Mr. M. M. Pattison Muir on Chemical Classification. 193^ 



Measurements of the diftusion, of the viscosity, and of the 

 transpiration of liquids have ah'eady thrown some light upon 

 the chemical composition, and so upon the classification, of 

 these liquids. 



" The coefficient of diffusion represents the rate at which 

 the equalization of chemical composition goes on by the 

 exchange of the molecules themselves." " The coefficient 

 of viscosity .... represents the rate at which the equalization 

 of velocity goes on by the exchange of the momentum of the 

 molecules"*. In each case, ^'if the temperature remain the 

 same, the rate of equalization is inversely proportional to the 

 density"!. But the density is proportional to the mass of 

 the molecules ; hence heavy molecules will diffuse more slowly 

 than lighter molecules. The velocity with which the mole- 

 cules diffuse, or the A^elocity with which they pass through 

 capillary tubes, will therefore be dependent upon the relative 

 masses of those molecules ; hence our knowledge of the mole- 

 cular weights of solid and liquid compounds may be very ma- 

 terially advanced by careful measurements of the coefficients 

 of diffusion, viscosity, and transpiration of various bodies. 

 And if this be so, much light would be thrown by such re- 

 searches upon the general problem of chemical classification J. 

 If from researches on diffusion, viscosity, and transpiration we 

 may hope for determinations of the relative magnitudes of 

 solid and liquid molecules, it is evident that such researches 

 will also throw light upon the question of the chemical com- 

 position of mixtures of several compounds w^hich are capable 

 of exerting actions upon one another, but which remain, along 

 with the products of their mutual actions, in a state of solu- 

 tion § . 



Kopp II and others have established the fact that determina- 

 tions of specific or molecula?' vohimes throw much light upon 

 the general composition of chemical compounds. The mole- 

 cular volume of the compound is of course directly proportional 

 to the molecular weight, and inversely proportional to the 

 density. If we determine the molecular volumes of a series 



* Clerk Maxwell, ^Theory of Heat,' 1st. edit. p. 302. 



t Clerk Maxwell, loc. cit. 



X See Graham's Classic Series of Researches ; also Loschmidt, Wien. 

 Akad. Ber. [2] Ixi. p. 367 &c. ; also in Jahresher. 1870, p. 58 ; also 

 Wretschko, Wien. Akad. Ber. .[2] Ixii. p. 575 &c. ; also in Jahresher. 

 1871, p. 51: Maxwell, Phil. Mag. [IV.] vol. xlvi. p. 468; Gueront, 

 Cotnptes Rendus, Ixxxiii. p. 1291 ; &c. &c. 



§ Mr. Hannay is also engaged in investigating the problems presented 

 by transpiration, or, as he prefers to call it, microrheosis of liquids. His 

 method is wonderfully exact. 



II Ann. Chem. Pharm. xcvi. pp. 153, 303; c. p. 91 &c. 

 Phil. Mag. S. 5. Vol 4. No. 24. Sept. 1877. 



