360 
PEOFESSOE TYNDALL ON THE ACTION OF BAYS 
beautiful and curious forms. The inner portions of its two longitudinal lobes were 
shaped like screws ; they moreover rotated like screws, moving as if they were pushed 
mechanically into the mass of cloud in front of them. The whole effect was very fine, 
and the action extremely vigorous. As might be expected from the density of the 
cloud, the normal polarization was almost nil. 
II. Air and iodide-of-allyl vapour . . 8 inches ; then 
Air and nitric acid 8 inches. 
The tube was optically empty at first, but the action, though not so brilliant as in the 
last case, was very prompt and energetic. A very coarse cloud was rapidly formed 
throughout the entire tube, upon the bottom of which the particles appeared to fall 
in showers. 
The cloud having apparently ceased to thicken, the lamp was suspended. On its 
accidental reignition a fine cloud, dense and luminous, was suddenly precipitated among 
the coarser particles. On suspending the lamp the finer cloud vanished, but the coarser 
particles remained. On reignition the fine white cloud was precipitated as before, en- 
tirely masking the coarser one by its superior density and closeness of texture. This 
action was repeated several times in succession. 
Allowing the parallel beam from the lamp to act for a time upon the cloud, on 
changing it to a convergent one the superior intensity of the light immediately caused a 
fine, dense, and luminous precipitation. By rendering the beam alternately parallel and 
convergent, this action could be reproduced several times in succession. 
III. Air and nitric acid ...... 1 inch ; then 
Air and iodide-of-allyl vapour . . 15 inches. 
Immediately after the starting of the lamp the action commenced, and spread through 
the entire tube in less than two minutes. The falling of the particles in vertical showers 
occurred here also. 
After it had acted for a time the lamp was extinguished, and the tube was permitted 
to remain quiescent for an hour. On reigniting the lamp the tube appeared to be quite 
empty. The cloud that had previously filled it had entirely disappeared. Half a 
minute’s action of the beam brought down upon it copious precipitation, a revival of 
the action occurring afterwards throughout the entire tube. 
Iodide of Isopropyl (CH(CH 3 ) 2 1). 
Contents of experimental tube. 
I. Air and iodide-of-isopropyl vapour . . 1 inch ; then 
Air and nitric acid 15 inches. 
After a moment of apparent emptiness a very splendid action set in. A cloud of 
exceeding brightness suddenly filled the space occupied by the convergent beam. The 
