440 Dr. J. W. Draper on the Distribution of 



On removing the screen and allowing the rays to fall on the 

 sensitive mixture in the actinometer, an expansion amounting to 

 half a degree was observed. In sixty seconds this expansion 

 ceased. 



The volume of the mixture now remained stationary, no ap- 

 parent change going on in it. At lengthy 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 contrac- 

 tion compensating for the half degree of expansion. 



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 chlorine and 

 hydrogen unite under the influence of a radiation, there are four 

 distinct periods of action : — 



1st. For a brief period the mixture expands. 



2nd. For a much longer period it then remains stationary in 

 volume, though still absorbing rays. 



3rd. Contraction, arising from the production of hydrochloric 

 acid, begins; at first it goes on slowly, then more and more 

 rapidly. 



4th. i\.fter that contraction is fully 'established, it proceeds 

 w4th uniformity, equal quantities of hydrochloric acid being pro- 

 duced 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 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 sensitive 

 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 disturb- 

 ing 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 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. 



It might be supposed that this instantaneous expansion is 

 due to a heat-disturbance, arising from the absorption of rays 



