286 Scientific Intelligence. 
4, PaorocraPHy.— 
6. The action of light upon a sensitive plate—At a recent session of 
the Photographic Society of Marseilles, one of the members stated his hay- 
ing failed to obtain a good development in some tannin plates which had 
been kept some twenty days after exposure in the camera, although some of 
the same lot of plates had developed good results, when the development 
took place within twenty-four hours of the exposure.—Mr. Vidal explains 
this phenomenon by a new theory of the action of the actinic rays upon 
a sensitized plate. He supposes that under the action of light a certain 
molecular change, of a transitory nature, takes place, but that, in accord 
ance with a general physical law, there is a tendency to return to the 
anterior molecular condition, and that in the process of time a plate ex- 
posed in the camera would, by returning to its original molecular condi- 
tion, lose all trace of its exposure, and be ready to receive an entirely 
new impression, the same as a plate which had not been ex} 
at all, Mr. Vidal concludes that the physical theory of the absorption of 
the actinic fluid by certain substances, such as the iodid of silver, 18 the 
one which best explains photographic reactions 
e have only to say, briefly, in regard to this theory, that we have no 
faith in it whatever; it is contrary to our photographic experience; which 
confirms the theory that the action of light upon the sensitized plate 18 4 
chemical and not a physical action. The loss of sensitiveness, or the 
lack of ability to develope well after keeping—can be readily and satis: 
factorily explained by referring it to causes familiar to all practical photo 
graphers. se 
II. METALLURGY. 
use of aluminum-bronze as a material for the construction of ast mtg 
eal and other philosophical instruments. Col ge remarks at, 
“the qualities of most importance in instrument making are, { 
strength ; (2) resistance to compression; (3) malleability ; (4) transit) 
strength or rigidity; (5) expansive ratio; (6) founding qualities ; ( 
behavior under files, cutting tools, &c.: (8) resistance to atmospheric 1% 
Hg es Sree ed AH (11) fitness 
Tensile strength.—The mean of experiments made by Mr. Anderso® 
d 
at the Royal Gun Factory, Woolwich, shows that the average pas 
neh, 
wo to 
m pression 
lied was 
59 tons 2 cwt. 1 qr. 4 + af 
came too much distorted to permit of more weight being applied ¥ 
an 
