CHAPTER VI

THE AGE OF MODERN CHEMISTRY The Birth of Modern Chemistry. § =71.= Chemistry as distinct from Alchemy and Iatro-chemistry commenced with Robert Boyle (see plate 15), who first clearly recognised that its aim is neither the transmutation of the metals nor the preparation of medicines, but the observation and generalisation of a certain class of phenomena; who denied the validity of the alchemistic view of the constitution of matter, and enunciated the definition of an element which has since reigned supreme in Chemistry; and who enriched the science with observations of the utmost importance. Boyle, however, was a man whose ideas were in advance of his times, and intervening between the iatro-chemical period and the Age of Modern Chemistry proper came the period of the Phlogistic Theory--a theory which had a certain affinity with the ideas of the alchemists. [Illustration: PLATE 15. PORTRAIT OF ROBERT BOYLE. _To face page 94_]] The Phlogiston Theory. § =72.= The phlogiston theory was mainly due to Georg Ernst Stahl (1660-1734). Becher (1635-1682) had attempted to revive the once universally accepted sulphur-mercury-salt theory of the alchemists in a somewhat modified form, by the assumption that all substances consist of three earths--the combustible, mercurial, and vitreous; and herein is to be found the germ of Stahl’s phlogistic theory. According to Stahl, all combustible bodies (including those metals that change on heating) contain _phlogiston_, the principle of combustion, which escapes in the form of flame when such substances are burned. According to this theory, therefore, the metals are compounds, since they consist of a metallic calx (what we now call the “oxide” of the metal) combined with phlogiston; and, further, to obtain the metal from the calx it is only necessary to act upon it with some substance rich in phlogiston. Now, coal and charcoal are both almost completely combustible, leaving very little residue; hence, according to this theory, they must consist very largely of phlogiston; and, as a matter of fact, metals can be obtained by heating their calces with either of these substances. Many other facts of a like nature were explicable in terms of the phlogiston theory, and it became exceedingly popular. Chemists at this time did not pay much attention to the balance; it was observed, however, that metals increased in weight on calcination, but this was “explained” on the assumption that phlogiston possessed negative weight. Antoine Lavoisier (1743-1794), utilising Priestley’s discovery of oxygen (called “dephlogisticated air” by its discoverer) and studying the weight relations accompanying combustion, demonstrated the non-validity of the phlogistic theory[89] and proved combustion to be the combination of the substance burnt with a certain constituent of the air, the oxygen. By this time Alchemy was to all intents and purposes defunct, Boerhave (1668-1738) was the last eminent chemist to give any support to its doctrines, and the new chemistry of Lavoisier gave it a final death-blow. We now enter upon the Age of Modern Chemistry, but we shall deal in this chapter with the history of chemical theory only so far as is necessary in pursuance of our primary object, and hence our account will be very far from complete. [89] It should be noted, however, that if by the term “phlogiston” we were to understand energy and not some form of matter, most of the statements of the phlogistics would be true so far as they go. Boyle and the Definition of an Element. § =73.= Robert Boyle (1626-1691) had defined an element as a substance which could not be decomposed, but which could enter into combination with other elements giving compounds capable of decomposition into these original elements. Hence, the metals were classed among the elements, since they had defied all attempts to decompose them. Now, it must be noted that this definition is of a negative character, and, although it is convenient to term “elements” all substances which have so far defied decomposition, it is a matter of impossibility to decide what substances are true elements with absolute certainty; and the possibility, however faint, that gold and other metals are of a compound nature, and hence the possibility of preparing gold from the “base” metals or other substances, must always remain. This uncertainty regarding the elements appears to have generally been recognised by the new school of chemists, but this having been so, it is the more surprising that their criticism of alchemistic art was not less severe. The Stoichiometric Laws. § =74.