300 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
as a whole, to the practical exclusion of the constituent atoms of the molecule. For 
if the atoms exert any influence, the mere passage from the liquid to the vaporous 
condition, which separates the molecules from each other, but leaves them individually 
intact, cannot destroy their powers of absorption. 
At an early stage of these researches the parallelism of liquid and vaporous absorp¬ 
tion forced itself upon my attention. Thus, my experiments on bisulphide of carbon 
vapour were connected with the deportment of liquid bisulphide, as set forth in 
Melloni’s table. The vapours, moreover, of chloride of sulphur and chloride of 
phosphorus, whose liquids stand in Melloni’s table next to bisulphide of carbon, were 
afterwards proved by me to possess a diathermancy corresponding to that of their 
liquids. After various references to this subject in preceding memoirs, a portion 
of the Bakerian Lecture for 1864 was devoted to its examination. Liquid layers 
enclosed between plates of transparent rocksalt were tested in regard to their dia¬ 
thermancy, and for the sake of control and verification they were employed in five, 
different thicknesses. The vapours of these liquids were examined in quantities pro¬ 
portional to the quantity of liquid, the same quality of heat being applied both to 
liquids and vapours. By these experiments, it seemed to me placed beyond a doubt, 
that the liberation of the molecule from the liquid condition did not destroy its 
absorbent power, the order of absorption being proved to be precisely the same for 
liquids and their vapours. Ten different substances were shown at the time here 
referred to to follow this rule. The list has since been extended, and I am not 
acquainted with a single real exception to the rule. Any reasoning, therefore, which 
ascribes a potent absorption to perfectly impalpable films, condensed on the surface of 
my apparatus, and which denies that absorption to the free molecules within the 
experimental tube, is in my opinion untenable. 
The relation between liquids and their vapours here indicated is very thorough. It 
extends beyond the field of experiment which we have hitherto had in view. I have, 
for example, published some researches on the action of rays of high refrangibility on 
gaseous matter, and have shown in a great variety of cases, that the molecules are 
shaken asunder by such rays. The actinic clouds, as I have called them, produced by 
this decomposition, reveal vividly the track of the beam by which they are generated, 
and render it easy to observe the distance to which the action penetrates. In the 
case of nitrite of amyl, for example, the power of decomposition is soon exhausted, the 
actinic cloud ceasing abruptly at a point about 18 inches from the place where the 
beam enters the vapour. An experimental tube 3 feet long, has therefore one half of 
its vapour shielded by the other half, and on reversing the tube, the shielded half 
comes instantly down as an actinic cloud. In the case of iodide of allyl vapour, on 
the other hand, the beam may pass through a charged experimental tube 5 feet long, 
fill it with an actinic cloud, and still effect decomposition in another tube placed 
beyond it. What is true of these vapours is true equally of their liquids ; for while 
a layer of the liquid nitrite -Jth of an inch thick prevents, when placed in the track of 
