therefore the properties of the elements are a function not of the weight but of the atomic number.

Written in other ways, the table makes it possible to discern analogies not only by groups, as has been seen, but also by periods and zones. Finally, its rationality emerged from the fact that the regularities and irregularities of the system are closely connected with the electronic constitution of the atoms. The classification is therefore useful to the scholar also from the didactic and mnemonic point of view.

IV. ENERGETIC ASPECTS OF CHEMICAL TRANSFORMATIONS

With the growth of knowledge, the chemist was no longer satisfied with knowing which and how many are the elementary constituents of matter, the chemical elements, and the stoichiometric laws that govern combinations and that are summed up in the knowledge of atomic weights, valences and chemical formulas; but he wished to investigate, concerning chemical reactions, what the conditions for their completion are, why they proceed in one direction rather than another, and by what laws the development and absorption of energy that accompanies them is regulated.

Fundamental for science was the discovery of the law of the conservation of energy and of the equivalence between the various forms of energy, made by J. R. Mayer (1842), confirmed by the experiments of J. P. Joule (1843) and generalized by H. von Helmholtz (1847). It states: an isolated system preserves its total energy unchanged, independently of the transformations undergone by the individual fractions of energy: mechanical, thermal, electrical, radiant, chemical and potential.

Observation had then shown that reactions accompanied by the development of heat occur with greater frequency, that is, the transformations of systems rich in internal energy into poorer ones. Explosive substances are examples of systems that readily transform with a great development of thermal and mechanical energy (which had previously been contained in the explosive substance in the form of chemical energy) to give more stable systems constituted in that case by the gases formed in the explosion. Water, limestone and silicates, which together form almost the entirety of the Earth's solid and liquid surface, are instead examples of substances poor in internal energy and therefore very stable: their decomposition into the elementary components requires a great expenditure of energy.

The elements that combine to give very stable substances possess a great mutual affinity. This is a term that comes to us from alchemy, already used by St. Albert the Great to designate the union between similar substances in conformity with the axiom of the ancients: «similia similibus». The term was retained because even at the beginnings of chemical science it was believed that kindred or similar elements united to give combinations in preference to dissimilar elements. It was later seen that this could be true for certain categories of substances while for others the opposite was true, combinations between elements having opposite characteristics, such as sodium and chlorine, calcium and oxygen, being very stable. But the word affinity has remained to indicate the aptitude of elements to enter into combination and in general of chemical substances to react with one another.

It was M. Berthelot, with his celebrated law of maximum heat development (1867), who provided a criterion for measuring affinity: «The chemical transformations that proceed spontaneously, that is, without the contribution of energy supplied from outside, tend to produce the system of substances whose formation is accompanied by the greatest development of heat».

The rule indicated by Berthelot worked well in many cases: it was not, however, of general application: it could not therefore constitute a natural law. The difficulty was resolved by J. van't Hoff, who replaced heat, as a measure of affinity, with the maximum work that can be obtained from the reaction when it is conducted under the ideal condition of reversibility. One of the most practical ways, but possible only in a few cases, of realizing reversible processes, so as to obtain the maximum work and therefore the measure of affinity, consists in transforming the chemical energy of the reaction into electrical energy, which is what happens in cells.

It is important to note how the affinity of a chemical process depends not only on the nature of the reaction, but also on the concentrations of the substances that react and that are formed and on the temperature, so that the direction in which a reaction proceeds also depends on such circumstances. The study of the relations that obtain between concentration of the substances, their specific heat, temperatures, energy exchanges and affinity is perhaps the finest chapter of physical chemistry, entirely founded on a fertile application of the second principle of thermodynamics. Further progress in this field was brought by Nernst in 1906 with the formulation of his theorem, or third principle of thermodynamics, which specifies the relations that obtain between thermal tone and maximum work at low temperatures, near absolute zero.

V. CHEMICAL REACTIONS IN THE LIVING BEING AND THE 2ND PRINCIPLE OF THERMODYNAMICS

The question of vital energy that agitated chemists and biologists of the 18th and 19th centuries is well known: it was asked whether a force different from those at play in ordinary physical and chemical processes was needed to produce organic substances. It was resolved experimentally with the synthesis of urea carried out by Wöhler in 1826 starting from an inorganic substance, ammonium cyanate, a synthesis followed by that of very numerous other organic substances. Today it is held that even the most complex chemical reactions essential for life follow the same laws as any other chemical reaction: there escapes science, however, that aspect of biochemistry (v.) by which reactions are provoked, ordered and utilized for the purposes of the growth and preservation of the individual or of the species.

