380 Prof. Draper on the Chemical Action of Light:' 



If a hot body, a, be placed in presence of a cold body, b, the 

 theoiy of the exchange of heat teaches that the temperature of 

 the latter will steadily rise until eqiiilibriiim takes place. The 

 moleciUes of a commiuiicate their vibratory motion to the tethei', 

 and this in its turn imparts an analogous movement to the mo- 

 lecules of b. For, as the ethereal medium is of vastly less den*- 

 sity than the vibrating molecules, each of their oscillations will 

 produce in it a determinate wa\e, which is propagated through 

 it according to the ordinary laws of undulations, in such a way 

 that the aether would be in repose after the wave had passed were 

 it not for the continuuig vibration of the molecules. At each 

 vibration the molecules of a lose a part of their vis viva, by the 

 quantity they have communicated to the siethereal wave, the in- 

 tensity or amplitude of the wave becoming gradually less and 

 less as this abstraction of force is going on. But the jether 

 being of uniform density and elasticity throughout each of its 

 particles, communicates the whole vis viva it has received to the 

 next adjacent, and would instantly come to rest were it not again 

 disturbed by the vibrations of the material molecules. 



These elementary considerations show how it is that a wave of 

 sound passes through the air, and of light through the jether, 

 and the particles of those media instantly come to rest ; but a 

 hot body or a A'ibrating string persists in its motions, which only 

 undergo a gradual decline. If the vibrating molecule was in a 

 medium of the same density, it wovdd impart to it all its motion 

 at once ; and in the same ^vay that a heavy molecule gradually 

 commimicates a consentaneousness of motion to the aether, so in 

 its turn does the aether to other systems of heavy molecules. 



'^^'hen therefore a ray of light falls, for example, on the per- 

 oxalate of iron, it compels the compound molecules thereof to 

 execute a A'ibratory motion. Each atom of carbon, of iron, of 

 oxygen, commences to vibrate for itself; and if, under the cir- 

 cumstances of the case, these movements do not change the rela- 

 tion of the constituents of the whole group, it continues to exist 

 unimpaired ; but if among these motions there be any which, by 

 reason of their peculiar frequency, amplitude, or other aiFections, 

 compel the constituent atoms to enter on motions which are dis- 

 cordant with one another, or to arrange themselves in new posi- 

 tions with respect to each other, the group can no longer exist 

 as it was, it must break up, and decomposition ensue. In the 

 case before us it breaks up into carbonic acid and protoxalate (rf 

 iron, bodies which are unaffected by the action of light. 



But this breaking up of the arrangement and constitution of 

 the group is ])erhaps the work of a single ray, — a ray of definite 

 refi-angibility. The simple fact, that the molecule which is 

 breaking up cannot vibrate in unison with the impinging ray, is 



