LATENT LIGHT. gj 



should assert that when a ray falls on a sensitive compound, its first effect is to pro- 

 duce an expansion. During a certain period, differing in different cases, no change 

 whatever takes place, the ray being constantly absorbed, and not appearing to produce 

 any visible effect. After a time, chemical action commences, at first more slowly, then 

 with a determined and constant rate of rapidity, equal quantities of the rays now pro- 

 ducing equal chemical effects. 



310. I may here anticipate what will presently be proved, that this generalization 

 can be sustained, and that, therefore, the conclusions which we have arrived at for this 

 particular case, hold also for all others, not only in the cases of chemical action pro- 

 duced by the tithonic rays, but also of chemical action brought about by the rays of 

 light. 



311. Let us return again to the study of the curve, or of the numbers contained in 

 the table. It is obvious that there are two portions of these which demand peculiar 

 attention; they are embraced in the second and fourth epochs referred to in (308). As 

 to the first and third, these are for the present of less interest. 



312. What, then, is the interpretation we are to put upon that part of the curve 

 which is between b and c ? or, in other words, what 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 go- 

 ing forward, without any corresponding apparent effect ensuing, and that once accom- 

 plished, chemical change begins? 



313. Is not this the same phenomenon which has been for a long time known in the 

 case of radiant heat 1 When a ray of heat falls on a mass of ice at 32 Fah., in which 

 a thermometer is imbedded, for a certain space of time no apparent rise of temperature 

 takes place, but the radiation continuing long enough, a physical change is accomplish- 

 ed ; the ice puts on a fluid form, and now the thermometer commences to ascend, 

 equal quantities of heat producing, for a certain period, proportionally equal effects. 

 Would not the table given in (306), or the curve projected in Jig. 125, answer as well 

 to express the phenomenon of the action of caloric upon ice, as of the tithonic rays on 

 a mixture of chlorine and hydrogen 1 



314. It was from the study of that phenomenon in the case of ice that the doctrine 

 of latent heat arose ; and do not these things teach us that just as a calorific ray becomes 

 latent under certain circumstances, so also does a tithonic ray, and, consequently, a ray 

 of light 1 I regard the phenomenon of that pause which is seen before chlorine and 

 hydrogen unite, and during which absorption is taking effect, as setting forth in a strong, 

 and clear, and prominent manner, that as radiant heat may become latent, so also may 

 tithonic rays, and also rays of light. 



315. Let us, in the next place, direct our attention to the second branch of the curve, 

 or to the fourth epoch, the changes of which are included between d and e. Rigor- 

 ously speaking, this is not a straight line. It only makes a sensible approach to one. 

 There are several causes which obviously interfere. When a given quantity of gase- 

 ous mixture is exposed to a radiant source, and the experiment we have been relating 

 performed, it is obvious that, as it proceeds, the volume of gas so exposed steadily di- 



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