102 FRAGMENTS OF SCIENCE. 



Solar light also effects the decomposition of the 

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

 tially darkened a small room in the Eoyal 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, covered with a black cloth, and 

 carried into the partially darkened room. On thrust- 

 ing one end of the tube into the cone of rays behind the 

 lens, precipitation within the cone was copious and im- 

 mediate. The vapour at the distant 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 pre- 

 cipitated. 



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 introduced 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 

 produces chemical action, but the absorbed portion. 



