338 PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
The differences here are inconsiderable, and lean to neither side ; within these limits, 
therefore, the calibration must be correct ; it shall be tested more severely in another 
part of this paper. 
H- 
We are now in a condition to compare the action of a series of volatile liquids with 
that of the vapours of those liquids upon radiant heat. 
Commencing with the substance of the lowest absorptive energy, and proceeding to 
the highest, we have the following order of absorption : — 
Liquids. 
Bisulphide of Carbon. 
Chloroform. 
Iodide of Methyl. 
Iodide of Ethyl. 
Benzol. 
Amylene. 
Sulphuric Ether. 
Acetic Ether. 
Formic Ether. 
Alcohol. 
Water. 
Vapours. 
Bisulphide of Carbon. 
Chloroform. 
Iodide of Methyl. 
Iodide of Ethyl. 
Benzol. 
Amylene. 
Alcohol. 
Formic Ether. 
Sulphuric Ether. 
Acetic Ether. 
Here, as far as amylene, the order of absorption is the same for both liquids and 
vapours. But from amylene downwards, though strong liquid absorption is in a general 
way paralleled by strong vapour absorption, the order of both is not the same. There 
is not the slightest doubt that next to water alcohol is the most powerful absorber in the 
list of liquids ; but there is just as little doubt that the position which it occupies in the 
list of vapours is the correct one. This has been established by reiterated experiments. 
Acetic ether, on the other hand, though certainly the most energetic absorber in the 
state of vapour, falls behind both formic ether and alcohol in the liquid state. Still, 
on the whole, I think it is impossible to contemplate these results without arriving at 
the general conclusion that the act of absorption is in the main molecular, and that the 
molecule maintains its power as an absorber and radiator when it changes its state of 
aggregation. Should, however, any doubt linger as to the correctness of this conclusion, 
it will speedily disappear. 
A moment’s reflection will show that the comparison here instituted is not a strict 
one. We have taken the liquids at a common thickness, and the vapours at a common 
volume and pressure. But if the layers of liquid employed were turned bodily into 
vapour, the volumes obtained would not be the same. Hence the quantities of matter 
traversed by the radiant heat are neither equal nor proportional to each other in the 
two cases; and to render the comparison strict they ought to be proportional. It is 
easy, of course, to make them so ; for the liquids being examined at a constant volume, 
their specific gravities give us the relative quantities of matter traversed by the radiant 
