DETITHOXIZING ACTION. 167 



more remarkable image emerged on mercurializing. This image, like the former, was 

 circular and black, but all around it for a certain space there was an annulus of nar- 

 row dimensions of pure unmercurialized silver, the deep black of which contrasted 

 strikingly with the blue black of the spectre, and its outer circumference was marked 

 by a faint whitening of the plate faint, but as sharp as it is possible to conceive. 



722. In a third trial, things were conducted as before, except that now chlorine, 

 diluted with atmospheric air, was used ; the spectre again came out, and did not differ 

 in any observable manner from that produced by iodine. 



723. In a fourth trial, the vapour of nitrous acid was used as a detithonizer. In 

 this case, the edges of the spectre commonly had a gradually shading outline, and only 

 in one instance did I find that sharpness of termination exhibited in the other cases. 



724. We therefore perceive that iodine, bromine, chlorine, and nitrous acid can de- 

 tithonize a surface on which light has fallen : they can undo what the tithonic rays have 

 done. 



72 3. In repeating these experiments, as, for example, the one by iodine, if the com- 

 mon iodine-box be used to effect the detithonization, two or three seconds of time is 

 all that is required. A longer period is demanded when the vapour is very weak, but 

 when strong the effect is almost instantaneous. 



726. This detithonization and production of spectral images can therefore be ac- 

 complished in an incredibly short space of time. 



727. I made trials with other substances, such as hydrogen gas and the vapour of 

 liquid muriatic acid. The former to a certain extent, though not near so powerfully 

 as the electro- negative bodies mentioned, could produce the change in question ; the 

 latter seemed to be without any perceptible action. 



728. To the list, with the other electro-negative substances, I believe oxygen ought 

 to be added ; for, on repeating the same experiment, and raising the temperature of the 

 plate in atmospheric air so as to maintain the tithonized surface at about 200 Fah. for 

 several minutes, a certain effect, which in an imperfect way resembled those already 

 described, was exhibited. Oxygen, therefore, diluted as in atmospheric air, at 200" Fah., 

 uiav be regarded as possessing, to a small extent, the property in question. 



729. Without multiplying the description of these experiments farther for the in- 

 genuity of any one who repeats them will suggest many modifications which may give 

 rise to striking results I will, in conclusion, give the reasons which have led me to 

 suppose that in all these phenomena two different principles are engaged vapour ac- 

 tion and radiation. 



730. I have stated that the ELECTRO-NEGATIVE bodies possess this detithonizing 

 quality in a very marked Hnanner. I do not wish it to be understood, however, that 

 there is any relation of~ants^uization between that particular class of substances and 

 the tithonic rays. It appears uNq^e that their peculiar quality, in the circumstances 

 described, may be traced to the fact that silver, an electro-positive substance, happens 

 to afford the sensitive surface. I have, however, prepared a paper which takes up the 

 consideration of the conditions and theory involved, and will not at present anticipate 

 what has to be offered when that paper shall be published. 



