428 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



through insolation increases in specific volume. More- 

 over, as the reunion of the chlorine atoms probably 

 produces heat, rays of high refrangibility will cause 

 chlorine to expand ; but it will contract to its original 

 volume when no longer under the influence of light. 



In corroboration of this conclusion, Budde found that 

 a differential thermometer filled with chlorine showed a 

 certain expansion when placed in the red or yellow rays, 

 but it gave an expansion six or seven times greater 

 when in the violet rays. With carbonic acid and ether 

 no such effect took place. 



It should not be forgotten, however, in considering 

 the bearing of these experiments, that chlorine merely 

 because it is yellowish -green will absorb rays of a com- 

 plementary that is, of an indigo and violet color and 

 become heated thereby. 



It has next to be determined whether the points of 

 maximum action that is, the points of maximum ab- 

 sorption correspond to the rays of emission of either or 

 both these gases, as they apparently ought to do under 

 Angstrom's law: "A gas when luminous emits rays of 

 light of the same refrangibility as those which it has the 

 power to absorb." 



Of the four rays characteristic of hydrogen there is 

 one the wave-length of which is 4340. It is in the in- 

 digo space. 



Pliicker gives for chlorine a ray nearly answering to 

 this. Its wave-length is 4338, and also another, 4346, the 

 latter being one of the best marked of the chlorine lines. 



There are, therefore, rays in the indigo which are ab- 

 sorbed both by hydrogen and by chlorine. The place 

 of these rays in the spectrum corresponds to that in 

 which the gases unite the place of maximum action for 

 their mixture. 



But the absorptive action of chlorine is not limited 



