DECOMPOSITION BY LIGHT. 99 



itself with vapour, after which the mixed air and 

 vapour, passing from the flask by the tube a, entered 

 the experimental tube, where they were subjected to 

 the action of light. 



The whole arrangement is shown in fig. 3, where L 

 represents the electric lamp, s s' the experimental tube, 

 pp' the pipe leading to the air-pump, and F the test- 

 tube containing the volatile liquid. The tube 1 1' is 

 plugged with cotton-wool intended to intercept the float- 

 ing matter of the air ; the bent tube T' contains caustic 

 potash, the tube T sulphuric acid, the one intended to 

 remove the carbonic acid and the other the aqueous 

 vapour of the air. 



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 ordinary daylight appears absolutely 

 clean, is often shown by the present mode of examina- 

 tion to be exceedingly filthy. 



The following are some of the results obtained with 

 this arrangement : 



Nitrite of amyl. 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, the cause of which 

 was then unknown, were observed to form near the 

 place of entry, being 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 mixture. For a moment the tube 

 was optically empty, nothing whatever being seen 

 within it ; but before a second had elapsed a shower of 



