102 FRAGMENTS OF SCIENCE. 



Solar light also effects the decomposition of the 

 nitrite-of-amyl vapour. On October 10, 1868, I 

 tially darkened a small room in the Koyal Institution, 

 into which the sun shone, permitting the light to enl 

 through an open portion of the window-shutter. In th( 

 track of the beam was placed a large plano-convex lens 

 which formed a fine convergent cone in the dust of th< 

 room behind it. The experimental tube was filled ii 

 the laboratory, covered with a black cloth, and carri< 

 into the partially darkened room. On thrusting one en< 

 of the tube into the cone of rays behind the lens, pi 

 cipitation within the cone was copious and immedij 

 The vapour at the distant end of the tube was in pai 

 shielded by that in front, and was also more feebly 

 acted on through the divergence of the rays. On re- 

 versing the tube, a second and similar cone was precipi- 

 tated. 



Physical Considerations. 



I sought to determine the particular portion of the 

 light which produced the foregoing effects. When, 

 previous 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 introfluced before the removal of the yellow 

 or the red, on taking the latter away prompt precipita- 

 tion occurred along the track of the blue beam. Hence, 

 in this case, the more refrangible rays are the most 

 chemically active. The colour of the liquid nitrite of 

 amyl indicates that this must be the case ; it is a feeble 

 but distinct yellow : in other words, the yellow portion 

 of the beam is most freely transmitted. It is not, 

 however, the transmitted portion of any beam which 



