Chemical Force in the Spectrum. 95 
to half a degree was observed. In 60 seconds this expansion 
ceased. 
The volume of the mixture now remained stationary, no 
apparent change going on init. At length, after the close of 
270 seconds, it was beginning to contract, and hydrochloric 
acid to form. 
At the end of 45 seconds more a contraction of half a degree 
had occurred; the volume of the mixture was, therefore, now 
the same as when the experiment began, this half degree of 
contraction compensating for the half degree of expansion. 
e rate of contraction of the gaseous mixture, that is, the 
rate at which its constituents were uniting, was then ascertained. 
rom these observations it appeared that when chlorine and 
hydrogen unite, under the influence of a radiation, there are 
four distinct periods of action. 
Ist. For a brief period the mixture expands, 2 
_ 2d. For a much longer period it then remains stationary in 
volume, though still absorbing rays. : 
ontraction arising from the production of hydrochloric 
acid begins; at first it goes on slowly, then more and more 
ra 
with uniformity, equal quantities of hydrochloric acid being 
produced in equal times by the action of equal quantities of the 
rays. 
pidly. 
4th. After that contraction is fully established, it proceeds 
fe h 
_ The prominent phenomena exhibited by a mixture of chlor- 
tne and hydrogén are a preliminary absorption and a subse- 
quent definite action. 
It may be remarked, since a similar preliminary absorption 
occurs in the case of other sensitive substances; that there is in 
Practical photography an advantage, both as respects time and 
Correctness in light and shadow, gained by submitting a sensi- 
tive surface to a brief exposure in a dim light, so as to pass it 
through its preliminary stage. 
The expansion referred to as taking place during the first of 
these periods, may be advantageously observed when the dis- 
turbing radiation is very intense. It is well seen when a Ley- 
this 
an instantaneous expansion, followed by an instantaneous con- 
traction. Not unfrequently the gases unite with an explosion. 
I have had several of these instruments destroyed in that 
manner. 
ner, Stee 
It might be su posed that this instantaneous expansion 18 
due to a heat dvnctane arising from the absorption of rays 
that are not engaged in producing the chemical effect. But 
this interpretation seems to be incompatible with the instanta- 
heously following contraction. Though it is admissible that 
