of Gases and Molecular Force. 521 



mutual potential energy of two compound molecules in con- 

 tact in the gaseous state is affected in the same way as the 

 yirial (or potential energy) of all the molecules when brought 

 close to one another in the liquid state. With our present 

 knowledge the simplest explanation of this fact is got by 

 supposing that in a compound gas each collision of two mole- 

 cules is of the nature of a brief pairing with formation of a 

 temporary bimolecule, rearrangement of atomic energy, and 

 alteration of constant of mutual energy, all in a reversible 

 manner when the molecules can get out of one another's 

 influence, as in a gas, but not reversible in the limited free 

 range of a molecule in a liquid. Thus, as in the theory of 

 viscosity we have been dealing only with the mutual po- 

 tential energy of two molecules in contact, the possibility of 

 pairing in this manner has not been excluded. In connexion 

 with a more complete testing of the theory of this paper, it 

 may be pointed out that approximate values of C have been 

 given on theoretical grounds for CH 4 , NH 3 , S0 2 , Cl 2 , HC1, 

 H 2 S, and C 2 N 2 , the experimental determination of which 

 would supply a further check on the sufficiency of the 

 theory. 



Although it is not proposed to deal fully in this paper with 

 the viscosity of vapours, still, as the cause of a difference in 

 the cases of gases and vapours has been pointed out, it may 

 be as well to indicate what is the degree of importance of 

 that cause, namely the deflexion of molecular paths produced 

 by molecular attraction without the occurrence of actual col- 

 lisions. The best way to do this will be to calculate the part 

 of the viscosity of vapours due to collisions, and compare it 

 with the total viscosity found by experiment. Determinations 

 of viscosity have been made for the following substances : — 

 Ethyl chloride by Obermayer, with the capillary -tube method 

 (Sitzb. Akacl. Wien, lxxiii.); ethyl oxide, ethyl alcohol, steam, 

 benzene, acetone, chloroform, and carbon disulphide, by Puluj, 

 with the vibration method (ibid, lxxviii.) ; and a number of 

 esters by 0. Schumann by the vibration method (Wied. Ann. 

 xxiii.) . A still greater number of esters had been previously ex- 

 amined by L. Meyer and 0. Schumann with the capillary-tube 

 method (Wied. Ann. xiii.), with results which brought out the 

 viscosities much larger than they were afterwards found by 

 Schumann with the vibration method ; and as the reason for 

 the discrepancy has not been demonstrated (though we can 

 see on theoretical grounds that it is probably due to the use 

 of pressures too near that of saturation), we will not use these 

 doubtful data. 



The viscosity of any substance as a gas at any temperature 



