82 PRELIMINARY ABSORPTION. 



minishes, and the effect of this must be apparently to diminish the force of the ray. 

 Moreover, during the progress of the trial, muriatic acid gas accumulates, for its ab- 

 sorption does not instantaneously take place ; the operation of this must be the reverse 

 of the former, or apparently to increase the effect. To a certain extent, but not per- 

 fectly, these sources of error may be avoided, as I have attempted, and practically we 

 may discuss the branch of the curve, d e, as though it were a straight line. 



316. This being understood, the result which is before us leads us obviously to this 

 important law, that for a given compound, equal quantities of tithonic rays, after the 

 preliminary latent absorption is over, give rise to equal chemical effects. 



317. In thus setting forth, in as prominent a manner as I am able, these two doctrines, 

 1st. Of the latent condition of the rays that are at first absorbed ; and, 2d. Of the def- 

 inite chemical action of those that are subsequently, I am again urging the same doc- 

 trine which three years ago (Ap., 595) I attempted to establish for iodide of silver. 



318. In passing, it may be observed, that one of the greatest practical difficulties in 

 the art of photography, more especially in taking portraits from life, is connected 

 with this matter of preliminary latent absorption. Artists know well, that to obtain a 

 perfect result is the exception, and an indifferent one is the rule. It is, indeed, rare 

 that the relation of light and shadow is perfectly observed ; the high lights most com- 

 monly come out unduly, the feebler lights are more slowly evolved, and very often never 

 come out at all. This arises from the circumstance that the sensitive surface does not 

 begin to change uniformly from the first instant of exposure, but the preliminary latent 

 stage of absorption has to be gone through ; with high lights and a brilliant illumination, 

 that period is passed over in an instant, and the second entered upon; but the feebler 

 lights have to expend themselves for a long while in passing through this stage, and 

 while the others have carried on their operation almost to completion, these have not 

 been able to leave a sensible trace of action. Theoretically, the remedy for this diffi- 

 culty would be, by a brief exposure to a transitory or dim light, to pass the sensitive 

 surface uniformly through its preliminary stage. 



319. These, therefore, are the prominent phenomena which are exhibited by a mix- 

 ture of chlorine and hydrogen, a latent preliminary absorption, and a subsequent def- 

 inite chemical action. We have observed them also in iodide of silver (317), and in 

 various other compounds. 



320. Let us direct our attention, in the next place, to what has happened to the ray. 

 We have already seen (295) that when, through a gaseous sensitive mixture, the beams 

 from a lamp are suffered to pass, and fall on the tithonometer, they are found to have 

 lost much of their chemical force. The beam has therefore become detithonized. 



(a.) The vessel described (293) was filled with atmospheric air over the trough, 

 and the chemical force of the ray passing through it from the lamp (fig. 123) was de- 

 termined. It was measured by the period required to cause the index to descend 

 through one division, and represented by 12 seconds. 



(Z>.) The vessel was now half filled with chlorine, derived from a mixture of muriatic 

 acid and peroxide of manganese, and the chemical force of the ray, after passing through 

 it, determined as before ; it now was represented by 251 seconds. 