= With the study of the relative weights in which substances combine, certain generalisations or “natural laws” of supreme importance were discovered. These stoichiometric laws, as they are called, are as follows:-- 1. “The Law of Constant Proportion”--_The same chemical compound always contains the same elements, and there is a constant ratio between the weights of the constituent elements present._ 2. “The Law of Multiple Proportions”--_If two substances combine chemically in more than one proportion, the weights of the one which combine with a given weight of the other, stand in a simple rational ratio to one another._ 3. “The Law of Combining Weights”--_Substances combine either in the ratio of their combining numbers, or in simple rational multiples or submultiples of these numbers._ (The weights of different substances which combine with a given weight of some particular substance, which is taken as the unit, are called the combining numbers of such substances with reference to this unit. The usual unit now chosen is 8 grammes of Oxygen.)[90] [90] In order that these laws may hold good, it is, of course, necessary that the substances are weighed under precisely similar conditions. To state these laws in a more absolute form, we can replace the term “weight” by “mass,” or in preference, “inertia”; for the inertias of bodies are proportional to their weights, providing that they are weighed under precisely similar conditions. For a discussion of the exact significance of these terms “mass” and “inertia,” the reader is referred to the present writer’s _Matter, Spirit and the Cosmos_ (Rider, 1910), Chapter I., “On the Doctrine of the Indestructibility of Matter.” As examples of these laws we may take the few following simple facts:-- 1. Pure water is found always to consist of oxygen and hydrogen combined in the ratio of 1·008 parts by weight of the latter to 8 parts by weight of the former; and pure sulphur-dioxide, to take another example, is found always to consist of sulphur and oxygen combined in the ratio of 8·02 parts by weight of sulphur to 8 parts by weight of oxygen. (The Law of Constant Proportion.) 2. Another compound is known consisting only of oxygen and hydrogen, which, however, differs entirely in its properties from water. It is found always to consist of oxygen and hydrogen combined in the ratio of 1·008 parts by weight of the latter to 16 parts by weight of the former, _i.e._, in it a definite weight of hydrogen is combined with an amount of oxygen _exactly twice_ that which is combined with the same weight of hydrogen in water. No definite compound has been discovered with a constitution intermediate between these two. Other compounds consisting only of sulphur and oxygen are also known. One of these (viz., sulphur-trioxide, or sulphuric anhydride) is found always to consist of sulphur and oxygen combined in the ratio of 5·35 parts by weight of sulphur to 8 parts by weight of oxygen. We see, therefore, that the weights of sulphur combined with a definite weight of oxygen in the two compounds called respectively “sulphur-dioxide” and “sulphur-trioxide,” are in the proportion of 8·02 to 5·35, _i.e._, 3 : 2. Similar simple ratios are obtained in the case of all the other compounds. (The Law of Multiple Proportions.) 3. From the data given in (1) above we can fix the combining number of hydrogen as 1·008, that of sulphur as 8·02. Now, compounds are known containing sulphur and hydrogen, and, in each case, the weight of sulphur combined with 1·008 grammes of hydrogen is found always to be either 8·02 grammes or some multiple or submultiple of this quantity. Thus, in the simplest compound of this sort, containing only hydrogen and sulphur (viz., sulphuretted-hydrogen or hydrogen sulphide), 1·008 grammes of hydrogen is found always to be combined with 16·04 grammes of sulphur, _i.e._, exactly twice the above quantity. (The Law of Combining Weights.) Berthollet (1748-1822) denied the truth of the law of constant proportion, and a controversy ensued between this chemist and Proust (1755-1826), who undertook a research to settle the question, the results of which were in entire agreement with the law, and were regarded as completely substantiating it. [Illustration: PLATE 16. [by Worthington, after Allen] PORTRAIT OF JOHN DALTON. _To face page 100_]] Dalton’s Atomic Theory. § =75.= At the beginning of the nineteenth century, John Dalton (see plate 15) put forward his Atomic Theory in explanation of these facts. This theory assumes (1) that all matter is made up of small indivisible and indestructible particles, called “atoms”; (2) that all atoms are _not_ alike, there being as many different sorts of atoms as there are elements; (3) that the atoms constituting any one element are exactly alike and are of definite weight; and (4) that compounds are produced by the combination of different atoms. Now, it is at once evident that if matter be so constituted, the stoichiometric laws must necessarily follow. For the smallest particle of any definite compound (now called a “molecule”) must consist of a definite assemblage of different atoms, and these atoms are of