July 14, 1892] 



NATURE 



243 



Amongst the articles written by eminent specialists, one, 

 the most important, is that contributed by Prof. J. J. 

 Thomson, of Cambridge, on the theories of the molecular 

 structure of bodies. It is from the interpretation of 

 chemical phenomena, by the help of exact physical re- 

 search, that we may most hopefully look for insight into 

 the true explanation of these phenomena. And although 

 the theory of the molecular constitution of matter now 

 universally held has been adopted as regards chemical 

 change ever since the publication of Dalton's new system 

 of chemistry in 1808, the crucial proof of its necessity has 

 only recently been given. Prof. Thomson briefly but 

 clearly explains the historical development of this proof. 

 The first attempt was made by Cauchy, founded on the 

 dispersion which light experiences when it passes through 

 transparent bodies. But this attempt was an incomplete 

 one, and a less ambiguous proof was given by Osborne 

 Reynolds in 1879, based upon the thermal effusion of 

 gases. Lord Kelvin, Loschmidt, and others have gone 

 still further, not only proving that matter possesses struc- 

 ture, but giving limits below which the " coarse-grained- 

 ness" of matter cannot lie. These conclusions are 

 founded upon considerations of several distinct sets of 

 phenomena, viz. surface-tension, the difference of potential 

 which occurs when two metals are placed in metallic con- 

 nection, the amount of polarization at the surface of an 

 electrode and of an electrolyte, and the viscosity, the 

 diffusion, and the conductivity for heat, of gases. The 

 discussion of the methods by which the limit is reached 

 in the case of surface tension is next clearly given, and 

 the result arrived at that a thickness of io~* cm. must 

 be comparable with the range of molecular action of the 

 water molecules. The results of the well-known researches 

 of Quincke on silver films and on capillary elevation, as 

 summarized in a lecture delivered before the Chemical 

 Society of London by Prof. Riicker in 1888, are then 

 explained, and the limits of molecular action deduced 

 from these experiments. Having given an idea of the 

 coarse-grainedness of matter, Thomson proceeds to con- 

 sider the various theories of that structure, and gives an 

 account of the most important of these by Lord Kelvin and 

 Lindemann. The evidence of molecular structure afforded 

 by the spectra of bodies, that concerning the arrange- 

 ment of the atoms in the molecule on the supposition 

 that the atoms are vortex-rings, and the electrical theory 

 of molecular structure, first brought forward by Helm- 

 holtz in his Faraday Lecture, are all clearly discussed ; 

 and the author's own researches on the conduction of 

 electricity by gases, which bear out the results of this 

 latter theory, are adverted to. The whole article, which 

 only extends over seven pages, forms an admirable ex- 

 position of a most important, if a somewhat difficult, 

 subject, and shows what chemistry gains from the work 

 of mathematical physicists. 



Another short but excellent article is that by Mr. Shen- 

 stone, on ozone, including, as it does, the most recent work 

 on the subject, as well as a resume of the older and better 

 known results. The question as to the relation existing 

 between the quantity of ozone produced and the potential 

 difference between the discharging surfaces, does not 

 appear to have as yet been settled, though Berthelot 

 finds that an increase of potential produces an increased 

 NO. 1185, VOL. 46] 



yield of ozone. Nor has the exact influence of tempera- 

 ture and pressure been properly made out, though it 

 appears that at a pressure of about 50 mm. ozone is 

 alternately produced and destroyed. These facts point 

 to the conclusion that, although much labour has already 

 been spent upon the investigation of ozone, much yet 

 remains to be accomplished before our knowledge of 

 " modified oxygen " is anything like complete. 



Of the recent progress made in our general chemical 

 conceptions, none are of greater, if any are of as great, 

 importance as the foundation of the periodic law by 

 Mendeleeff in 1869. A Dictionary which failed to give an 

 account, not only of the nature of this law, but also of its 

 rise and development, would indeed be incomplete. Mr. 

 Douglas Carnegie's article, however, does justice to his 

 subject, and I am glad to see he has not ignored the 

 extensions made by my lamented pupil and friend, 

 Carnelly, which are truly said to be as much in advance 

 of the earlier views of Dumas and Gladstone as the 

 periodic law is in advance of the earlier disconnected 

 schemes of classification. And I agree with the writer in 

 his remarks that if these extensions must be regarded as 

 bold speculations, they indicate the direction in which 

 investigations on the rationale of the periodic law, and 

 of the nature of the elements, will probably have to be 

 prosecuted before we can hope to arrive at any explana- 

 tion of the law, or of the nature of the chemical elements 

 themselves. 



The article on " Metals (rare) " is, of course, contributed 

 by Mr. Crookes. It contains an account of the contri- 

 butor's own researches on the splitting up of the rare 

 earth metals. Many of the metals described in our trea- 

 tises, and in the Dictionary itself, are probably mixtures. 

 Some years ago I proved that an element termed philip- 

 pium was in reality a mixture of two others, viz. terbium 

 and yttrium, and Mr. Crookes's researches have since 

 confirmed my results. It is, however, quite true, as 

 Crookes observes, that until we know what terbium and 

 yttrium themselves are, we have not got to the bottom of 

 the question. And from his own work it does not appear 

 very likely that the chemists of this generation will bottom 

 this subject, for the more Mr. Crookes works on the 

 separation of these bodies the more complex does the 

 question of identification appear to become. Those who 

 wish to form an idea of the character of work of this kind 

 will do well to study the article. 



A notable characteristic of this Dictionary is the summa- 

 tion of the properties of the different allied groups of 

 chemical elements. Thus in this volume we find an ex- 

 cellent article by one of the editors, Mr. Pattison Muir, on 

 the nitrogen group of elements. The relationships be- 

 tween the corresponding compounds of two different 

 members are clearly set forth in tabular form, and thus 

 the reader is able at a glance to compare the analogies 

 and differences which these compounds exhibit both in 

 composition and properties. 



Prof. Armstrong's article on isomerism bears out 

 the author's reputation for clear statement and com- 

 plete knowledge of his subject. He fully discusses its 

 historical development, strengthening his statements by 

 valuable quotations from the writings of chemists of 

 eminence, and brings the matter up to the latest views 



