CONTRIBUTIONS TO MOLECULAR PHYSICS. 439 



different layers of each, varying in thickness from 0-02 of 

 an inch to 0*27 of an inch. The liquids were enclosed, 

 not in glass vessels, which would have materially modi- 

 fied the incident heat, but between plates of transparent 

 rock-salt, which only slightly affected the radiation. 

 The source of heat throughout these comparative experi- 

 ments consisted of a platinum wire, raised to incandes- 

 cence by an electric current of unvarying strength. The 

 quantities of radiant heat absorbed and transmitted by 

 each of the liquids at the respective thicknesses were first 

 determined. The vapours of these liquids were subse- 

 quently examined, the quantities of vapour employed 

 being rendered proportional to the quantities of liquid 

 previously traversed 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 absorber 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 multi- 

 tude 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-hot platinum 

 spiral to pass through it ; benzol absorbs 43 and trans- 



