.),, RAYS ARE ABSORBED. 



ary for a considerable time, after a certain period has elapsed it will be seen on a sud- 

 den to start and commence rising. Observe now how far it will rise during a period 

 which is equal to the time that elapsed between the first exposure and the beginning 

 of the rise, and it will be seen that one fourth or half of the gases will disappear. 



908. It is obvious that from the first moment of exposure the rays must have been 

 exerting their influences on the mixture. As will presently be proved, absorption has 

 been all along taking place. There are, therefore, two distinct phenomena exhibited 

 by this experiment. There is a period during which, though large quantities of the 

 dark rays are disappearing, no visible change is produced ; there is a second period, 

 during which absorption is accompanied by a remarkable chemical effect, the produc- 

 tion of muriatic acid. From these things we gather that a definite amount of the 

 tithonic rays must disappear and become latent before muriatic acid can form. The 

 phenomenon is not unlike that of the disappearance of a definite quantity of heat in the 

 passage of ice into the condition of water. 



909. A mixture of chlorine and hydrogen does not, therefore, instantly give rise to 

 the production of muriatic acid on exposure to the light, but, as a preliminary condition, 

 a certain definite amount of absorption must take place. 



910. 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 mix- 

 ture, after it has been tithonized 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. Rays are absorbed in producing this Change. 



911. I have thus far assumed that the rays which bring about these are absorbed; 

 the following is the proof which I have to offer : 



912. 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. 



913. 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 which it may be caused to traverse. The 

 active rays have been absorbed ; they disappear from the beam, and are lost in producing 

 their first effect. 



914. A beam of light becomes detithonized in producing a chemical effect; the 

 beam, as well as the medium on which it acts, becomes changed. I have a series of 

 results which proves that this takes place for a great variety of compound bodies. 



