June 9, 192 1] 



NATURE 



451 



known, and has been the subject of occasional 

 investigation at various times, even from the 

 ■earliest periods of systematic scientific inquiry. 

 It occupied the attention of the first Italian 

 academies, and was among the matters experi- 

 mentally studied by the Fellows during- the early 

 years of the Royal Society. Until comparatively 

 jecent times, however, work on the subject was 

 :sporadic, intermittent, and directed mainly to the 

 -investigation of particular cases rather than to 

 .the elucidation of general principles. The neces- 

 sities of modern chemical manufactures have 

 •created a demand for further and more accurate 

 iknowledge, inasmuch as the whole course of a 

 •chemical reaction and its economic aspect may 

 •depend upon it. Many instances of this fact 

 might be cited. One of the most recent, and also 

 •one of the most striking, is seen in the case of 

 the synthetic production of ammonia from its 

 •elements under the influence of catalysts, in which 

 vthe question of the appropriate pressure is of 

 iundamental importance. 



Prof. Ernst Cohen, of the van't Hoff Labora- 

 tory of the University of Utrecht, and his col- 

 laborator, Dr. VV. Schut, have placed chemists 

 and physicists under a great obligation by their 

 •compilation of the book now under review. It 

 does not profess to be a text-book on the subject. 

 It is, as stated, a compilation of the facts known, 

 or allowed to transpire, scattered through the 

 A'olumes of some fifty different periodicals, and 

 was originally made for the convenience of 

 workers on the subject of piezo- (or pressure) 

 chemistry in the laboratory which Prof. Cohen 

 •directs. The material thus accumulated has 

 been arranged in a systematic and orderly 

 manner. We have, first, a description of the 

 methods of creating and measuring high pres- 

 sures ; next, a general discussion of compressi- 

 bility, its methods, direct and indirect, with some 

 -account of their relative merits and defects. Then 

 follows a full historical description of the several 

 attempts to obtain accurate values of certain 

 fiduciary constants generally necessary in piezo- 

 metric work — viz. the compressibility coefficients 

 of glass, mercury, water, and incidentally of ice, 

 and their relation to temperature. Each section 

 is accompanied by bibliographical references to 

 the original sources of information. 



The authors then treat of the compressibility 

 •coefficients of the various elements and such of 

 their compounds as have been studied. Special 

 attention is, of course, paid to the work of 

 Richards, of Harvard, and his co-workers, and 

 the question of the compressibility of atoms and 

 NO. 2693, VOL. 107] 



the existence of interatomic spaces in solids and 

 liquids is shortly discussed, mainly in the light of 

 the American chemist's published views on the 

 subject. .\s regards liquids, an attempt is made 

 to group them in conformity with their chemica' 

 relationships. Thus all the hydrocarbons art. 

 brought together, as are the alcohols, esters, 

 halogen compounds, acids, etc., obviously with 

 the view of facilitating the detection of general 

 principles. It must be confessed, however, that 

 as yet the data afforded by different investigators 

 are too discrepant to afford a satisfactory basis 

 for generalisations. This is due mainly to imper- 

 fections in the method of observation, and in a 

 less degree in some cases to insufficient care 

 in the purification of the liquids employed. Accu- 

 rate work, like that of Bridgman, suffers by asso- 

 ciation with that of earlier inquirers whose 

 measurements were largely of the pioneering 

 order. 



Although definite numerical values are lacking 

 in many cases, certain conclusions may be said 

 to be fairly well established. Thus, for example, 

 Bartoli has shown that the compressibility-coeffi- 

 cient in an homologous series of the liquid 

 paraffins decreases with the increase of molecular 

 weight. That the same is true of the aromatic 

 hydrocarbons appears from the observations of 

 Richards and his co-workers. Measured at equal 

 temperatures and pressures, benzol is more com- 

 pressible than toluol, and toluol than xylol. The 

 various isomerides of xylol have, however, dif- 

 ferent compressibilities, o-xylol being less com- 

 pressible than m-xylol, which in its turn is more 

 compressible than /)-xylol, and still less compress- 

 ible than ethyl-benzol. Identical thermometric 

 temperatures, strictly speaking, are not absolute 

 evidence of a comparable physical condition. 

 Before any sound deductions can be made it will 

 be necessary to establish what is a valid com- 

 parable condition. There is a considerable volume 

 of work on the compressibility of liquids, but its 

 treatment and discussion are vitiated by the cir- 

 cumstance that this point has hitherto been in- 

 sufficiently appreciated. For a fuller account of 

 the relation of compressibility to the chemical 

 nature and constitution of liquids. Prof. Cohen's 

 volume must be consulted. 



The influence of pressure upon the expansion- 

 coeflficients of substances — solids, liquids, solu- 

 tions, and alloys — and upon surface tension and 

 melting-point has been studied by many ob- 

 servers. Their work has been systematically col- 

 lated by the authors, and its outcome discussed. 

 The case of water is of special interest, on 



