346 
PROFESSOR TYNDALL ON THE ACTION OF RAYS 
the light, but which in the presence of other substances is readily decomposed. During 
the earlier stages of this inquiry a vast number of experiments were made with benzol 
and commercial hydrochloric acid. The results well illustrate actinic action, but they 
are not to be accepted as indicative of the action of pure hydrochloric acid. Indeed with 
the pure acid and benzol vapour there is no visible action. 
On the 16th of November, 1868, 2 inches of air and benzol vapour were sent into the 
experimental tube, and afterwards the tube was filled with air which had bubbled through 
the commercial acid. My notes, written at the time, describe the action of light upon 
the mixture as producing a cloud of an exquisite sky-blue colour, only more luminous 
and ethereal than the sky. The figure of the cloud was also very wonderful. 
This cloud was permitted to remain for fifteen hours in the experimental tube unin- 
fluenced by light. iVfter this interval it was found still floating, being composed of 
curiously shaped granular sections joined together by others of more delicate hue and 
texture. The renewed light set the cloud immediately in motion, the "granular parts 
disappeared, and the whole for a length of 18 inches resumed its primitive delicate hue 
and texture. In some portions it became white or whitish grey, but at others it was 
a pure firmamental blue. It became very dense as the light continued to act, and finally 
developed itself into a form of astonishing complexity and beauty. 
The experimental tube had then a current of dry air swept through it, and it was 
afterwards exhausted. 2 inches of the benzol vapour were admitted as before, and dry 
air was added until the tube was full. It required five minutes’ action of the light to 
develope the faintest visible cloud ; even after ten minutes’ action the cloud was very 
faint*. The tube was again cleansed and exhausted, 2 inches of the benzol vapour were 
admitted, followed by air and hydrochloric acid until the tube was full. On starting 
the light chemical action began almost immediately, and ended by the formation of a 
cloud throughout the tube. The influence of the commercial hydrochloric acid is here 
demonstrated. The interaction of nitric acid and benzol will be immediately referred to. 
Bisulphide of carbon is also an illustration in point. Alone or mixed with air it 
resists the action of the light ; in the presence of hydrochloric or of nitric acid it is 
responsive to that action. On the 17tli of November, 1868, for example, the pure vapour 
was admitted into the experimental tube until a depression of 2 inches of the mercury 
column was observed. A powerful light was permitted to act for twelve minutes upon 
the vapour, but no action was observed. A quantity of air which had passed through 
aqueous hydrochloric acid was then admitted into the tube. Six minutes subsequent 
action of the light developed a cloud of considerable density. Toluol and other sub- 
stances might here be mentioned in further illustration of this mode of decomposition. 
But I pass over hundreds of these earlier experiments which were made chiefly to instruct 
myself and to secure me from error. Some definite results will be given further on. 
* It was certainly due to a residue of the previous charge. 
