76 FRAGMENTS OF SCIENCE. 



the beam by the vapor occurs, which diminishes the chem- 

 ical action in the posterior portions. In some experiments 

 the precipitated cloud only extended halfway down the 

 tube. When, under these circumstances, the lamp was 

 shifted so as to send the beam through the other end of the 

 tube, copious precipitation occurred there also. 



Solar light also effects the decomposition of the nitrite- 

 of-amyl vapor. On October 10, 1868, I partially darkened a 

 small room in the Royal 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 con- 

 vergent cone in the dust of the room behind it. The ex- 

 perimental tube was filled in the laboratory, covered with a 

 black cloth, and carried into the partially darkened room. 

 On thrusting one end of the tube into the cone of rays be- 

 hind the lens, precipitation within the cone was copious 

 and immediate. The vapor 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 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 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 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. Blue, 

 as the complementary color to yellow, is here absorbed, and 

 hence the, more energetic action of the blue rays. 



This reasoning, however, assumes that the same rays are 

 absorbed by the liquid and its vapor. The assumption is 