definite weight: whence the law of constant proportion. One atom of one substance may combine with 1, 2, 3 . . . atoms of some other substance, but it cannot combine with some fractional part of an atom, since the atoms are indivisible: whence the law of multiple proportions. And these laws holding good, and the atoms being of definite weight, the law of combining weights necessarily follows. Dalton’s Atomic Theory gave a simple and intelligible explanation of these remarkable facts regarding the weights of substances entering into chemical combination, and, therefore, gained universal acceptance. But throughout the history of Chemistry can be discerned a spirit of revolt against it as an explanation of the absolute constitution of matter. The tendency of scientific philosophy has always been towards Monism as opposed to Dualism, and here were not merely two eternals, but several dozen; Dalton’s theory denied the unity of the Cosmos, it lacked the unifying principle of the alchemists. It is only in recent times that it has been recognised that a scientific hypothesis may be very useful without being altogether true. As to the usefulness of Dalton’s theory there can be no question; it has accomplished that which no other hypothesis could have done; it rendered the concepts of a chemical element, a chemical compound and a chemical reaction definite; and has, in a sense, led to the majority of the discoveries in the domain of Chemistry that have been made since its enunciation. But as an expression of absolute truth, Dalton’s theory, as is very generally recognised nowadays, fails to be satisfactory. In the past, however, it has been the philosophers of the materialistic school of thought, rather than the chemists _quâ_ chemists, who have insisted on the absolute truth of the Atomic Theory; Kekulé, who by developing Franklin’s theory of atomicity or valency[91] made still more definite the atomic view of matter, himself expressed grave doubts as to the absolute truth of Dalton’s theory; but he regarded it as _chemically_ true, and thus voices what appears to be the opinion of the majority of chemists nowadays, namely, there are such things as chemical atoms and chemical elements, incapable of being decomposed by purely chemical means, but that such are not absolute atoms or absolute elements, and consequently not impervious to all forms of action. But of this more will be said later. [91] The term “valency” is not altogether an easy one to define; we will, however, here do our best to make plain its significance. In a definite chemical compound we must assume that the atoms constituting each molecule are in some way bound together (though not, of course, rigidly), and we may speak of “bonds” or “links of affinity,” taking care, however, not to interpret such terms too literally. Now, the number of “affinity links” which one atom can exert is not unlimited; indeed, according to the valency theory as first formulated, it is fixed and constant. It is this number which is called the “valency” of the element; but it is now known that the “valency” in most cases can vary between certain limits. Hydrogen, however, appears to be invariably univalent, and is therefore taken as the unit of valency. Thus, Carbon is quadrivalent in the methane-molecule, which consists of one atom of carbon combined with four atoms of hydrogen; and Oxygen is divalent in the water-molecule, which consists of one atom of oxygen combined with two atoms of hydrogen. Hence, we should expect to find one atom of carbon combining with two of oxygen, which is the case in the carbon-dioxide--(carbonic anhydride)--molecule. For a development of the thesis, so far as the compounds of carbon are concerned, that each specific “affinity link” corresponds in general to a definite and constant amount of energy, which is evolved as heat on disruption of the bond, the reader is referred to the present writer’s monograph _On the Calculation of Thermo-Chemical Constants_ (Arnold, 1909). The phenomena of valency find their explanation in modern views concerning the constitution of atoms (see § 81). The Determination of the Atomic Weights of the Elements. § =76.= With the acceptance of Dalton’s Atomic Theory, it became necessary to determine the atomic weights of the various elements, _i.e._, not the absolute atomic weights, but the relative weights of the various atoms with reference to one of them as unit.