106 EFFECTS of HEAT 
part of the cradle, the remaining fpace being filled with two 
pieces of chalk, cut for the purpofe; the uppermoft of thefe 
being excavated, fo as to anfwer the purpofe of an air-tube. 
The pieces thus added, were computed to weigh about 300 
grains. There was no pyrometer ufed ; but the heat was guefled 
to be about 30°. After the barrel had ftood during a few minutes 
in its delivering pofition, the whole lead, with the rod and 
cradle, were thrown out with a {mart report, and with con- 
fiderable force. The lowermoft piece of chalk had fearcely 
been acted upon.by heat. The upper part of the other piece was 
in a ftate of marble, with fome remarkable facettes. | The car- 
bonate, in the little tube, had fhrunk very much during the 
firft action of heat, and had begun to fink upon itfelf, by a fur- 
ther advancement towards liquefaction. The mafs was divi- 
ded into feveral cylinders, lying confufedly upon each other ; 
this divifion arifing from the manner in which the pounded 
chalk was rammed into the tube in fucceflive portions. In 
feveral places, particularly at the top, the carbonate was very 
porous, and full of decided air-holes, which could not have 
been formed but in a foft fubftance; the globular form ‘and 
fhining furface of all thefe cavities, clearly indicating fu- _ 
fion. The fubftance was femitranfparent ; in fome places yel- 
low, and in fome colourlefs.. When broken, the folid parts 
fhewed a faline fraéture, compofed’ of innumerable facettes. 
The carbonate adhered, from end to end, to the tube, and in- 
corporated with it, fo as to render it impoflible to afcertain 
what lof had been fuftained. In general, the line of conta& was 
of a brown colour; yet there was no room for fufpecting the 
prefence of any foreign matter, except, perhaps, from the iron- 
rod which was ufed in ramming downthe chalk. But, in fubfe- 
quent experiments, I have obferved the fame brown or black: 
colour at the union of the carbonate with the porcelain tubes, 
where the powder had been purpofely rammed with a piece of 
wood ; 
