402 Prof, Tyndall on the Action of Free Molecules on 



memoirs were exerted by the vapour molecules, and not by 

 liquid layers deposited on my plates of salt. The hypothesis 

 of such layers, however, being completely disposed of, we can 

 carry the experiments a step further. Assuming that the 

 absorption does not change with change of density, so long 

 as the quantity remains constant, it would follow that 1 mer- 

 cury inch of vapour in the long tube ought to quench as much 

 heat as 3*5 inches in the short one. The same conclusion 

 ought, of course, to hold good when we compare 2 inches of 

 vapour in the long tube with 7 inches in the short one. 



The experiments have been made, and with the following 

 results : — 



Sulphuric-ether vapour. 

 Short experimental tube. 

 Pressure. Deflection. Absorption per 100. 



3-5 inches . . 24° 30 



7 „ . . 30 37-5 



Totatheat . 50 

 Again. 



3-5 inches . . 24° 30 



7 „ . . 31-5 39-4 



Total heat . 50 

 Taking the _ means of these two experiments, we have the 

 absorptions — 



For 3*5 inches .... 30 per cent. 

 For 7 „ . . . . 38-5 „ 

 Such is the result obtained with the short tube. We now 

 turn to the long one : — 



Long experimental tube. 

 Pressure. Deflection. Absorption per cent, 



linch. ... 23° 30-3 



Total heat . 49 

 2 inches ... 31 38-8 



Total heat . 50 



These results are almost identical with those obtained with 

 the short tube and greater pressures. 



The source of heat in this case was a dull lime-light. In 

 subsequent experiments the light was brightened. Here is 

 the result : — 



Long experimental tube. 

 Pressure. Absorption. 



1 inch 22-3 per cent. 



2 inches 29*5 „ 





