Description of a differential Thermometer. ‘927 
The two outermost necks are furnished with funnels, and 
the central one with a tube bent a little more than at right 
angles, and passing through ice to the bottom of a bottle 
surrounded by the same. The acid and alcohol ought to 
be very strong. Let a gill of the latter be poured into the 
bottle, and then add as much acid as will make it boil brisk- 
ly. When the effervescence relaxes, add more acid until 
the addition of this produces no great effect. Then add 
more alcchol, and again more acid, till the bottle becomes 
about one third full. The ether will be rapidly formed and 
collected in the bottle into which the recurved tube leads. 
This tube is represented in the plate of about one third of 
the proper length. There should be a triangular wooden 
trough adapted to it for holding ice or snow. 
It might be an improvement if another neck were added — 
through which the residual liquor might be drawn out. 
With this addition, the distillation of ether might be con- 
ducted in a way analogous to that of the distillation of whis- 
key by the celebrated Scotch still. 
Art. XVIII. Description of a differential Thermometer, 
by W. Howarpn, M. D. Adjunct Professor of Anatomy 
in the University of Maryland.* 
THis instrument is in imitation of Mr. Leslie’s differen- 
tial thermometer, but is on a different principle. In his, 
the degree of heat is measured by the expansion of air, but 
in the present one by the increase of expansive force of the 
vapour of ether or spirit of wine in vacuo, which affords a 
test of great delicacy, and is easily constructed. 
+ A tube (A) being first made with a ball at each extremi- 
ty, in one of which is left a small orifice, a portion of ether 
er spirit of wine is then introduced, and heat being applied, 
is brought to a state of active ebullition. At this moment 
the orifice is closed with a piece of wax, and finally hermet- 
ically sealed by the biowpipe. ‘The tube may then be care- 
*¥rom the London Quarterly Journal of Science, Literature and the Arts. 
1 See the figure at the end of the volume. 
