92 VISCOSITY OF QASES 245 



tion in an easy way, by recalling attention to the fact that 

 the saturated vapour which enters the tube cannot, with a 

 high back-pressure, expand so much as still to follow with 

 sufficient exactness the gaseous laws, while, with smaller 

 back-pressure, it is brought by expansion still more nearly 

 into the state of a perfect gas. Schumann, 1 who had 

 taken part in carrying out these observations, followed up 

 this explanation still further by saying outright that the 

 originally saturated vapour must, during its expansion, form 

 drops of liquid which it carries on with it, and that thence 

 it follows that the mass transpired comes out as too great, 

 and consequently the coefficient of viscosity as too small. 



To make this explanation of the process perfectly clear 

 and convincing we have only to remember that a saturated 

 vapour which expands must cool thereby, and consequently 

 partially condense into a liquid. The capillary tube used 

 inLothar Meyer's experiments was, of course, contained 

 in a tube surrounded by the vapour of benzol of the same 

 pressure and temperature, so ithat it seemed to be ensured 

 against cooling. But if we consider that the thick wall of a 

 fine capillary tube offers a considerable obstruction to the 

 passage of heat, it in no way seems improbable that a slight 

 lowering of the temperature might have occurred within 

 the capillary tube, and that, therefore, a slight amount of 

 vapour might have been condensed. This mass of vapour 

 precipitated in the form of drops will then settle on the walls 

 of the tube, and spread over them as a thin liquid layer. 



That the transpired mass becomes greater by means of 

 this disturbance of the experiment follows at once from the 

 fact that the density of the liquid is very much greater than 

 that of the vapour. But we might raise the objection that 

 the friction which the liquid experiences as it flows along the 

 bottom of the tube is also much greater than that which 

 the vapour undergoes. This objection is, however, answered 

 by the fact that the coefficient of friction of a substance is 

 not by any means so greatly altered by the passage from 

 the vapour into the liquid state as its density is. Thus, for 

 instance, the coefficient of friction of water in the liquid 



1 Wied. Ann. 1884, xxiii. p. 393. 



