May II, 1876] 



NATURE 



25 



! different, but have the same pressure, the reduced volumes 



of the gases diffused in opposite directions through the 

 septum are inversely as the square roots of their specific 

 gravities. 



If one or both of the vessels is of invariable volume, 

 the interchange of gas will cause an inequality of pressure, 

 the pressure becoming greater in the vessel which con- 

 tains the heavier gas. 



If a vessel contains a mixture of gases, the gas diffused 

 from the vessel through a porous septum will contain a 

 larger proportion of the lighter gas, and the proportion of 

 the heavier gas remaining in the vessel will increase 

 during the process. 



The rate of flow of a gas through a long capillary tube 

 depends upon the viscosity or internal friction of the gas, 

 a property quite independent of its specific gravity. 



The phenomena of diffusion studied by Dr. v. Wro- 

 blewski are quite distinct from any of these. The septum 

 through which the gas is observed to pass is apparently 

 quite free from pores, and is indeed quite impervious to 

 certain gases, while it allows others to pass. 



It was the opinion of Graham that the substance of the 

 septum is capable of entering into a more or less intimate 

 combination with the substance of the gas ; that on the 

 side where the gas has greatest pressure the process of 

 combination is always going on ; that at the other side, 

 where the pressure of the gas is smaller, the substance of 

 the gas is always becoming dissociated from that of the 

 septum ; while in the interior of the septum those parts 

 which are richer in the substance of the gas are commu- 

 nicating it to those which are poorer. 



The rate at which this diffusion takes place depends 

 therefore on the power of the gas to combine with the 

 substance of the septum. Thus if the septum be a film 

 of water or a soap bubble, those gases will pass through 

 it most rapidly, which are most readily absorbed by 

 water, but if the septum be of caoutchouc the order of the 

 gases will be different. The fact discovered by St. Claire- 

 Ueville and Troost that certain gases can pass through 

 plates of red hot metals, was explained by Graham in 

 the same manner. 



Franz Exner ^ has studied the diffusion of gases through 

 soap bubbles, and finds the rate of diffusion is directly as 

 the absorption-coefficient of the gas, and inversely as the 

 square root of the specific gravity. 



Stefan ^ in his first paaer on the diffusion of gases has 

 shown that a law of this form is to be expected, but he 

 says that he will not go further into the problem of the 

 motion of gases in absorbing medium, as it ought to form 

 the subject of a separate investigation. 



Dr. V. Wroblewski has confined himself to the investi- 

 gation of the relation between the rate of diffusion and 

 the pressure of the diffusing gas on the two sides of the 

 membrane. The membrane was of caoutchouc, o"co34 

 cm. thick. It was almost completely impervious to air. 

 The rate at which carbonic acid diffused through the 

 membrane was proportional to the pressure of that gas, 

 and was independent of the pressure of the air on the 

 other side of the membrane, provided this air was free 

 from carbonic acid. The connection between this result 

 and Henry's law of absorption is pointed out 



I "Pogg. Ann.," Bd. 153. 



^ " Ueber das Gleichgewicht u. d. Diffusion von Gasgemengen." Sitzb. 

 der k. Akad. (Wien), Jan. 5, 1871. 



The time of diffusion of hydrogen through caoutchouc 

 is 3'6 times that of an equal volume of carbonic acid. 

 The diffusion of a mixture of hydrogen and carbonic acid 

 takes place as if each gas diffused independently of the 

 other at a rate proportional to the part of the pressure 

 which is due to that gas. 



We hope that Dr. v. Wroblewski will continue his re- 

 searches, and make a complete investigation of the 

 phenomena of diffusion through absorbing substances. 



J. Clerk Maxwell 



MACALISTER'S ''ANIMAL MORPHOLOGY'' 

 All Introductioti to Animal Morphology and Sysiematic 

 Zoology. Part I.— Invertebrata. By Prof. Alexander 

 Macalister, M.B. (Longmans, Green, and Co., 1876.) 



HOW many of those who are not of an extra syste- 

 matic turn of mind, when they review their read- 

 ing in any special line of research, have continually to 

 regret that they have not had the industry to abstract as 

 well as to classify the various monographs and papers they 

 have perused, and to preserve them in a united form for 

 future reference. Those of us who are zoologists may lay 

 aside some of our misgivings on this score ; for one among 

 us, an exhaustive reader and an acute appreciator of the 

 relative importance of facts, has so widely distributed his 

 literary investigations, at the same time that he has made 

 it a principle to keep a memorandum of those points 

 which have most impressed him, that he has felt justified 

 —quite correctly, as all his readers we are convinc ed will 

 agree — in placing his compilation at the disposal of the 

 scientific public. The volume on the Invertebrata, now 

 before us, fills between four and five hundred closely 

 printed octavo pages. 



It is evident that a work constructed on the principles 

 above indicated must be of too exhaustive and too 

 abstruse a nature for the commencing student. It would 

 be impossible for any author so to combine primary 

 definitions and first principles with elaborate detail as to 

 produce a book which would appeal to the tyro as well as 

 the advanced zoologist. Prof. Macalister's "Introduc- 

 tion to Animal Morphology " must be therefore looked 

 upon as an introduction to the science proper, to be read 

 by the second-year student, or to be interleaved for 

 further annotation by the specialist. To teachers ot 

 Zoology it will be found invaluable on account of the 

 great fund of information it contains in a highly con- 

 densed form, also because in nearly all cases the name 

 of the authority for each important fact is associated (in 

 brackets) with his observation. In such a work we think 

 that no better method could have been employed. It 

 would have greatly overloaded the pages if full references 

 had been given ; and now that the invaluable Catalogue 

 of Scientific Papers, published by the Royal Soc ety — in 

 which the publications are arranged under the names 

 of authors — is within reach of all, in the libraries of 

 the learned societies, if not elsewhere, it is a matter of 

 no great difficulty for anyone who is particularly inter- 

 ested in any special detail, to find which is, and refer to 

 the monograph or shorter communication in which the 

 point in question is embodied. 



There is a small detail in association with the printing 

 of the work, a modification of which in the second volume 



