248 FRAGMENTS OF SCIENCE. 



vapors under the action of the unsifted beam, you have in 

 stantly this fine luminous cloud precipitated. 



The light of the sun also effects the decomposition of 

 the uitrite-of-amyl vapor. A small room in the Royal 

 Institution, into which the sun shone, was partially dark 

 ened, the light being permitted 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-dark 

 ened 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 distant end 

 of the tube was shielded by that in front ; but on revers 

 ing the tube, a second and similar splendid cone was pre 

 cipitated. 



Now let us pause for a moment and glance at the 

 ground over which we have passed. We have defined a 

 vapor as an aggregate of. molecules mutually repellent, 

 but hindered from indefinitely retreating from each other 

 by an external pressure. We have defined a molecule as 

 an aggregate of atoms maintained in positions of equi 

 librium by the equalized action of two opposing forces, 

 and always oscillating to and fro across those positions. 

 We have defined a beam of light as a train of innumerable 

 waves, and have illustrated their chemical action. We 

 have learned that it is not the magnitude or power of the 

 waves, so much as their periods of recurrence, that renders 

 them effectual as chemical agents. We have also seen 

 how the luminous beam is sifted by the vapor which it 

 decomposes, and deprived of those rays which are com 

 petent to effect the decomposition. The effects, moreover, 

 obtained with the electric beam are also produced by the 

 beams of the sun. 



