Steam Engine, 
for the escape of vapour. When the water boils, the 
thermometer, surrounded by steam, will rise to 212^^, 
which is precisely the temperature of the water beneath ; 
yet water, placed on a fire, continues to receive heat, very 
abundantly, even when boiling hot ; and as this heat is 
not appreciable by the thermometer, it must exist in the 
steam, in a latent state. 
Perfectly formed steam is entirely invisible. We may 
satisfy ourselves of this by boiling strongly a small 
quantity of water in a flask ; for complete transparent 
cy will exist in the upper part of the vessel. It is only 
when it begins to be condensed, that steam becomes vi» 
sible. We have a proof also, of the same fact in the 
thick fogs, which are produced by a sudden transition from 
warm to cold weather ; the vapour, which was impercep» 
tible at the higher temperature, being condensed and rent 
dered visible by the lower. 
III. The boiling point of the snme fluid varies,, under 
different degrees of atmospheric pressure. Thus, water, 
which h ,s been renaoved from the fire, and ceased to boil, 
has its ebullition renewed, when it is placed under a re- 
ceiver, the air of w^hicb is quickly exhausted by an afr 
pump. Alcohol and ether, confined under an exhausted 
receiver, boil violently at the temperature of the atmos- 
phere. In general, liquids boil in vacuo,, with about 140*^ 
less of heat, than are required under a mean pressure of 
the atmosphere. (Black’s Lectures, i. 151.) Even the 
ordinary variations in tfre weight of the air, as measured 
by the barometer, are sufficient to make a difference in 
die boiling point of water of about 5® between the two 
extremes.^ On ascending considerable heights, as to the 
lops of mountains, the boiling point of water gradually 
*■ Sir G. Shuckbiirgh, in Philosophical Transactions, IxxL' 
