354 
PROFESSOR TYNDALL ON THE ACTION OP RAYS 
I hoped during the present year to be able to go over again a vast amount of ground 
rendered debateable by the discovery of such irregular actions as those here recorded. 
An accident in the Alps has unfortunately disqualified me from doing this. But as I 
find that ardent workers have already entered this new field of inquiry, I think it right 
to lay before the Society this first part of my researches. I omit not only descriptions of 
the deportment but even the names of the vast majority of the substances with which I 
have experimented; confining myself to eight or ten closely examined and well-esta- 
blished cases of actinic decomposition, and putting aside for reconsideration all such 
matters as might vainly occupy the attention of the Society. 
§ XII. 
The vapours of the substances mentioned in this section were sent into the tube in the 
♦ 
manner described in § III. They were mixed, in the proportions stated, with air which 
had been permitted to bubble through aqueous nitric acid, and the effect produced by 
exposure to the condensed beam of the electric lamp is in each case described. 
Toluol (C 7 H 8 ) : — A transparent colourless liquid. 
Contents of experimental tube. 
I. Air with toluol vapour ... I inch ; then 
Air with aqueous nitric acid . 15 inches. 
On igniting the lamp the experimental tube was optically empty. 
After thirty seconds the track of the beam through the experimental tube became 
blue ; the blue was about as pure as that of an ordinary cloudless sky in England. After 
two minutes the colour began to change to a whitish blue. 
The light discharged normally by the blue cloud continued to be perfectly polarized 
for four minutes after the first appearance of the cloud. A rich residual blue was after- 
wards observed when the Nicol was in its position of minimum transmission. 
At the end of ten minutes the residual colour was no longer blue, but bluish white. 
Hence the light which first exhibited perfect polarization, and which first escaped from 
perfect polarization, was blue. 
At the end of fifteen minutes a very beautiful cloud-figure was developed. The denser 
portions of the cloud were very luminous. 
II. Air and toluol vapour ... 8 inches ; then 
Air and aqueous nitric acid . . 8 inches. 
The experimental tube was optically empty for a moment at starting, but the action 
was so rapid that in two or three seconds the tube was filled with a heavy cloud. At 
the beginning the colour of the cloud was blue. The incipient cloud which whirled 
round the beam discharged for two or three seconds perfectly polarized light ; but the 
perfection of the polarization ceased almost immediately. 
