4'74> Prof. Draper on the Interference Spectrum, 



falls on a mass of ice at 32° F., in which a thermometer is 

 imbedded, for a certain space of time no apparent rise of tem- 

 perature takes place, but the radiation continuing long enough, 

 a physical change is accomplished ; the ice puts on a fluid 

 form, and now the thermometer commences to ascend, equal 

 quantities of heat producing for a certain period equal effects. 

 Would not the table just given, or a curve projected from it, 

 answer as well to express the phjcnomena of the action of ca- 

 loric upon ice, as of the tithonic rays on a mixture of chlorine 

 and hydrogen ? 



It was from the study of that phasnomenon 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 cer- 

 tain circumstances, so also does a tithonic ray, and conse- 

 quently a photic ray ? I regard the phaenomenon of the 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. 



We have further to remark, that the consideration of the 

 fourth period leads us to the important law, that for a given 

 compound, equal quantities of the tithonic rays, after the pre- 

 liminary latent absorption is over, give rise to equal effects. 



In thus setting forih these two doctrines, — 1st, of the latent 

 condition of the rays which are first absorbed ; and 2nd, of 

 the definite action of those absorbed subsequently, I am again 

 urging the same doctrine which four years ago I attempted to 

 establish for iodide of silver. 



Let us direct our attention in the next place to what has 

 happened to the ray. We have already seen that when, 

 through a gaseous sensitive mixture, the beams from a lamp 

 are suffered to pass and fall on the lithonometer, they are 

 found to have lost much of their chemical force. The beam 

 has therefore become detithonized. 



A glass trough, whose parallel sides were 2'6 inches apart, 

 was filled with atmospheric air over a small pneumatic trough, 

 and the chemical force of a ray passing through it from an 

 Argand lamp was determined by the tithonometer. It was 

 measured by the period required to cause the index to descend 

 through one division, and represented by 12 seconds. 



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, deter- 

 mined as before. It was now represented by 25^ seconds. 

 To the chlorine an equal volume of hydrogen was now 



