246 PHENOMENA DEPENDENT ON MOLECULAR PATHS 92 



state at mean temperatures is only about 120 times greater 

 than that of water- vapour, while the density of the liquid 

 water is about 1,000 times greater than that of water- vapour 

 under atmospheric pressure. From this example we easily 

 understand how a wet vapour seems to have a smaller 

 viscosity than a dry saturated vapour. 



This behaviour substantially occurs also in the case of 

 mercury vapour, the viscosity of which has been determined 

 by Synesius Koch. Since the mercury vapour entered 

 into the capillary tube in a saturated state in Koch's 

 experiments, we must assume that, in these measures too, 

 some of the vapour condensed in the tube into little drops, 

 and that, consequently, the transpired masses came out too 

 large, and, therefore, the values of the viscosity too small l ; 

 and, indeed, they will have come out the smaller the nearer 

 the vapour was to condensing, and, therefore, the lower its 

 temperature. Hence the consequence would be that the 

 observations would give a greater variation of the viscosity 

 with temperature than the theory could explain. The same 

 might, in like manner, occur with many others of the 

 vapours mentioned in 87. 



1 Compare also 108. 



