CONTRIBUTIONS TO MOLECULAR PHYSICS. 387 



different layers of each, varying in thickness from 0.02 

 of an inch to 0.27 of an inch. The liquids were en- 

 closed, not in glass vessels, which would have material- 

 ly modified the incident heat, but between plates of 

 transparent rock-salt, which only slightly affected the 

 radiation. The source of heat throughout these com- 

 parative experiments consisted of a platinum wire, 

 raised to incandescence by an electric current of un- 

 varying strength. The quantities of radiant heat ab- 

 sorbed and transmitted by each of the liquids at the 

 respective thicknesses were first determined. The 

 vapours of these liquids were subsequently examined, 

 the quantities of vapour employed being rendered pro- 

 portional to the quantities of liquid previously trav- 

 ersed by the radiant heat. The result was that, for 

 heat from the same source, the order of absorption of 

 liquids and of their vapours proved absolutely the 

 same. There is no known exception to this law; so 

 that, to determine the position of a vapour as an ab- 

 sorber or a radiator, it is only necessary to determine 

 the position of its liquid. 



This result proves that the state of aggregation, as 

 far at all events as the liquid stage is concerned, is of 

 altogether subordinate moment a conclusion which 

 will probably prove to be of cardinal importance in 

 molecular physics. On one important and contested 

 point it has a special bearing. If the position of a 

 liquid as an absorber and radiator determine that of its 

 vapour, the position of water fixes that of aqueous 

 vapour. Water has been compared with other liquids 

 in a multitude of experiments, and it has been found, 

 both as a radiant and as an absorbent, to transcend 

 them all. Thus, for example, a layer of bisulphide of 

 carbon 0.02 of an inch in thickness absorbs 6 per cent., 

 and allows 94 per cent, of the radiation from the red- 



