220 Scientific Intelligence. 
due to exceedingly minute traces of aqueous vapor from the plug 
of the stop-cock. The quantity of ponderable matter is so small, 
and the ease of measuring$'the velocity of cooling such, that it 
seems to offer an extremely good test for the quality of a vacuum. 
In an experiment, under 760 mms. pressure, the time was 225 
seconds, under 1°3 mms. 364, and continually increasing the ex- 
haustion the times 664, 555, 602 and 712°5 seconds were obtained. 
The last number was the result of drying the apparatus in an oil 
bath at 200° C., and shutting it off at that temperature. To 
prove that we thus obtain an actual vacuum with regard to heat 
conduction, a thermometer was manufactured which by slips of 
glass could be put into two different envelopes. With medium 
pressures the times of cooling were nearly as one to two, but with 
the last vacuum 576 and 576. Replacing the air by hydrogen, 
gave 588 and 578, and with carbonic acid 586 and 578. In the 
was 6, and in the other 2°5. eir experiments, therefore, cannot 
be looked upon as rigorously demonstrating the law founded 
: hich 
- heat _of mercury on the temperature.—Phil. Vag., 1, 585 
oe. Roy. Pruss. Acad., 1875, p. | pate 
y a pluviometer the results are much too small. e leaves 
condense far more than surrounding bodies, and their temperature 
may fall six or eight degrees below the air, showing that ear 
emissive power is much greater than that of the metal surfaces 0 
ter. 
cube was 
ered with 
the leaves, e deflections were measured by a mirror an ? 
and a twentieth of a degree was easily observed. On trying 5° ; 
eral kinds of leaves it appeared that their emissive poweT did no 
differ greatly, was the same on both sides, and had an average 
value of 94, that of lampblack being 100. 
0 
thin sheet of copper riveted to a steel wpring. One face wae 
ered with lampblack, the other with the leaf to be examined. 
c40) 