[92] We cannot in this place enter upon a discussion of the various difficulties, both of an experimental and theoretical nature, which were involved in this problem, save to remark that the correct atomic weights could be arrived at only with the acceptance of Avogadro’s Hypothesis. This hypothesis, which is to the effect that equal volumes of different gases measured at the same temperature and pressure contain an equal number of gaseous molecules, was put forward in explanation of a number of facts connected with the physical behaviour of gases; but its importance was for some time unrecognised, owing to the fact that the distinction between atoms and molecules was not yet clearly drawn. A list of those chemical substances at present recognised as “elements,” together with their atomic weights, will be found on pp. 106, 107. [92] Since hydrogen is the lightest of all known substances, the unit, Hydrogen = 1, was at one time usually employed. However, it was seen to be more convenient to express the atomic weights in terms of the weight of the oxygen-atom, and the unit, Oxygen = 16 is now always employed. This value for the oxygen-atom was chosen so that the approximate atomic weights would in most cases remain unaltered by the change. Prout’s Hypothesis. § =77.= It was observed by a chemist of the name of Prout, that, the atomic weight of hydrogen being taken as the unit, the atomic weights of nearly all the elements approximated to whole numbers; and in 1815 he suggested as the reason for this regularity, that all the elements consist solely of hydrogen. Prout’s Hypothesis received on the whole a very favourable reception; it harmonised Dalton’s Theory with the grand concept of the unity of matter--all matter was hydrogen in essence; and Thomas Thomson undertook a research to demonstrate its truth. On the other hand, however, the eminent Swedish chemist, Berzelius, who had carried out many atomic weight determinations, criticised both Prout’s Hypothesis and Thomson’s research (which latter, it is true, was worthless) in most severe terms; for the hypothesis amounted to this--that the decimals in the atomic weights obtained experimentally by Berzelius, after so much labour, were to be regarded as so many errors. In 1844, Marignac suggested half the hydrogen atom as the unit, for the element chlorine, with an atomic weight of 35·5, would not fit in with Prout’s Hypothesis as originally formulated; and later, Dumas suggested one-quarter. With this theoretical division of the hydrogen-atom, the hypothesis lost its simplicity and charm, and was doomed to downfall. Recent and most accurate atomic weight determinations show clearly that the atomic weights are not exactly whole numbers, but that, nevertheless, the majority of them (if expressed in terms of O = 16 as the unit) do approximate very closely to such. The Hon. R. J. Strutt has recently calculated that the probability of this occurring, in the case of certain of the commoner elements, by mere chance is exceedingly small (about 1 in 1,000),[93] and several attempts to explain this remarkable fact have been put forward. Modern scientific speculations concerning the constitution of atoms tend towards a modified form of Prout’s hypothesis, or to the view that the atoms of other elements are, in a manner, polymerides of hydrogen and helium atoms. As has been pointed out, it is possible, according to modern views, for elements of different atomic weight to have identical chemical properties, since these latter depend only upon the number of free electrons in the atom and not at all upon the massive central nucleus. By a method somewhat similar to that used for determining the mass of kathode particles (see § 79), but applied to positively charged particles, Sir Joseph Thomson and Dr. F. W. Aston discovered that the element neon was a mixture of two isotopic elements in unequal proportions, one having an atomic mass of 20, the other (present only to a slight extent) having an atomic mass of 22. Dr. Aston has perfected this method of analysing mixtures of isotopes and determining their atomic masses.[94] The results are of great interest. The atomic weight of hydrogen, 1·008, is confirmed. The elements helium, carbon, nitrogen, oxygen, fluorine, phosphorus, sulphur, arsenic, iodine and sodium are found to be simple bodies with whole-number atomic weights. On the other hand, boron, neon, silicon, chlorine, bromine, krypton, xenon, mercury, lithium, potassium and rubidium are found to be mixtures. What is specially of interest is that the indicated atomic mass of each of the constituents is a whole number. Thus chlorine, whose atomic weight is 35·46, is found to be a mixture of two chemically-identical elements whose atomic weights are 35 and 37. Some of the elements, _e.g._, xenon, are mixtures of more than two isotopes. [93] Hon. R. J. STRUTT: “On the Tendency of the Atomic Weights to approximate to Whole Numbers,” _Philosophical Magazine_, [6], vol. i. (1901), pp. 311 _et seq._ [94] F. W. ASTON: “Mass-spectra and Atomic Weights,” _Journal of the Chemical Society_, vol. cix. (1921), pp. 677 _et seq._ It is highly probable that what is true of the elements investigated by Dr. Aston is true of the remainder. It appears, therefore, that the irregularities presented by the atomic weights of the ordinary elements, which have so much puzzled men of science in the past, are due to the fact that these elements are, in many cases, mixtures. As concerns hydrogen, it is only reasonable to suppose that the close packing of electrically charged particles should give rise to a slight decrease in their total mass, so that the atomic weights of other elements referred to H = 1 should be slightly less than whole numbers, or, what is the same thing, that the atomic weight of hydrogen referred to O = 16 should be slightly more than unity. The “Periodic Law.” § =78.