336 
PROFESSOR TYNDALL ON THE ACTION OF RAYS 
The nitrite vapour was then intentionally subjected to the action of the light. The 
beam employed was convergent. As the vapour reached the point of greatest concen- 
tration of the beam cloudy matter was there precipitated, which was afterwards whirled 
by the moving air into the more distant parts of the tube. The cloud thus carried away 
was incessantly renewed, and after the mixed air and vapour had ceased to enter, preci- 
pitation occurred all along the cone of rays in front of the focus. 
The lamp was then extinguished, and the mixture of air and nitrite vapour permitted 
to enter the tube in the dark. When the tube was full the condensed beam was sent 
through it. For a moment the light seemed to pass through air only; but after a 
moment’s pause a white cloud fell suddenly upon the conical portion of the beam, 
causing it to flash forth almost like an illuminated solid. 
When the beam, previous to allowing it to enter the vapour, was caused to pass through 
a red or yellow glass, the action though visible was feeble ; it was much more energetic 
when the beam passed through a blue glass. I sent a convergent beam through a red 
glass and observed the feeble effect. A blue glass was then added, and by the concert of 
both the light was completely cut off. On withdrawing the red glass, a very beautiful 
blue cloud came down upon the conical beam. The experiment proved that in this 
case, as in so many others, the blue rays are the “ chemical rays.” 
Solar light, as might be expected, produces all the effects of the electric light, and in 
regions more favoured than London may be employed in continuous researches of this 
nature. When the parallel beams of the sun are duly concentrated, the precipitation 
which they invoke in passing through nitrite-of-amyl vapour is copious and immediate. 
§ III- 
The simple apparatus employed in these experiments will be at once understood by 
reference to fig. 1. S S' is the glass experimental tube which has varied in length from 
1 to 5 feet, and which may be from 2 to 3 inches in diameter. From the end S the 
pipe pjjf passes to an air-pump. Connected with the other end we have the flask F, 
containing the liquid whose vapour is to be examined ; then follows a U-tube, T, filled 
with fragments of clean glass wetted with sulphuric acid; then a second U-tube, T', 
containing fragments of marble wetted with caustic potash ; and finally a narrow 
straight tube 1 1', containing a tolerably tightly fitting plug of cotton-wool. To save 
the air-pump gauge from the attack of such vapours as act on mercury, as also to 
facilitate observation, a separate barometer tube was employed. 
Through the cork which stops the flask F two glass tubes, a and b, pass air-tight. 
The tube a ends immediately under the cork ; the tube b, on the contrary, descends 
to the bottom of the flask and dips into the liquid. The end of the tube b is 
drawn out so as to render very small the orifice through which the air escapes into the 
liquid. 
The experimental tube S S' being exhausted, a cock at the end S' is carefully turned 
on. The air passes slowly through the cotton-wool, the caustic potash, and the sulphuric 
