1879J 
447 
Proceedings of Societies . 
During the aurora’s greatest brilliancy the magnet was observed 
during five minutes to be undisturbed.” The aurora was visible 
on forty-nine days between Ocftober 22nd, 1875, and February 
27th, 1876. The observations from which the foregoing mag- 
netical results have been obtained were made by Commander 
(now Captain) A. H. Markham and Lieut. G. A. Giffard of the 
Alert, and Lieuts. R. H. Archer and R. B. Fulford, of the 
Discovery. 
Physical Society, March 22, 1879. — Prof. W. G. Adams, 
President, in the chair. 
Capt. Abney, R.E., F.R.S., read a paper “ On obtaining Photo- 
graphic Records of Absorption Spedtra.” Absorption spedtra 
have hitherto been recorded by the difficult process of hand 
copying; but the discovery by Capt. Abney of a silver salt 
sensitive to all rays in different degrees renders the photographic 
method available. The records thus obtained are photographs 
of the spedtrum of the naked light of the source, and of that of 
the same light reduced by insertion of the absorbing material in 
its track, and these are taken parallel, so that the dark absorption 
lines* can be readily compared. Examples of these were thrown 
on the screen. This method can be used as a new weapon in 
attacking solar physics, and determining whether or not com- 
pound bodies exist in the sun. Absorption spedtra to compare 
with the sun’s can be got for compound bodies by burning the 
matter in question in a flame in front of the slit, and passing a 
bright light through the flame. 
Prof. Guthrie, F.R.S., then read a paper “ On the Fradture of 
Colloids,” as illustrated by experiments on the breakage of glass 
plates, either by pressure or heating at the centre or round the 
circumference. Circular plates of glass pressed at centre or 
circumference break in radial lines. However supported, a plate 
breaks in the same fashion if heated in the same way. If heated 
in the middle the crack is peak-shaped, like an obelisk on a 
double pedestal, two cracks forming the outline, with sometimes 
a third down the middle. The two cracks unite before they reach 
the edge on one side, and (as afterwards pointed out by Prof. W. 
G. Adams) the three extremities of the two cracks all meet the 
edge at right angles to it. The crackage varies with the size 
and shape of the plates, the flame, and kind of glass ; but the 
type is the same for all. Cracks cross each other. Prof. Guthrie 
defined a crack as the line where the ratio of cohesion to strain 
is least, and likened it to the lightning flash. 
April 26. — Mr. C. V. Boys gave an account of some experi- 
ments made by Dr. Guthrie and himself on the subjedt of Arago’s 
rotation. The experiments were begun with a view to determine 
if the drag on a copper disk when a magnet is made to revolve 
beneath it, or on the magnet if the disk is made to revolve above 
