100 FRAGMENTS OF SCIENCE. 



particles was precipitated on the beam. The cloud 

 thus generated became denser as the light continued 

 to act, showing at some places vivid iridescence. 



The lens of the electric lamp was now placed so as 

 to form within the tube a strongly convergent cone of 

 rays. The tube was cleansed and again filled in dark- 

 ness. When the light was sent through it, the pre- 

 cipitation 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 ob- 

 serve, 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. 



When dry 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 ef- 

 fect 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 vapour itself. When it was permitted to enter 



