1869.] Physics. 145 



the tube from end to end. For a moment the tube was optically 

 empty, nothing whatever was seen within it ; but before a second 

 had elapsed, a shower of hquid spherules was precipitated on the 

 beam, thus generating a cloud within the tubes. This cloud became 

 denser as the light continued to act, showing at some places a 

 vivid iridescence. The effect was the same when the air and vapour 

 were allowed to enter the tube in diffused daylight. The cloud, 

 however, which shone with such extraordinary radiance in the 

 electric beam, was invisible in the ordinary light of the laboratory. 



When di-y oxygen was employed to carry in the vapour, the 

 effect was the same as that obtained with air. When dry hydrogen 

 was used as a vehicle the action was also the same. The effect, 

 therefore, is not due to any interaction between the vapour of the 

 nitrite and its vehicle. This was further demonstrated by the 

 deportment of the vapoiu* itself. When it was permitted to enter 

 the experimental tube unmixed with air or any other gas, the effect 

 was substantially the same. Hence the seat of the observed action 

 is the vapour itself. With reference to the air and the glass of the 

 experimental tube, the beam employed in these experiments was 

 perfectly cold. It had been sifted by passing it through a solution 

 of alum, and through the thick double-convex lens of the lamp. 

 When the unsifted beam of the lamp was employed the effect was 

 still the same ; the obscm-e calorific rays did not afppear to interfere 

 with the result. When, previous to entering the experimental 

 tube, the beam was caused to pass through a red glass, the effect 

 was greatly weakened, but not extinguished. This was also the 

 case with various samples of yellow glass. A blue glass being 

 introduced before the removal of the yellow or the red, on taking the 

 latter away augmented precipitation occurred along the track of 

 the blue beam. Hence, in this case, the more refrangible rays 

 are the most chemically active. 



When the quantity of nitrite \apour is considerable and the 

 light intense, the chemical action is exceedingly rapid, the particles 

 j)recipitated being so large as to whiten the luminous beam. Not 

 so, however, when a well-mixed and highly-attenuated vapour fills 

 the experimental tube. The effect now to be described was obtained 

 in the greatest perfection when the vapour of the nitrite was 

 derived from a residue of the moisture of its hquid, which had been 

 accidentally introduced into the passage through which the dry 

 air flowed into the experimental tube. In this case the electric 

 beam traversed the tube for several seconds before any action was 

 visible ; decomposition then visibly commenced, and advanced slowly. 

 Tlie particles first precipitated were too small to be distinguished 

 by a hand lens; and, when the light was very strong, the cloud 

 appeared of a milky blue. When, on the contrary, the intensity 

 was moderate, the blue was pure and deep. In Briicke's important 



VOL. YI. L 



