MEMOIR XIX.] MODIFIED CHLORINE. 275 



disappearance of a definite quantity of heat in the pas- 

 sage of ice into the condition of water. 



A mixture of chlorine and hydrogen does not, therefore, 

 instantly give rise to the production of hydrochloric acid 

 on exposure to the light, but as a preliminary condition 

 a certain definite amount of absorption must take place. 



Now if this were a mere molecular disturbance, such 

 as might be brought about by the action of heat, we 

 should expect to find it transient and speedily passing 

 away. Such, however, is far from being the case. As 

 with simple chlorine, so with this mixture : after it has 

 been modified it loses its quality very slowly. I have 

 observed that after a week or more has elapsed since it 

 was first exposed to the light, it commences to contract 

 when placed in a feeble gleam. 



IV. Hays are Absorbed in Producing this Change. 



I have thus far assumed that the rays which bring 

 about these changes are absorbed ; the following is the 

 proof which I have to offer : 



Over a tube half an inch in diameter and six inches 

 long, closed at its upper extremity and open at its lower, 

 invert a jar of the same length and one inch and a half 

 in diameter. Fill the tube and the jar at the salt-water 

 trough, about two thirds full, with the same mixture of 

 chlorine and hydrogen. Expose them to diffuse day- 

 light. Now it is clear that no rays can gain access to 

 the tube except after having passed through the gaseous 

 mixture in the jar. After a certain space of time the 

 level of the liquid in the jar commences to rise, but that 

 in the tube will remain much longer wholly stationary. 



It therefore appears that a beam which has passed 

 through a mixture of chlorine and hydrogen has lost, to 

 a great extent, the quality of bringing about the union 

 of a second portion of the mixed gases through w r hich it 



