18 J. Trowbridge—Molecular Change produced by 
of an alcoholic solution of shellac and sandarach, and this may 
play a very important part in the process. ess, however, 
the results obtained are superior in rapidity, or quality, the in- 
convenience, involved in the preparation of special collodion 
and special baths, will prevent its replacing the former method, 
in which all the solutions employed are such as are in common 
: e employment of argentic iodide alone is suggested 
for photographing the transit of Venus, and it is also stated 
that with bromo-iodized films, it is necessary to protect them 
from the light, even before sensitizing them, on account of the 
effect of light on the argentic bromide. The writer in his first 
experiments also confined himself to the use of iodized col- 
lodion and potassic iodide, for a similar reason, but was 
subsequently convinced by numerous experiments, that this 
direct, or photo-chemical, action of light on argentic bromide, 
when a bromo-iodized collodion is employed, as shown in the 
bluish cast of the film, is also either entirely removed in the 
subsequent desensitizing, or plays no part in the subsequent 
development, and is too feeble of itself, if it is not entirely 
removed in fixing, to merit practical consideration. Neither 
was it found necessary to prolong the immersion of plates in 
the silver bath when coated with a collodion containing a 
bromide, as it seems it is necessary in ne’s process to 
prolong the immersion of plates coated with the silver col- 
lodion, when the bath contains potassic bromide with the 
iodide. 
Art. IIL—Brief Contributions from the Physical Laboratory of 
Harvard College. No. [X.—On a Molecular Change produced by 
the passage of Electrical Currents through Iron and Steel bars ; 
by JoHN TROWBRIDGE. 
coil, A, of fine copper wire was slipped upon the core to a dis- 
tance of 25 cm. from the face of tbe : set a helix B. This 
coil of fine wire was connected with a Thomson’s reflecting gal- 
vanometer. Thesecond magnetizing helix of coarse wire, C, was 
so arranged that its action neutralized the inducing effect of the 
