352 
PEOFESSOE TYNDALL ON THE ACTION OF EAYS 
acid a second time, an exposure of twenty minutes was found to produce no action. 
This and many other similar experiments demonstrate the inertness of pure hydrochloric 
acid. The inert acid of the foregoing experiment was permitted to remain in the expe- 
rimental tube all night. Next morning, when the beam was permitted to play upon it, 
a blue streak became visible in less than a minute. In ten minutes the tube was filled 
with a delicate cloud. This was an almost every-day occurrence at the time here 
referred to. There must have been something in this tube in the morning which 
was not there on the preceding night. An infinitesimal residue had crept out of the 
stopcocks, or the hydrochloric acid had acted on the cerate employed to render the tube 
air-tight. 
And here I would allude in passing to an effect which at a future stage of this inquiry 
will be found suggestive of the mechanism by which the complex cloud-forms are pro- 
duced. I touched the top and bottom of the experimental tube for a moment with my 
two fingers ; the cloud, which was of exceeding lightness, immediately showed responsive 
convection. It was wonderfully sensitive to the slightest local change of temperature. 
Once started in this simple way the motions of the cloud went on, and ended in the 
development of a splendid cloud-figure. 
The influence of a minute residue is also strikingly illustrated by the following fact. 15 
inches of mixed hydrochloric acid and air, exposed for fifteen minutes to a powerful beam, 
showed not the slightest trace of action. A small pellet of bibulous paper, not half the 
size of a pea, was moistened with the iodide of allyl. I held the pellet between my 
fingers till it became almost dry, then inserted it into a connecting piece, and sent a little 
air over it into the experimental tube. On stopping the flow of air a blue cloud began 
to form immediately, and in five minutes the rich colour had extended quite through 
the experimental tube. This cloud was 3 feet long and discharged a good body of light, 
but for some minutes it could be completely quenched by the Nicol. At the end of 
fifteen minutes a white massive cloud filled the experimental tube. Considering the 
amount of matter concerned in the production of this nebula, it seemed like the deve- 
lopment of a cloud-world out of nothing. 
But this is not all. The pellet of bibulous paper was removed, and the experimental 
tube was cleansed by allowing a current of dry air to sweep through it. The current 
passed through the connecting piece in which the pellet of bibulous paper had rested. The 
supply of air was at length cut off and the experimental tube exhausted. 15 inches of 
hydrochloric acid were sent into the tube through the same connecting piece. It is here 
to be noted, 1°, that the whole quantity of iodide of allyl absorbed by the pellet was 
exceedingly small ; 2°, that I had allowed almost the whole of this small quantity to 
evaporate ; 3°, that the pellet had been cast away and the tube in which it had rested 
had been rendered the conduit of a strong current of pure air. It was such a residue 
as could linger after all this in the connecting piece that was carried by the hydrochloric 
acid into the tube, and there acted on by the light. 
A minute after the ignition of the lamp chemical action declared itself by the forma- 
