DECOMPOSITION BY LIGHT 109 



other end of the tube, copious precipitation occurred there 

 also. 



Solar light also effects the decomposition of the nitrite- 

 of-arnyl vapor. On October 10, 1868, I partially dark- 

 ened a small room in the Koyal Institution, into which 

 the sun shone, permitting the light to enter through an 

 open portion of the window- shutter. In the track of the 

 beam was placed a large plano-convex lens, which formed 

 a fine convergent cone in the dust of the room behind it. 

 The experimental tube was filled in the laboratory, cov- 

 ered with a black cloth, and carried into the partially 

 darkened room. On thrusting one end of the tube into 

 the cone of rays behind the lens, precipitation within the 

 cone was copious and immediate. The vapor at the dis- 

 tant end of the tube was in part shielded by that in front, 

 and was also more feebly acted on through the divergence 

 of the rays. On reversing the tube, a second and similar 

 cone was precipitated. 



Physical Considerations 



I sought to determine the particular portion of the 

 light which produced the foregoing effects. When, pre- 

 vious 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 prompt precipitation occurred 

 along the track of the blue beam. Hence, in this case, 

 the more refrangible rays are the most chemically active. 

 The color of the liquid nitrite of amyl indicates that this 

 must be the case; it is a feeble but distinct yellow: in 



