162 Miscellanies. 
returned to a perfectly fluid state. At 7 o’clock, the thermome- 
ter was still as before at —40°. The mercury in the phials was 
unchanged. ‘That on the piece of charcoal exhibited the same ap- 
pearances as at the last observation, only in a less marked degree, 
and it sooner became fluid. Soon after this, the sun rose, and of 
course the attempt was discontinued. 
To Pror. Sittiman.—Dear Sir,—I send you above, the ex- 
tract from my minutes of an observation of the effect of natural 
cold on mercury, on the Kennebeck. A few weeks later, having 
been supplied by the kindness of the late Mr. Vaughan with sever- 
al excellent thermometers of Troughton’s manufacture, I attempted 
to ascertain how much effect was attributable to the cooling of the 
surface by radiation, in a similar state of atmosphere: and on one 
occasion found a difference of 18 degrees between a thermometer 
on charcoal on the ground, and another suspended freely in the air, 
18 or 20 feet above it—one being 36°, the other 18° below zero. 
This was in a clear night, and subsequently when clouds appeared, 
the difference diminished, until at the commencement of snow the 
two instruments agreed. Yours very truly, 
E. Hau, Jr. 
Boston, Wednesday Evening, March 23, 1836. 
CHEMISTRY. 
1. Cause of Decrepitation; by M. Bavprimont.—Most au- 
_thors on this subject have attributed decrepitation to the vaporiza- 
tion of water contained between the lamine which compose the sub- 
stance operated upon. This opinion not appearing to me well 
founded, I dried at a low temperature and by various means, differ- 
ent anhydrous substances capable of decrepitation, but found that 
notwithstanding the most perfect desiccation, they still decrepitated 
when suddenly heated. The tendency of the decrepitating body 
to a separation of its parts in the direction of its cleavage faces, 
leads me to the following conclusion: that decrepitation is owing 
principally to the bad conducting power of the several substances, 
the outer layers expanding by heat, without a corresponding inter- 
nal expansion, and consequently splitting off with a kind of explosion. 
There are instances, however, in which the substance is volatile and 
for this reason decrepitates; but this is not in general the case with 
decrepitable substances.—L Institut, No. 158. si 
