and the Absorj)tion of the Tithonic Rays, 4-73 



2ncl. For a much longer period it then remains wholly sta- 

 tionary, neither expanding nor contracting, though the rays 

 are constantly falling on it and it is absorbing them. 



3rd. Contraction arising from the production of muriatic 

 acid begins, commencing at first slowly, and then more and 

 more rapidly. 



4th. And, after that contraction has fairly set in, it goes on 

 with uniformity; equal quantities of muriatic acid being pro- 

 duced in equal times by the action of equal quantities of the 



The following Table represents such a result : — 



If we project these observations, laying off" the quantities of 

 the gases that have united on the axis of abscissas, and repre- 

 senting the times by the ordinates, they will give us a curve 

 the discussion of which exhibits the leading phaenomena of 

 absorption by this sentient mixture; for the ordinates of that 

 curve represent the quantities of the tithonic rays, and its ab- 

 scissas the corresponding chemical effects. 



What now is the interpretation we are to give of the fact, 

 that when a sensitive compound is exposed to a given ray, it 

 does not change all at once, but a certain period must elapse, 

 during which absorption is going forward without any corre- 

 sponding apparent effect ensuing, and that once accomplished 

 chemical change begins ? 



Is not this the same phcenomenon which has been for a long 

 time known in the case of radiant heat? When a ray of heat 



