DETITHONIZING ACTION. 1(57 



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. 



725. 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 

 lo 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 Fab. 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., 

 may 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 manner. I do not wish it to be understood, however, that 

 there is any relation of antagonization between that particular class of substances and 

 the tithonic rays. It appears to me 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. 



