364 
PKOEESSOR TYNDALL ON THE ACTION OE EAYS 
Iodide of allyl 8 inches. 
Nitric acid 8 inches. 
Looked at for an instant after the vapour and acid had entered, with the white light 
of the electric lamp, the experimental tube was seen to be optically empty. The opaque 
solution was immediately introduced, and the vapour was subjected to the action of the 
dark rays for ten minutes. 
The opaque solution was then removed for an instant, and the tube was seen to be op- 
tically empty. The strong calorific rays had produced no action. 
The cell containing the blue liquid was then introduced ; in less than half a minute 
the action became visible, and augmented rapidly. In three minutes a cloud stretched 
quite through the tube from end to end. The scattering of the blue light by the coarse 
particles of this cloud produced a very pretty effect. 
Benzol 4 inches. 
Nitric acid 4 inches. 
Looked at for an instant after the admission of the vapour and acid the tube was 
optically empty. The opaque solution was introduced, and the invisible rays permitted 
to act for ten minutes. The solution was then removed, and the tube was examined for 
a moment with white light. It was optically empty. The blue liquid being interposed, 
visible action commenced 24 minutes afterwards*, and in ten minutes a cloud was formed 
throughout the tube. A repetition of this experiment confirmed the inaction of the 
calorific rays, and showed the action of the blue rays to be visible a minute after the 
introduction of the ammonia-sulphate cell. 
Toluol 8 inches. 
Nitric acid 8 inches. 
Looked at for an instant after the admission of the vapour and acid, the tube was 
found optically empty. Ten minutes’ action of the calorific rays produced no effect. 
The blue liquid was then interposed, and in two minutes a cloud was visible upon the 
feeble blue beam. At the end of ten minutes this cloud stretched throughout the tube. 
Iodide of ft propyl . » . 8 inches. 
Nitric acid 8 inches. 
The tube was optically empty at the commencement. At the end of ten minutes’ 
exposure to the calorific rays the tube was also empty. The blue cell was introduced, 
but in two minutes after its introduction, no cloud appearing, the cell was removed for 
an instant. The action had begun, though the coarse particles of the actinic cloud were 
too sparsely distributed to be seen by the weak blue light. The experiment was repeated. 
As before, ten minutes’ action of the calorific rays proved quite ineffectual. In one 
minute after the introduction of the blue liquid, no cloud being visible in the tube, 
the cell was removed. A crowd of particles were then seen upon the cone of light. 
* No doubt it bad previously commenced, but it was invisible in the feeble light. 
