DECOMPOSITION BY LIGHT 105 



mental tube, where they were subjected to the action of 

 light. 



The whole arrangement is shown in Fig. 3, where L rep- 

 resents the electric lamp, s s' the experimental tube, p p' 

 the pipe leading to the air-pump, and F the test-tube con- 

 taining the volatile liquid. The tube t I' is plugged with 

 cotton-wool intended to intercept the floating 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 vapor of the air. 



The power of the electric beam to reveal the existence 

 of anything within the experimental tube, or the impuri- 

 ties of the tube itself, is extraordinary. When the ex- 

 periment is made in a darkened room, a tube which in 

 ordinary 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 arrangement: 



Nitrite of Amyl. The vapor 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 afterward whirled through the tube. 



The tube being again exhausted, the mixed air and 

 vapor 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 particles was precipitated on the 

 beam. The cloud thus generated became denser as the 



