146 Dr. Hare on the Cause of Heat. 
weight by gravity, during each vibration. In the vibrations 
conceived to constitute heat, there is no generating power to 
make up for this loss. A body preserves the expansion 
communicated by heat in vacuo, where, insulated from all 
other matter, the only momentum, by which the vibrations 
of its particles can be supported, must have been received 
before its being thus situated. If we pour mercury into a 
glass tube shaped like a shepherd’s crook, the hook being 
downwards, the fluid will be prevented from occupying that 
part of the tube where the air is in such position as not to 
escape. In this case, according to the hypothesis in ques- 
tion, the mercury is prevented from entering the space the 
air occupies, by a series of impalpable gyratory movements ; 
so that the collision of the aerial particles against each oth- 
er, causes each to occupy a larger share of space in the man- 
ner above illustrated by the descending weight and inter- 
posed body. The analogy will be greater, if we suppose a 
row of interposed bodies alternately striking against each 
other, and the descending weight ; or we may imagine a vi- 
bration in all the particles of the interposed mass equal in ag- 
gregate extent and force to that of the whole, when per- 
forming a common movement. If the aggregate extent of 
the vibration of the particles very much exceed that which 
when performed in mass would be necessary to preserve a 
certain space, it may be supposed productive of a substance 
like the air by which the mercury is resisted. But whence 
is the momentum adequate in such rare media to resist a 
pressure of a fluid so heavy as mercury, which in this case 
performs a part similar to that of the weight, cited for the 
purpose of illustration? If it be said that the mercury and 
class being at the same temperature as the air, the particles 
of these substances vibrate in a manner to keep up the aerial 
pulsations ; I ask, when the experiment is tried in an ex- 
hausted receiver, what is to supply momentum to the mer- 
cury and glass? There is no small difficulty in conceiving 
ander the most favourable circumstances, that a species of 
motion, that exists according to the hypothesis as the cause 
of expansion in a heated solid, should cause a motion pro- 
ductive of fluidity or vaporization, as when by means of a 
hot iron, we convert ice into water, and water into vapour. 
How inconceivable is it that the iron boiler of a steam en- 
gine should give to the particles of water, a motion so 
