1868.] 



Chemical Reactions produced by Light. 



93 



through the tube b, and escaped by the small orifice at the lower end of b 

 into the liquid. Through this it bubbled, loading itself with vapour, 

 after which the mixed air and vapour, passing from the flask by the tube 

 entered the experimental tube, where they were subjected to the action 

 of light. 



The power of the electric beam to reveal the existence of anything within 

 the experimental tube, or the impurities of the tube itself, is extraordinary. 

 When the experiment is made in a darkened room, a tube which in ordi- 

 nary daylight appears absolutely clean is often shown by the present mode 

 of examination to be exceedingly filthy. 



The following are some of the results obtained with this arrange- 

 ment : — 



Nitrite of amyl (boiling-point 91° to 96° C). — The vapour of this liquid 

 was in the first instance permitted to enter the experimental tube while 

 the beam from the electric lamp was passing through it. Curious clouds were 

 observed to form near the place of entry, which were afterwards whirled 

 through the tube. 



The tube being again exhausted, the mixed air and vapour were allowed 

 to enter it in the dark. The slightly convergent beam of the electric light 

 was then sent through 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 liquid spherules was precipitated on the 

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

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

 descence. 



The beam of the electric lamp was now converged so as to form within 

 the tube, between its end and the focus, a cone of rays about eight inches 

 long. The tube was cleansed and again filled in darkness. When the 

 light was sent through it, the precipitation upon the beam was so rapid and 

 intense that the cone, which a moment before was invisible, flashed suddenly 

 forth like a solid luminous spear. 



The effect was the same when the air and vapour were allowed to enter 

 the tube in diffuse daylight. The cloud, however, which shone with such 

 extraordinary radiance under the electric beam, was invisible in the ordinary 

 light of the laboratory. 



The quantity of mixed air and vapour within the experimental tube could 

 of course be regulated at pleasure. The rapidity of the action diminished 

 with the attenuation of the vapour. When, for example, the mercurial 

 column associated with the experimental tube was depressed only five inches, 

 the action was not nearly so rapid as when the tube was full. In such cases, 

 however, it was exceedingly interesting to observe, after some seconds of 

 waiting, a thin streamer of delicate bluish-white cloud slowly forming 

 along the axis of the tube, and finally swelling so as to fill it. 



