168 DETITHONIZING ACTION. 



731. The action, then, which these different gases and vapours exhibit is so intense 

 as to mask the feebler effect of radiation. Thus, it takes several hours' exposure 

 in the dark, and after a long subsequent process of mercurialization, to prove the true 

 radiant effect, a slow detithonization, which could be brought about by vapour action 

 in an instant. But whoever has seen the symmetrical, or, rather, geometrical lines 

 that are left when the slower process is followed, must be struck with the persuasion 

 that the phenomena he witnesses is obeying geometrical laws, and is not due to the 

 irregular action of a dilute and varying current of vapour. 



732. Thus, on repeating carefully the experiment cited at the close of my last paper, 

 in which a lens is laid on a tithonized surface and left in the dark, I found that after 

 the mercurial process was completed, the plate exhibited a dark central spot sur- 

 rounded by a white annulus. On drawing upon paper a section of the lens and the 

 sensitive surface, I found that a line drawn from the extreme edge of the white annu- 

 lus to the edge of the lens was a tangent to the lens at that point ; that a line drawn 

 from the extreme edge of the central dark spot would, after reflexion by the convex 

 surface of the lens, be found precisely on the edge of the white annulus ; the edge of 

 the annulus and the edge of the spot thus having a true catoptrical relation to the curva- 

 ture of the lens. 



733. Now, although in laboratories such as that in which my experiments are con- 

 ducted, the vapours of these different electro-negative bodies are unquestionably pres- 

 ent, and may produce a part of the phenomena witnessed, yet, inasmuch as that phe- 

 nomenon follows laws that are apparently of a strict geometrical kind, and to those 

 floating vapours we could hardly assign anything like symmetrical results guided, 

 also, by the analogy of cooling bodies, which lose part of their heat through radiation, 

 and part through the current action of the air, and part through the conducting power 

 of their supports, I have been led to take the view of the phenomena in question 

 which I have set forth. 



