90 MODE OF ACTION OF THE TITHONIC RAYS. 



362. The iodide of silver is a body which copiously absorbs blue tithonic rays, a 

 quality arising from its chemical relations and constitution. It is unaffected by the 

 pure yellow and red tithonic rays, when they operate alone, and diffused light is exclu- 

 ded. Consequently, it can give no indications dependant on the presence or absence 

 of them. The sunlight to it is monochromatic, or nearly so, for it is decomposed by 

 blue, indigo, and violet rays only. The phenomena, therefore, which it will exhibit 

 when in thin plates, are such as would be exhibited by thin transparent plates of air, 

 or water, or glass, on which a monochromatic light is falling. If monochromatic light 

 falls upon plates of glass of variable thickness, the reflected beam simply changes in 

 intensity, and, if the language of science permitted us to describe bodies in their opti- 

 cal relations as being more sensitive to light when they failed to reflect it to our eye, 

 and less sensitive the more copious the reflexion, we can understand that the plate of 

 glass would pass through all orders of sensitiveness as its thickness varied. At one 

 time it would reflect all the incident beam, and as it increased in thickness the ray 

 would diminish in intensity, and finally disappear, and, with a still farther increase, the 

 brilliancy of the beam would again be reassumed, and so on through successive periods. 



363. Now this is absolutely the same phenomenon as that exhibited by iodide of silver 

 injilms of variable thickness when exposed to the spectrum. It depend on the ideal col- 

 oration of the iodide. 



364. Let us examine, in the next place, what should be the event when bromide of 

 silver is used instead of iodide. 



In order to enable us to predict the result, we have simply to consider what would 

 occur if a film of lampblack, or of any other perfectly absorbent body, could be obtain- 

 ed, of suitable thinness, and exposed to the luminous spectrum. For, to use a some- 

 what objectionable, but perhaps emphatic expression, bromide of silver is the lamp- 

 black of the tithouic rays. It is obvious that a perfectly absorbent film would uni- 

 formly appear black, no matter what its thickness might be. The tints of thin plates 

 arising in interferences among reflected or transmitted rays must, in this instance, be 

 absent. A film of bromide of silver must, therefore, have a uniform sensitiveness a 

 .sensitiveness which is independent of its thickness. 



365. Having thus explained the laws of absorption and the doctrine of ideal color- 

 ation in this and the preceding chapter, it remains only to add a few words on the 

 mode of action of the tithonic rays. 



366. There is no reason to believe that oxygen, hydrogen, or nitrogen gases, in 

 masses of ordinary magnitude, exert any perceptible absorptive effect on light, heat, or 

 the tithonic rays. These bodies, therefore, and all others having the same relation, 

 can exert no action on each other, even though they are under the influence of the 

 most intense radiation. 



367. A mixture of oxygen and hydrogen gases, exposed to a brilliant light, can never 

 produce water, because neither of its constituents has the power of absorbing the in- 

 cident rays. 



368. But a mixture of chlorine and hydrogen gas explodes in an instant under the 

 influence of light, because the chlorine can exert a powerful absorbent action. 



