130 EXPERIMENTS on EVAPORATION. 
2. If its heat be increafed a certain degree above that of 
the air to which it is expofed, a vifible vapour or fmoke 
will arife fromit, which will appear more or lefs in quan- 
tity in proportion to the heat. 
3. If it be heated to 212°, while expofed to the pref- 
fure of the Atmofphere, or to g8° in vacuo, {mall tran{- 
parent globules are formed fuddenly, and with a crackling 
noife, in that part of it which firft receives the heat; thefe 
globules, which are compofed of elaftic vapour, afcend 
through the water as quickly as air would do, if in the 
fame circumftances : as foon as they efcape from water in- 
to air, which is colder, they are converted from tran{pa- 
rent elaftic vapour, into vilible inelaftic vapour or fmoke, 
which paffes through the air as other vilible vapour does: 
the formation and paflage of thefe bubbles through the wa- 
ter, produces that motion in it which we call boiling. * 
Any perfon may be convinced of this, by applying a can- 
die to the bottom of a flafk or thin glafs veffel which has 
a {mall quantity of water in it. 
The evaporation produced by immerfing moift bodies or 
ice, in cold air, refembles the fecond kind which I have 
defcribed (or that which produces {moke,) in feveral re- 
fpects. In order to make water {moke, you need only 
render it warmer than the air to which itis expofed; thus, 
to give a very familiar example, a dith of tea, when frit 
poured out, {mokes at the fire fide, when it has loft fome 
Ok 
* T have ftated that water will boilin vacuo, with a heat of 98° upon the authority of Mr. 
Watt; but anelaftic vapour will arife from water in vacuo when the heat is much lower— 
Some Gentlemen have related in the Philofophical Tranfactions, that when they were making 
experiments with the Barometer in an exhautted receiver, an elaftic vapour arofe from the 
moilt leathers, and comprefled the mercury in the Barometer. They alfo refer to the experi- 
ments of Lord Cavendifh, and from thefe they fay it appears, that water of 72° yielded an 
elaftic vapour when the receiver was fo much exhaufted, that the Barometer funk to ¢ of an 
inch, or when 1-40 of the common preflure of the Atmofphere remained ; and that when the 
Barometer {unk to £ of an inch, or that r20 only of the common preffure remained, the fame 
kind of vapour arofe from water of the temperature of 47°. ‘This fluid therefore when its tem 
perature is 41°. or upwards may be confidered as in a conftant nifus to affume the form of 
elaftic vapour, which nifus is counteracted by the weight of the atmofphere. See Naixn’s ac- 
counts ef experiments with the air pump, in Phil. Tranfactions, part 2d, 1777. — 
