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622 matter in a fourth state previously unrecognized (1879). The fundamental character of the "discovery was not realized, how- ever, until it was interpreted by Sir J. J. Thomson (1897), after Rontgen (1895) had shown that peculiar pulsations (X-rays) were excited by the impact of the discharge against a solid surface.

From 1852 onwards, the year in which Frankland first made known the simple theory of atomic valency upon which hitherto all structural formulae have been based, chemists spent laborious days in verifying the Daltonian theory of atoms, itself a most wonderful prediction of genius. They have been engaged in de- fining atomic properties and in the comparative study of the ele- ments; also they have been at infinite pains to elucidate molecular structure, in the hope of explaining the properties of compounds generally in terms of such structure. The work done is of colossal proportions. Success was attending their efforts in most direc- tions; and a finished stable system was almost in prospect, when, with little notice, although the storm had long been brewing, their peace of mind was disturbed by the rudest possible intrusion from the side of physics. It is true that a note of warning came through the discovery of the radio-active properties of uranium by Becquerel; but it was not until the high-explosive shell radium was let loose that all preconceived views of atomic sanctity and sanity were scattered to the winds.

Although no one regarded the elements as strictly " element- ary " -the only explanation of Mendeleeff's generalization was that they were genetically related and therefore of complex struc- ture it had always been supposed that they were infinitely stable, only to be decomposed, if at all, by resort to extreme meas- ures. In radium, however, an " element " was suddenly found that was ever undergoing disruption and yet it was impossible to control its decay, either to hasten or diminish the rate. Even more marvellous was the character of the change particularly as illustrating the dependence of molecular idiosyncrasies on structure. Radium is a metallic material, resembling barium; the first weighty product of its slow spontaneous decomposition, to- gether with the inert gas helium, was found to be a highly vola- tile and inert gas emanation now known as radium (or niton) having none of the properties of a metal; this latter, however, also underwent change and very rapidly, a helium molecule being again obtruded. This downward course was progressively continued, until at last what seemed to be lead was obtained.

Radium has been proved to be but a child of uranium, the most weighty of the known primary materials (238), though produced from it at a rate far slower even than that at which radium itself commits suicide. Thorium, the oxide of which plays so great a part as chief component of the " mantle " now generally used for incandescent gas-light, has also been shown to be a member of the Suicide Club (see RADIO-ACTIVITY).

Faraday, who early made clear the essential unity of chemical and electrical action, in the researches in which he laid the foun- dations of electro-chemistry, discovered that, in electrolysis, def- inite electric charges were carried by the moving atoms of mat- ter; gradually the view grew up that the charge carried by the atom was related to the principal valency of the element. After a considerable interval, Helmholtz, in his Faraday lecture to the Chemical Society (1871), sought to draw the logical conclusion from Faraday's facts: he pictured chemical combination as the consequence of atomic charges of electricity and chemical inter- action as involving the exchange and neutralization of such charges. Johnstone Stoney, in 1881, baptized these charges electrons. The hypothesis did not altogether satisfy chemists, more particularly on account of the strange variations in the valency of some elements and because it did not seem to afford an explanation of so-called residual affinity. The chemist, be it said parenthetically, ever has the feeling that the physicist and he are not in full sympathy and that the physicist has a tendency to treat the phenomena somewhat too broadly, if not superfi- cially to disregard the fine shades of difference which the chem- ist learns to evaluate through constant intimate intercourse with materials and his introspective habit of mind.

The electronic hypothesis only began to take firm hold of the imagination after Crookes had called attention to the special

properties of the negative electric discharge in high vacua, when Sir J. J. Thomson formulated the view that the Crookes cathode discharge was not particulate in the ordinary sense but composed of moving particles of electricity (electrons) little more than i/i, 800 of the mass of the hydrogen atom. Physicists tell us now that not only is matter atomic which many scarcely be- lieved 50 years earlier and electricity also atomic; but that atomic matter itself is made up of sub-atoms of electricity, and that, if the properties peculiar to the elements and peculiar to their com- pounds are to be explained, attention must now be turned to the determination of the electronic structure of the atom (see MAT- TER, CONSTITUTION or).

Mindful of the long struggle he has waged in determining structure, the chemist foresees that it will not be an easy task for physicists to penetrate the mysteries of sub-atomic structure by experimental means and to arrive at a general agreement as to the validity of their conclusions. The new discoveries are such, however, as he has long awaited and he is profoundly grateful to his physicist colleagues for having taken up the quest at a stage beyond which he could scarcely hope to travel the methods to be adopted, the kind of logic required, being so different from those proper to chemistry.

We cannot, in fact, overlook the differences which separate the practice of the different branches of science, nor can we disregard the existence of different types of mind suited to one or the other discipline. The line of demarcation, if not the stumbling block, is mathematics: the position of the chemist, in this respect, is mid- way between that of the physicist and the biologist. The popular saying, " too much learning has made him mad," may be paral- leled by the statement that too much mathematics may deprive the chemist of his practical ability, especially of his constructive power; and mathematics seem to be anathema to the biologist and naturalist. Just as it takes all sorts to make a world, so it takes all sorts to solve the infinitely varied problems of science. The attempt to train all by similar methods is bound to end in failure ; if it be persevered in, ultimately only the uneducated will be able to do original work.

The new discoveries are those, we say, that the chemist has long awaited. He has often speculated on the constitution of matter and supposed it to be built up of some primordial constitu- ent. He has long thought that the elements are in some way genetically connected, on account of the striking "periodic" relationship they exhibit. He has not been satisfied with the weights he has been forced to assign to many atoms, feeling in- tuitively that it was not right that even an atom should be in- flicted with a weight that had not the dignity of an integer at least this has been an impression in the minds of those who were fully alive to the wonderful regularities and relationships mani- fest among the compounds of carbon. Lastly, he has also been prepared to believe that in some cases he might be dealing with mixtures almost impossible to separate: tellurium is a particular case in point, while nickel and cobalt afford another; probably something similar occurs in the case of chlorine.

The facts, however, go beyond all dreams. As the study of the products of radio-active change proceeded, it became necessary to recognize that although each had peculiar radio-active charac- teristics, the products in a number of cases were not distinguish- able chemically; gradually the conception grew up of elements differing in atomic mass but indistinguishable chemically termed isotopes by Prof. Soddy.

The constant presence of lead in radio-active minerals of vari- ous geological ages containing uranium led Boltwood to suggest that lead was probably the ultimate product of spontaneous breakdown in the uranium-radium series. Soddy, speculating on the position of the radio-active elements in the periodic system, came to a similar conclusion as to the origin of the lead in tho- rium minerals; but on this assumption it appeared probable, taking into account the reduction in atomic mass at the several stages, that the leads from the two sources would be homologous (isotopic). The atomic weight to be expected was in the one case 206, in the other 208. Examining thoric-lead, Soddy and Hyman found the value 207-7, whilst common lead gave 207-2.