But if the question has been resolved from the qualitative standpoint, as has been said, with the synthesis of innumerable organic substances (about 1 million so far, 1940-49), other aspects of the question are not yet defined. A theme that has been raised also in recent times is the following: does life obey or not the second principle of thermodynamics (v.)?

Attention was fixed on a higher plant which, with two products of low energy content, water and carbon dioxide, and with small quantities of other substances, builds an extremely differentiated organism constituted by substances of high energy content, such as cellulose, starch, sugars, fats, proteins, lignin, etc. At the end of the growth process, the energy reached and consumed, noble energy theoretically entirely convertible into work, is found in three forms: of that accumulated in the plant in the form of chemical energy of the organic substances, part is noble energy convertible into work and part is degraded energy no longer convertible into work but only into heat. The remainder has been dispersed into the environment in the form of heat during the growth process: overall there has been a degradation of energy. A plant, while obeying the first principle of thermodynamics (or of the conservation of energy), would contravene the second principle if it were capable of building its own tissues starting from carbon dioxide and water by utilizing, at least in part, the heat, at uniform temperature, of the environment in which it lives: which does not happen, as has been seen through the example given, for higher plants and which has not been demonstrated even for the lower forms of life.

Science cannot explain life, a complex phenomenon, one aspect of which escapes its competence: it can only affirm that the individual elementary physical and chemical processes, from the totality of which results the aspect of life accessible to it, obey the laws of physical chemistry.

VI. CONSTITUTION OF MOLECULES

Theoretical chemistry today, having resolved the stoichiometric and energetic questions, concerns itself with the structure of molecules and with the relationships between molecular constitution and the physical and chemical properties of substances.

Numerous relationships of a quantitative character are known, especially those derived from the application of the second law of thermodynamics. They link together some of the following quantities: 1) the temperature at which the chemical process takes place, 2) the maximum work that this can supply, 3) the equilibrium constant of the reaction, 4) the variation of this constant with variation of temperature, 5) the latent heat of fusion and 6) the cryoscopic constant, 7) the latent heat of evaporation and 8) the ebullioscopic constant, 9) the specific heat of the substances that participate in the physical or chemical process, etc.

Such relationships, essentially physical in nature, do not represent sufficient material to be able to construct a deductive, rational science, but they cover a modest sector of chemistry, which still retains some characteristics of a descriptive and empirical science.

It must be recognized, however, that the results obtained from experimentation with physical and chemical means on substances and on chemical processes represent today an imposing mass of knowledge, both from the speculative and the practical point of view.

By bombarding crystals with X-rays of known wavelength and examining the diffraction patterns that these rays form upon leaving the crystal, it has been possible to measure the exact distances between the atoms in the molecule and the minimum distances at which the atoms of contiguous molecules can approach one another (v. figures).

With the use of infrared, visible and ultraviolet light (v. OPTICS) absorption spectra, fluorescence spectra and Raman spectra have been obtained which permit the measurement of the various molecular electronic levels, of the oscillation frequency of atoms and atomic groups, of the rotation constants of molecules, etc. And since in certain cases one succeeds in calculating such values when the following are known: the arrangement of the atoms in the molecule, the atomic weights and the magnitude of the interatomic forces that oppose the removal of the atoms from their equilibrium position, it has been possible to obtain useful information on the real structure of molecules by assimilating them to models.

From the measurement of the dielectric and magnetic constants (v. ELECTROLOGY) of substances and from the measurement of the variations that they undergo under the action of electric and magnetic fields respectively, it has been possible to calculate the electric and magnetic moments of molecules and to derive useful deductions on their shape, on the distribution of electric charges and on the shape of magnetic fields.

The results of these and of other numerous researches have permitted in many cases giving satisfactory answers to some questions relating to the structure of molecules and to the properties of substances.

When molecules (or ions) unite to form crystals they solve a "problem of minimum": compatibly with the shape of the molecules and with the intermolecular forces, they assume the arrangement to which the minimum internal energy corresponds and often, but not always, also the minimum volume.

The atoms in uniting into molecules must respect a greater number of conditions, not all well defined, among which are recalled valence, the direction of the valences, affinity and stability: these are nothing other than different aspects under which the properties of the atoms are revealed.

