MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 



The rate of contraction of the gaseous mixture that 

 is, the rate at which its constituents were uniting was 

 then ascertained. 



From these observations it appeared that when chlo- 

 rine and hydrogen unite, under the influence of a radia- 

 tion, there are four distinct periods of action : 



1st. For a brief period the mixture expands. 



2d. For a much longer period it then remains station- 

 ary in volume, though still absorbing rays. 



3d. Contraction arising from the production of hydro- 

 chloric acid begins. At first it goes on slowly, then 

 more and more rapidly. 



4th. After that contraction is fully established, it pro- 

 ceeds with uniformity, equal quantities of hydrochloric 

 acid being produced in equal times by the action of 

 equal quantities of the rays. 



The prominent phenomena exhibited by a mixture of 

 chlorine and hydrogen are a preliminary absorption and 

 a subsequent definite action. 



It may be remarked, since a similar preliminary ab- 

 sorption 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 sensitive surface to a brief ex- 

 posure in a dim light, so as to pass it through its pre- 

 liminary stage. 



The expansion referred to as taking place during the 

 first of these periods may be advantageously observed 

 when the disturbing radiation is very intense. It is well 

 seen when a Leyden-jar is discharged in the vicinity of 

 the actinometer. Though this light lasts but a very 

 small fraction of a second, it produces an instantaneous ex- 

 pansion, followed by an instantaneous contraction. Not 

 unfrequently the gases unite with an explosion. I have 

 had several of these instruments destroyed in that manner. 



