90 ATOMS, MOLECULES, 



other words the same number of molecules — as 3J 

 inches in the short one ; while 2 inches of vapour in 

 the long tube would be equivalent to 7 inches in the 

 short one. 



The experiments have been made with the vapours 

 of two very volatile liquids, namely, sulphuric ether 

 and hydride of amyl. The sources of radiant heat were, 

 in some cases, an incandescent lime cylinder, and in 

 others a spiral of platinum wire heated to bright red- 

 ness by an electric current. One or two of the measure- 

 ments will suffice for the purposes of illustration. First, 

 then, as regards the lime-light : — For 1 inch of pressure 

 in the long tube, the absorption was 18*4 per cent, of 

 the total beam ; while for 3-5 inches of pressure in the 

 short tube, the absorption was 18*8 per cent., or almost 

 exactly the same as the former. For 2 inches pressure, 

 moreover, in the long tube, the absorption was 25*7 per 

 cent. ; while for 7 inches, in the short tube, it was 25*6 

 per cent, of the total beam. Thus closely do the ab- 

 sorptions in the two cases run together — thus empha- 

 tically do the molecules assert their individuality. As 

 long as their number is unaltered, their action on 

 radiant heat is unchanged. Passing from the lime- 

 light to the incandescent spiral, the absorptions of the 

 smaller equivalent quantities in the two tubes were 

 23-5 and 23*4 per cent. ; while the absorptions of the 

 larger equivalent quantities were 32*1 and 32*6 per 

 cent, respectively. This constancy of absorption, when 

 the density of a gas or vapour is varied, I have called 

 ' the conservation of molecular action/ 



But it may be urged that the change of density in 

 these experiments has not been carried far enough to 

 justify the enunciation of a law of molecular physics. 

 The condensation into less than one-third of the space 

 does not, it may be said, quite represent the close file 



