340 
PROFESSOR TYNDALL ON THE ACTION OF RAYS 
is the disgust removed by the reflection that at a distance from the focus, though we do 
not see the dirt, we are breathing precisely the same air. The difficulty of wetting it 
before referred to, may render this suspended matter comparatively harmless to the 
lungs, but where these are fragile its mere mechanical irritation must go for something. 
Perhaps a respirator of cotton-wool might in some cases be found useful*. 
§ V. 
I now return to the nitrite of amyl. The action of light upon the vapour of this 
substance is exceedingly prompt and energetic. It may be illustrated by simply blowing 
the vapour into a concentrated sunbeam. Or the experiment may be made to take 
the following form : — Connecting the tube b of the flask F with the pipe of a bellows, 
after inflating the latter a sharp tap upon its board sends a puff of vapour through 
the tube a into the air. In a moderately lighted space nothing is seen ; but when the 
puff is projected into a concentrated sunbeam, or into the beam from the electric lamp, 
on crossing the limiting boundary of light and shade it is instantly precipitated as a 
white ring. The ring has of course the same mechanical course as the smoke-rings puffed 
from the mouth of a cannon, but it is latent until revealed by actinic precipitation f. 
In every one of the numerous experiments made with the nitrite of amyl, the 
chemical energy appeared to exhaust itself in the frontal portion of the experimental 
tube. A dense white cloud would fall for a distance of 12 or 15 inches upon the beam, 
while beyond this distance the tube would appear almost empty. This absence of action 
might naturally be ascribed to the diffusion of the beam beyond the focus ; but when 
the light was so converged as to bring the focus near the distant end of the tube the effect 
was the same. When, moreover, a concave mirror received a parallel beam which had 
traversed the tube, and returned it into the vapour in a high state of luminous concen- 
tration, the light was ineffectual. The passage of the beam through a comparatively 
small depth of the vapour appeared to extract from it those constituents which produced 
decomposition. That the vapour was present at the distant end of the tube, was proved 
by the fact that both with the sun and with the electric light the reversal of the tube 
instantly brought down a heavy cloud. As regards the chemical rays nitrite of amyl is 
the blackest substance that I have yet encountered. It rapidly extinguishes them, 
leaving behind a beam of sensibly undiminished photometric intensity, but powerless as 
a chemical agent as far as the nitrite is concerned. 
In these experiments air was employed as the vehicle of the nitrite-of-amyl vapour. 
By varying the quantity sent into the experimental tube, it was possible to vary in a 
remarkable manner the character of the resulting decomposition. The most splendid 
diffraction colours could be thus produced, and the finest texture could be imparted to the 
clouds. When pure oxygen or pure nitrogen was used, the effect was almost the same 
* Since this paper was forwarded to the Royal Society these experiments have been greatly extended. See 
Proceedings of the Royal Institution, January 1870. — [J. T., July 1870]. 
t By a special arrangement it is easy to obtain such rings 2 inches and more in diameter. 
