Mr. Joule on the Thermo-electricity of Ferruginous Metals. 227 
such that with the junction antimony and bismuth, a thermometric 
effect not greater than ,35, of a degree Centigrade could be esti- 
mated. In determining the thermo-electric position of the metals, it 
was necessary to increase the resistance of the instrument a hundred- 
fold, by placing in the circuit a coil of fine wire. In thermo-electric 
arrangement steel was found to be nearer copper than iron was. By 
hardening, steel was raised almost to the place of copper. Cast iron 
was found to surpass copper; so that the junction cast iron and 
copper is reverse to that of wrought iron and copper, and the 
arrangement cast iron and wrought iron is much more powerful than 
copper and wrought iron. A new test of the quality and purity of 
ferruginous metals is thus indicated, which will probably be found of 
value to the arts. 
The experiments on the stretching of solids showed, in the case of 
the metals, a decrease of temperature when the stretching weight was 
applied, and a heating effect when the weight was removed. An iron 
wire = of an inch in diameter was cooled 1 of a degree Centigrade 
when stretched by a weight of 775 lbs. Similar results were ob- 
tained with cast iron, hard steel, copper, and lead. The thermal 
effects were in all these cases found to be almost identical with those 
deduced from Professor Thomson’s theoretical investigation, the par- 
ticular formula applicable to the case in question being H= ; x Pe, 
where H is the heat absorbed in a wive one foot long, ¢ the absolute 
temperature, J the mechanical equivalent of the thermal unit, P the 
weight applied, and e the coefficient of expansion per 1°. With 
gutta-percha also a cooling effect on extension was observed; but a 
reverse action was discovered in the case of vuleanized india- 
rubber, which became heated when the weight is laid on, and cooled 
when the weight was removed. On learning this curious result, 
Professor Thomson, who had already intimated the probability of 
a reverse action being observed under certain circumstances with 
india-rubber, suggested to the author experiments to ascertain 
whether vulcanized india-rubber stretched by a weight is shortened 
by increase of temperature. Accordingly, on trial, it was found that 
this material, when stretched by a weight capable of doubling its 
length, has that length diminished by one-tenth when its temperature 
is raised 50° Centigrade. This shortening effect was found to increase 
rapidly with the stretching weight employed ; and, exactly according 
with the heating effects observed with different stretching weights, 
entirely to confirm the theory of Professor Thomson. 
February 12.—W. R. Grove, Esq., V.P., in the Chair. 
The following communications were read :— 
“On the Photography of the Moon.” By William Crookes, Esq. 
The subject of lunar photography is one which has engaged the 
attention of scientific men almost from the first announcement of the 
possibility of fixing the images in the camera. Owing to the extreme 
difficulty of satisfying all the conditions of the problem to be solved, 
Q 2 