= A remarkable property of the atomic weights was discovered, in the sixties, independently by Lothar Meyer and Mendeléeff. They found that the elements could be arranged in rows in the order of their atomic weights so that similar elements would be found in the same columns. A modernised form of the Periodic Table will be found on pp. 106, 107. It will be noticed, for example, that the “alkali” metals, Lithium, Sodium, Rubidium and Cæsium, which resemble one another very closely, fall in Column 1; the “alkaline earth” metals occur together in Column 2; though in each case these are accompanied by certain elements with somewhat different properties. Much the same holds good in the case of the other columns of this Table; there is manifested a remarkable regularity, with certain still more remarkable divergences (see notes appended to Table on pp. 106, 107). This regularity exhibited by the “elements” is of considerable importance, since it shows that, in general, the properties of the “elements” are _periodic_ functions of their atomic weights; and, together with certain other remarkable properties of the “elements,” distinguishes them sharply from the “compounds.” It may be concluded with tolerable certainty, therefore, that if the “elements” are in reality of a compound nature, they are all, in general, compounds of a like nature distinct from that of other compounds. THE PERIODIC TABLE OF THE CHEMICAL ELEMENTS. +-------+-------+-------+-------+-------+-------+-------+-------+--------+ | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ | |[Hydro-| | | | | |Hydro- | | | |gen][a]| | | | | |gen | | | |[H = | | | | | |H = | | | |1·008] | | | | | |1·008 | | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ |Helium |Lithium|Gluci- |Boron |Carbon |Nitro- |Oxygen |Fluo- | | | | |num | | |gen | |rine | | |He = |Li = |Gl = |B = |C = |N = |O = |F = | | |4·00 |6·94 |9·1 |10·9 |12·005 |14·008 |16·00 |19·0 | | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ |Neon |Sodium |Magne- |Alumin-|Silicon|Phos- |Sulphur|Chlo- | | | | |sium |ium | |phorus | |rine | | |Ne = |Na = |Mg = |Al = |Si = |P = |S = |Cl = | | |20·2 |23·00 |24·32 |27·1 |28·3 |31·04 |32·06 |35·46 | | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ |Argon |Potas- |Calcium|Scan- |Tita- |Vana- |Chro- |Manga- |Iron | | |sium[b]| |dium |nium |dium |mium |nese |Fe = | | | | | | | | | |55·84[c]| |A = |K = |Ca = |Sc = |Ti = |V = |Cr = |Mn = |Cobalt | |39·9 |39·10 |40·07 |45·1 |48·1 |51·0 |52·0 |54·93 |Co = | | | | | | | | | |58·97 | | | | | | | | | |Nickel | | | | | | | | | |Ni = | | | | | | | | | |58·68 | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ | |Copper |Zinc |Gallium|Germa- |Arsenic|Sele- |Bromine| | | | | | |nium | |nium | | | | |Cu = |Zn = |Ga = |Ge = |As = |Se = |Br = | | | |63·57 |65·37 |70·1 |72·5 |74·96 |79·2 |79·92 | | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ |Krypton|Rubi- |Stron- |Yttrium|Zirco- |Colum- |Molyb- | ? |Ruthe- | | |dium |tium | |nium |bium |denum | |nium | |Kr = |Rb = |Sr = |Y = |Zr = |Cb = |Mo = | |Ru = | |82·92 |85·45 |87·63 |89·33 |90·6 |93·1 |96·0 | |101·7 | | | | | | | | | |Rhodium | | | | | | | | | |Rh = | | | | | | | | | |102·9 | | | | | | | | | |Palla- | | | | | | | | | |dium | | | | | | | | | |Pd = | | | | | | | | | |106·7 | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ | |Silver |Cadmium|Indium |Tin |Antimo-|Tellu- |Iodine | | | | | | | |ny |rium |[d] | | | |Ag = |Cd = |In = |Sn = |Sb = |Te = |I (or | | | |107·88 |112·40 |114·8 |118·7 |120·2 |127·5 |J) = | | | | | | | | | |126·92 | | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ |Xenon |Cæsium |Barium |Lantha-|Cerium | ? | ? | ? | ? | | | | |num |[e] | | | | | |Xe = |Cs = |Ba = |La = |Ce = | | | | | |130·2 |132·81 |137·37 |139·0 |140·25 | | | | | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ | | ? | ? | ? | ? | ? | ? | ? | | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ | ? | ? | ? | ? | ? |Tanta- |Tung- | ? |Osmium | | | | | | |lum |sten | |Os = | | | | | | |Ta = |W = | |190·9 | | | | | | |181·5 |184·0 | |Iridium | | | | | | | | | |Ir = | | | | | | | | | |193·1 | | | | | | | | | |Platinum| | | | | | | | | |Pt = | | | | | | | | | |195·2 | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ | |Gold |Mercury|Thal- |Lead |Bismuth|Polo- | ? | | | | | |lium | | |nium | | | | |Au = |Hg = |Tl = |Pb = |Bi = |(210) | | | | |197·2 |200·6 |204·0 |207·20 |208·0 | | | | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ |Emana- | ? |Radium |Acti- |Thorium|Ekatan-|Uranium| ? | ? | |tion | | |nium | |talum | | | | |(Niton)| |Ra = | ? |Th = | ? |U = | | | | 222·0 | |226·0 | |232·15 | |238·2 | | | +-------+-------+-------+-------+-------+-------+-------+-------+--------+ NOTES. There are several somewhat different forms of this Periodic Table. This is one of the simplest, but it lacks certain advantages of some of the more complicated forms. The atomic weights given are those of the International Atomic Weights Committee for 1920-1. They are calculated on the basis, Oxygen = 16. The number of decimal places given in each case indicates the degree of accuracy with which each atomic weight has been determined. The letter or letters underneath the name of each element is the symbol by which it is invariably designated by chemists. The number above each column indicates the valency which the elements of each group exhibit towards oxygen. Many of the elements are exceptional in this respect. [a]: The exact position of Hydrogen is in dispute. [b]: The positions of Argon and Potassium have been inverted in order that these elements may fall in the right columns with the elements they resemble; [d]: so also have the positions of Tellurium and Iodine. [c]: The whole of “Group 8” forms an exception to the Table. [e]: There are a number of ill-defined rare earth metals with atomic weights lying between those of Cerium and Tantalum. They all appear to resemble the elements of “Group 3,” so that their positions in the Table cannot be decided with accuracy. It is now some years since the late Sir William Crookes attempted to explain the periodicity of the properties of the elements on the theory that they have all been evolved by a conglomerating process from some primal stuff--the protyle--consisting of very small particles. He represented the action of this generative cause by means of a “figure of eight” spiral, along which the elements are placed at regular intervals, so that similar elements come underneath one another, as in Mendeléeff’s table, though the grouping differs in some respects. The slope of the curve is supposed to represent the decline of some factor (_e.g._, temperature) conditioning the process, which process is assumed to be of a recurrent nature, like the swing of a pendulum. After the completion of one swing (to keep to the illustration of a pendulum) whereby one series of elements is produced, owing to the decline of the above-mentioned factor, the same series of elements is not again the result as would otherwise be the case, but a somewhat different series is produced, each member of which resembles the corresponding member of the former series. Thus, if the first series contains, for example, helium, lithium, carbon, &c., the second series will contain instead, argon, potassium, titanium, &c. The whole theory, though highly interesting, is, however, by no means free from defects. The Corpuscular Theory of Matter. § =79.= We must now turn our attention to those recent views of the constitution of matter which originated to a great extent in the investigations of the passage of electricity through gases at very low pressures. It will be possible, however, on the present occasion, to give only the very briefest account of the subject; but a fuller treatment is rendered unnecessary by the fact that these and allied investigations and the theories to which they have given rise have been fully treated in several well-known works, by various authorities on the subject, which have appeared during the last few years.[95] [95] We have found Prof. Harry Jones’ _The Electrical Nature of Matter and Radioactivity_ (1906), Mr. Soddy’s _Radioactivity_ (1904), and Mr. Whetham’s _The Recent Development of Physical Science_ (1909) particularly interesting. Mention, of course, should also be made of the standard works of Prof. Sir J. J. Thomson and Prof. Rutherford. When an electrical discharge is passed through a high-vacuum tube, invisible rays are emitted from the kathode, generally with the production of a greenish-yellow fluorescence where they strike the glass walls of the tube. These rays are called “kathode rays.” At one time they were regarded as waves in the ether, but it was shown by Sir William Crookes that they consist of small electrically charged particles, moving with a very high velocity. Sir J. J. Thomson was able to determine the ratio of the charge carried by these particles to their mass or inertia; he found that this ratio was constant whatever gas was contained in the vacuum tube, and much greater than the corresponding ratio for the hydrogen ion (electrically charged hydrogen atom) in electrolysis. By a skilful method, based on the fact discovered by Mr.