206 



SCIENCE. 



[Vol. IV., No. 83. 



them in the hydrokinetic system, by jointed rigid con- 

 necting-links, we may arrange for configurations of 

 stable equilibrium. Thus, without fly-wheels, but 

 with fluid circulations through apertures, we may 

 make a model spring-balance, or a model luminifer- 

 ous ether, either without or with the rotational 

 quality corresponding to that of the true luminifer- 

 ous ether in the magnetic fluid ; in short, do all by 

 the perforated solids, with circulations through them, 

 that we saw we could do by means of linked gyro- 

 stats. But something that we cannot do by linked 

 gyrostats, we can do by the perforated bodies with 

 fluid circulation: we can make a model gas. The 

 mutual action at a distance, repulsive or attractive 

 according to the mutual aspect of the two bodies 

 when passing within collisional distance of one 

 another, suffices to produce the change of direction 

 of motion in collision, which essentially constitutes 

 the foundation of the kinetic theory of gases. 



There remains, however, as we have seen before, 

 the difficulty of providing for the case of actual im- 

 pacts between the solids. 



Let us annul the solids, and leave the liquid per- 

 forming irrotational circulation round vacancy, in 

 the place of the solid cores which we have hitherto 

 supposed ; or let us annul the rigidity of the solid cores 

 of the rings, and give them molecular rotation accord- 

 ing to Helmholtz's theory of vortex motion. As to 

 whether, however, when the vortex theory of gases is 

 thoroughly worked out, it will or will not be found 

 to fail in a manner analogous to the failure already 

 pointed out in connection with the kinetic theory 

 of gases composed of little elastic solid molecules, 

 one cannot at present speak with certainty. 



PROGRESS OF CHEMISTRY SINCE 1848. 1 



With the death of Berzelius in 1848 ended a well- 

 marked epoch in the history of chemistry: with that 

 of Dumas — and, alas! that of Wurtz also — in 1884 

 closes a second. 



The differences between what may properly be 

 termed the ' Berzelian era,' and that with which the 

 name of Dumas will forever be associated, show 

 themselves in many ways, but in none more marked- 

 ly than by the distinct views entertained as to the 

 nature of a chemical compound. 



According to the older notions, the properties of 

 compounds are essentially governed by the qualitative 

 nature of their constituent atoms, which were sup- 

 posed to be so arranged as to form a binary system. 

 Under the new ideas, on the other hand, it is mainly 

 the number and arrangement of the atoms within the 

 molecule which regulate the characteristics of the 

 compound, which is to be looked on, not as built up 

 of two constituent groups of atoms, but as forming 

 one group. Another striking difference of view be- 

 tween the chemistry of the Berzelian era and that of 



1 Abstract of an address to the chemical section of the Brit- 

 ish association at Montreal, Aug. 28, 1884, by Professor Henry 

 Enfield Roscoe, Ph.D., LL.D., F.R.S., F.C.S., president of 

 the section. 



what we sometimes term the ' modern epoch,' is illus- 

 trated by the so-called ' substitution theory.' Dumas, 

 to whom we owe this theory, showed that chlorine 

 can take the place of hydrogen in many compounds, 

 and that the resulting body possesses characters simi- 

 lar to the original. But there is another change of 

 view, dating from the commencement of the Dumas 

 epoch, which has exerted an influence, equal, if not 

 superior, to those already named on the progress of 

 chemistry, and that is, as to the use of equivalent 

 or molecular weights. 



The theory of organic radicals, developed by Liebig 

 so long ago as 1834, received numerous experimental 

 confirmations in succeeding years. Bunsen's classi- 

 cal research on cacodyl, proving the possibility of 

 the existence of metallo-organic radicals capable 

 of playing the part of a metal, and the isolation of 

 the hydrocarbon ethyl by Frankland in 1849, laid 

 what the supporters of the theory deemed the final 

 stone in the structure. 



The fusion of the radical and type theories, chiefly 

 effected by the discovery in 1849 of the compound 

 ammonias by Wurtz, brings us to the dawn of mod- 

 ern chemistry. Henceforward organic compounds 

 were seen to be capable of comparison with simple 

 inorganic bodies, and hydrogen capable of replace- 

 ment not only by chlorine or by a metal, but by an 

 organic group or radical. 



At the Edinburgh meeting of this association in 

 1850, Williamson read a paper on ' Eesults of a re- 

 search on aetherification,' which not only included a 

 satisfactory solution of an interesting and hitherto 

 unexplained problem, but was destined to exert a 

 most important influence on the development of our 

 theoretical views: for he proved, contrary to the then 

 prevailing ideas, that ether contains twice as much 

 carbon as alcohol, and that it is not formed from the 

 latter by a mere separation of the elements of water, 

 but by an exchange of hydrogen for ethyl ; and this 

 fact, being in accordance with Avogadro's law of 

 molecular volumes, could only be represented by 

 regarding the molecule of water as containing two 

 atoms of hydrogen to one of oxygen, one of the 

 former being replaced by one of ethyl to form alco- 

 hol, and the two of hydrogen by two of ethyl to 

 form ether. Then Williamson introduced the type 

 of water (subsequently adopted by Gerhardt) into 

 organic chemistry, and extended our views of the 

 analogies between alcohols and acids by pointing- 

 out that these latter are also referable to the water- 

 type, predicting that bodies bearing the same rela- 

 tions to the ordinary acids as the ethers do to the 

 alcohols must exist, — a prediction shortly afterwards 

 (1852) verified by Gerhardt's discovery of the anhy- 

 drides. 



Again, in 1852, we note the first germs of a theory 

 which was destined to play an all-important part in the 

 progress of the science; viz., the doctrine of valency, 

 or atomicity; and to Frankland it is that we owe this 

 new departure. But whether we range ourselves 

 with Kekule, who supports the unalterable character 

 of the valency of each element, or with Frankland, 

 who insists on its variability, it is now clear to most 



