312 
PROFESSOR TYNDALL OR THE ACTION OF FREE MOLECULES ON 
a reflecting cone, received the rays which passed through the experimental tube. The 
other face, also provided with a cone, received the rays from a “ compensating cube, 5 ’ 
used, as formerly, to neutralise the radiation from the source, and to bring the needle 
of the galvanometer to zero when the experimental tube was exhausted. On the 
entrance then of any absorbent gas or vapour the equilibrium was destroyed, the 
needle moved from zero, and from the observed permanent deflection the absorption 
was calculated. Other qualities of heat, and other experimental tubes than that here 
described, were afterwards introduced into the inquiry. 
I here give the measurements executed in 1880 with the vapours of nine different 
liquids, in experimental tubes of the dimensions above given. 
Table I. 
Vapours. 
Liquids. 
Pressures. 
( 
Leslie’s cube 
Leslie’s cube 
Red hot 
vacuum. 
free. 
spiral. 
Bisulphide of Carbon . 
• 
•48 
in. itier. 
4-4 
5-0 
7-6 
Chloroform. 
• 
•36 
33 
12-8 
12-9 
28-8 
Benzol. 
•32 
33 
14-8 
15-0 
44-5 
Iodide of Ethyl .... 
•36 
33 
18-4 
19-3 
47-0 
Iodide of Methyl 
•46 
33 
25-0 
26-2 
59-0 
Amylene. 
•26 
33 
261 
27-2 
65-0 
Sulphuric Ether .... 
•28 
33 
35-0 
35-6 
71-0 
Acetic Ether. 
•29 
33 
43-3 
437 
77-5 
Formic Ether .... 
•36 
33 
43-3 
44-0 
78-0 
The " pressures ” in this table are chosen with a view to the comparison of liquids 
and vapours. They express quantities of vapour which are proportional to the 
quantities of matter in the respective liquids at a common thickness. The two next 
columns contain the absorptions per 100, of the heat from two Leslie’s cubes, the one 
with a vacuum in front of it, the other placed in free air, and well protected from air- 
currents. The close agreement of the two columns proves the “ front chamber’' to be a 
superfluity. It also illustrates the coincidence to be attained in these measurements 
when they are carefully made. In the last column I have placed the absorptions 
exerted by liquid layers of the respective substances at a common thickness of one 
millimeter. The source of heat here was an incandescent platinum spiral. The order 
of* absorption of liquids and vapours is the same. 
This order is, as might be expected, undisturbed, when we apply heat of the same 
quality to liquids and vapours respectively. This is shown by the following table 
