324 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
other. The shorter tube was constructed with the view of proving that the absorp¬ 
tions I had recorded in previous memoirs were exerted by the vapour molecules, and 
not by liquid layers deposited on my plates of salt. The hypothesis of such layers 
being, however, 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 mercury inch of vapour in the long 
tube ought to quench as much heat as 3'5 inches in the short one. The same con¬ 
clusion 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 :— 
Again:— 
Sulphuric ether vapour. 
Short experimental tube. 
Pressure. 
Deflection. 
Absorption per 100. 
3*5 inches . . 
24° 
30 
o 
O 
CO 
37*5 
Total heat . 
50° 
3‘5 inches . . 
24° 
30 
7 
i ,, . . . 
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. 
1 inch . . 
Total heat 
2 inches . . 
Total heat 
Deflection. Absorption per cent. 
23° 30-3 
49° 
31° 38-8 
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. 