Valence has already been discussed in connection with the periodic classification of the elements. The three hydrogen atoms in the molecule of ammonia, brought forward then as an example, do not assume any arbitrary position around the nitrogen atom, nor do they assume a position of maximum symmetry, corresponding namely to the three vertices of an equilateral triangle the center of which is occupied by the nitrogen atom: they place themselves instead at the base vertices of a triangular pyramid, the fourth vertex of which is occupied by the nitrogen atom, so that the hydrogen atoms do not stand around the nitrogen atom but all on one side. In water (v.) the two hydrogen atoms do not lie on a straight line with the oxygen atom but the first two form with this an angle of 104°-31'. In methane, on the other hand, the four hydrogen atoms stand around the carbon atom in the positions corresponding to maximum symmetry, namely at the vertices of a regular tetrahedron at the center of which stands the carbon atom.

Affinity, of which mention has already been made, considered as the greater or lesser tendency that equal or different atoms have to unite to form a true chemical bond, varies first of all with the nature of the atoms but also depends on other circumstances: in particular on the type of bond, as will be said below, and on the nature and arrangement of the other atoms in the same molecule: it is measured by the energy that must be expended to remove the two atoms to infinity, that is to break the bond, the other conditions remaining unaltered. Some typical metalloids (H, N, O₂) in the elemental state form very stable diatomic molecules and constitute therefore examples of very energetic bonds between two equal atoms. Carbon is the only element whose atoms have marked aptitudes for uniting in chains even very long or in rings or systems of rings even very complex: a less pronounced aptitude in this sense is also possessed by silicon and sulfur. Oxygen has the capacity to unite with all the elements, naturally excluding the zero-valent ones of the helium series which do not give stable combinations with any element.

The stability of chemical combinations depends in a particular way on temperature and on the heat of formation as well as on other circumstances. Every substance has a limit temperature of stability above which it decomposes into other more stable and often simpler substances. The influence of the heat of formation on the stability of substances has already been mentioned: substances that are formed from the elements with a strong evolution of heat are stable at ordinary temperatures, unstable, on the other hand, those that are formed with absorption of heat. The thermal tonality varies with the temperature at which the reaction takes place.

The forces that hold the atoms together in molecules are of various types: the most important are those corresponding to electrovalence and to covalence. There is an electric valence or typical heteropolar bond when one of the two atoms yields an electron to the other: between the two, the first positive, the second negative, an electrostatic attraction is established: this is the case of sodium chloride Na Cl. There is a covalence or homopolar bond when the two atoms share one electron each so that the two atoms remain bound to each other by a pair of electrons which in a certain way belongs simultaneously to both atoms. The attraction that results is not expressible by means of classical physics schemes but is explained only with mathematical means proper to quantum wave mechanics.

The forces that establish an attraction between molecules in crystals or in liquids, and even in the gaseous state when the molecules approach beyond a certain limit, are not essentially different from those that are established between atoms of the same molecule; but since the electric charges are for the most part already neutralized in the molecule and the electrons are already engaged in intramolecular bonds, the attractive forces that can develop are less strong than those between atoms of the same molecule, and equilibrium with the repulsive forces is established at a greater distance. When the intermolecular attractions reach the same order of magnitude as the intramolecular ones or exceed them, condensations or polymerizations or other chemical reactions can occur.

VII. FUTURE OF CHEMISTRY

Chemistry is distinguished from physics by having its own formal object and its own means of research: but from a strictly philosophical and systematic point of view it can also be considered as a branch of physics, inasmuch as the former does not possess philosophical problems of its own distinct from those of the latter, as instead do, for example, biology and astronomy; all the more so since the boundaries between physics and chemistry are becoming ever more imprecise with the progress of research. A time will come when from present-day chemistry a rational chemistry may arise, constituted in large part by procedures of higher mathematics; and experience will then have no other purpose than to seek the values to be introduced into those procedures and to check the predictions they will furnish. But there are no indications that would lead one to foresee this as imminent. Experimental and mathematical means are still lacking for resolving questions which, as they are posed today, present themselves as of extraordinary complexity. For many phenomena one can give the explanation, but not the exact quantitative formulation: even while knowing the certain molecular structure of many substances, one cannot predict the exact values of some of their physical and chemical properties. As a practical science, on the other hand, chemistry may be considered to have reached a high degree of development: it has provided technology with innumerable procedures that have contributed to profoundly transforming the social life of man.

Cesco Toffoli