320 PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
absorption immeasurably small, gradually augment the thickness of the absorbing 
layer until it becomes a column 38 inches long, and find, throughout, the increase of 
absorption running hand in hand with the increase of the length of the absorbent 
layer. This result is utterly irreconcilable with the hypothesis that liquid layers on 
the plates of salt played any important part in my experiments. 
With regard to vapour-hesion,” I have to make the following further remarks. 
Let a thin plate of polished rocksalt be placed upon, or against, the face of the thermo¬ 
pile, with its lampblack removed, so as to expose a clean metallic surface. Let an open 
glass tube end about a quarter of an inch above the plate of salt; and through this 
tube let a current of mixed air and vapour be gently urged downwards against the 
plate. No fine spray, which might readily arise from bubbling through a liquid, is to 
be permitted to mingle with the vapour. My vapours, I may say, have been usually 
formed, without bubbling, in large flasks, each containing a portion of a volatile liquid, 
the vapour of which was permitted to diffuse in the air of the flask. Fletcher’s foot- 
bellows were employed to urge the vapours forward. The needle being at zero, or 
thereabouts, on causing the mixed current to impinge upon the rocksalt, the needle 
immediately swings aside, the deflection varying from 20° to 80° and more, according 
to the delicacy of the galvanometer and the quality of the vapour. 
From such experiments, which prove heat to be liberated when vapour comes into 
contact with the salt, the condensation of the vapour has been inferred. I accept the 
inference. 
This, however, does not imply the acceptance of the inference from this inference, 
that the condensed films exert the action ascribed to them on radiant heat. To test 
whether they do so or not I had a circular plate of rocksalt mounted on its edge, and 
so placed that a beam of heat passed through it normally to a distant thermopile. The 
source of heat was an incandescent spiral, aided by a concave mirror; and the total 
radiation, after passing through the salt and impinging on the pile, produced a deflec¬ 
tion of 51°. This radiation was accurately neutralised by a compensating cube, 
the needle under the operation of the two opposing forces pointing to 0°. By an 
arrangement which every experimenter can imagine for himself, sheets of air, laden 
with various vapours, could be poured in succession over the plate of salt. If, under 
these circumstances, absorbent films were formed, the equilibrium would be destroyed 
and the needle would move from 0°. My slit, which was the flattened base of an 
open tin cone, was placed across the upper part of the plate of rocksalt, so that the 
vapours, which were all heavier than air, should fall over the plate downwards. Here 
are the results obtained when the following vapours were permitted to form films 
upon the plate:— 
