Chemical Force in the Spectrum. 93 
he 
duces 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. 
n corroboration of this conclusion Budde found that a differ- 
ential thermometer filled with chlorine showed a certain expan- 
sion when placed in the red or yellow rays, but it gave an 
expansion six or seven times greater when in the violet rays. 
ently ought to do under Angstrém’s law. “A gas when lumi- 
nous 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 indigo space. 
ticker gives for chlorine a ray nearly answering to this. 
Its wave-length is 4838, 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 absorbed 
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 to a few 
isolated lines. The gas removes a very large portion of the 
Spectrum. Subsequent experiments must determine whether 
each of these lines of absorption is also a line of maximum 
chemical action. 
The chlor-hydrogen actinometer, referred to in previous para- 
graphs as depending for its indications on the union of chlorine 
and hydrogen, furnishes the means of ascertaining many facts 
