342 
PROFESSOR TYNDALL ON THE ACTION OF RAYS 
action hacl been continued for some time, the fine purple colour of iodine exhibited itself 
at the end of the tube most distant from the source of light. When hydrogen was the 
vehicle, the clouds were particularly lustrous and beautiful. Here and there also, amid 
the white and coarser sections of a cloud, spaces of delicate blue would reveal them- 
selves, reminding one of the colour of a pure sky. The words “ wonderful,” “ beautiful,” 
“lustrous,” and others of a similar nature, occur frequently and naturally in my notes 
of this period ; for in those earlier experiments the cloud-forms obtained were so amazing, 
and their colours and textures so fine, as to rivet attention upon them alone. 
With long-continued action the colour due to the discharge of iodine became very 
intense. It was strong enough to empurple the beam which passed through the air 
of the laboratory after its transmission through the experimental tube, and to colour 
deeply a white screen on which the beam was permitted to fall. In what condition was 
this iodine 1 It could be liberated by a beam deprived almost wholly of its calorific 
rays. The temperature of the experimental tube was indeed so moderate that a quantity 
of iodine placed within it and permitted to saturate the space with its vapour, produced 
:a barely perceptible flush on a piece of white paper placed there expressly to detect it. 
The far more deeply coloured iodine revealed by the beam in the actinic cloud must, I 
think, have been for the most part liquid, and not vaporous iodine. I say liquid, be- 
cause the substance was probably dissolved by the particles of the cloud with which it 
was so intimately mixed. Di-ally 1, for example, is a powerful solvent of iodine, and 
it was probably one of the products of decomposition. 
The iodide of isopropyl also capitally illustrates the action of light upon vapours. It 
is more slowly acted upon than either the nitrite of amyl or the iodide of allyl ; never- 
theless, in sufficient quantity, its decomposition is very brisk and energetic. Purified air 
which had bubbled through the liquid iodide was conducted into the experimental tube. 
When the pressure was 1 inch of mercury, the light playing upon the vapour for five 
minutes produced no action ; but when it was 10 inches a blue cloud made its appear- 
ance in two minutes, and in ten minutes it had almost filled the tube. When the 
pressure was 20 inches, the action commenced more quickly, and the cloud generated 
was more dense. The whirling motions of this cloud appeared to be more brisk than 
that of the others examined. With 30 inches of the mixed air and iodide the action 
began in a quarter of a minute, and in five minutes a dense cloud was formed through- 
out the tube. The purple of the discharged iodine was also very plain in this cloud. 
§ VII. 
In the preliminary notice of these experiments laid before the Royal Society in June 
1868, considerable stress is laid upon the fact that the same rays are absorbed by the 
nitrite of amyl in the liquid and in the vaporous state. A layer of the liquid not more 
than one-eighth of an inch in thickness was found competent to withdraw from a powerful 
beam all the constituents which could effect the decomposition of its vapour. The action 
of the nitrite resembles in this respect that of the sulphate of quinine on the rays which 
