682 
Effects of ments of Mr Dalton, as low as 5° or 27° below the 
Heat. 
Experi- 
ments of 
Dalton, 
Black, 
Blagden. 
Yaporiza- 
‘tion, 
usual point of congelation. In order to succeed in 
giving water this unusual degree of cold, without con- 
verting it into ice, it is necessary that it be kept ina 
state of the’most complete rest ; for the least agitation 
either prevents it from falling lower than 32°, or if it 
be brought down below this point, it instantly begins 
to freeze, and the fluid part rises to 32°.. Black found 
that by carefully boiling the water, so as to expel from 
it all the air which is generally dissolved in it, it was 
less easy to reduce the temperature below the ordi 
degree; and Blagden discovered, that when water is 
thus artificially cooled, no circumstance had more ef- 
fect in producing its sudden congelation, than intro- 
ducing into it a small particle of ice. JHence when the 
freezing of water has once commenced, it proceeds 
with so much rapidity in these experiments ; and it 
may perhaps also, in some measure, account for the ef- 
fect which water confined in close vessels has in burst« 
ing the vessel when it freezes. In these cases the 
fluid may probably descend some degrees below the 
usual freezing point, before the congelation com+ 
mences; a slight agitation, or other incidental cireum- 
stance, then causes the freezing to commence, and the 
first spiculze of ice that are formed, tend to convert the 
remaining part of the water into the solid state so ra+ 
pidly, as to produce a greater expansion, than if the 
particles had arranged themselves more quietly in the 
crystalline form. 
The effect of a continued addition of heat to a liquid 
is, as we have remarked above, to convert the liquid 
into the state of vapour. This, like the former, is a sud- 
den change, and, like it, is attended with a complete 
alteration in the physical properties of the substance so 
changed. A certain portion of caloric added to a body, 
- insinuates itself among the particles, and places them so 
far from each other, as to dimimish the attraction which 
unites them into the solid form, without, however, en- 
tirely destroying their epgregation ; while a larger por- 
tion removes them toa still greater distance, and seems as, 
if it entirely placed them beyond the reach of their mu- 
tual attraction. The most obvious property of the body, 
when in this vaporized state, is its power of expanding 
HEAT. 
serve to produce a corresponding inerease of its bulk 
and this ap we have already rand, is oer 
ble for the same quantity of heat, and is likewise simi- 
lar in all the different gaseous bodies, = 
2 abe int of vaporization, like that of solidification, 
rs 
bodies that always appear in the state of gas when not 
combined with other substances, and no degree of cold 
which has yet been applied to them has been able to 
deprive them of their elastic fluidity. Yet from analo« 
gy we conclude, that these bodiés have nothing in their 
nature essentially different from those which we observe 
to assume either the liquid or the pseu state, accord- 
ing to the Compete ey in which they are placed ; and 
we conceive, that, had we the power of employing a 
degree of cold sufficiently intense, they might be ren- 
dered liquid, ae ns solid, Ether iy the lowest, and 
mercury the highest vapour points of any liquids juids with 
which ia are pata be A gp If we were always immersed 
in a temperature above that at which ether boils, we 
should never see it in a fluid state; and if we had not 
been able to produce parE eles oS above 670°, at which 
this 
mercury boils, we should have had no ¢ tion of 
substance being reducible into the state of a vapour. 
The terms jived and volatile, like solid and liquid, are 
therefore to be regarded as merely relative, and as not 
indicating the absolute quality of the bodies, but point- 
ing out the forms under which they usually 
themselves to our notice. But although we regard this a : 
as the most correct view. of the ania iek many che." ° 
mists of eminence have ; there is some es- 
sential or radical difference between the two kinds of 
aeriform fluids, and have called them by different names; 
‘to those that are easily reduced to the liquid state the 
term vapour has been exclusively applied, while the 
others have been called gases, or permanently elastic 
fluids. The discussion of this question belongs more to 
chemistry, than to the immediate subject of this article; 
but we farther remark concerning it, that when, by 
the effect of chemical affinity, these are reduced to 
the liquid or solid form, heat is extricated; thus proving 
‘that it entered into their constitution, and was necessa~ 
ry to their existence, 
almost every kind of liquid. There are some point 
te eee 
A io 
An important circumstance connected with the con- Vaporiz 
version of liquids into the state of elastic fluidity, is, pris 
that it is influenced by the degree of pressure to which py essur 
the liquid is subjected. External pressure thus coun- 
teracts the repulsive power of heat, and as it were heeee ih 
itself in an indefinite degree, until its particles be 
brought nearer together by an external compressing 
force. If this external force be removed, it quickly re- 
gains its former dimensions; and hence we regard it as 
Elasticity of a body possessed of a perfect degree of elasticity. It is 
Sa eed vee Ape he 
vapours, upon the elastic power which different substances pos- the particles in a state of forcible approximation. fferel 
sess, when they are converted by heat into the gaseous follows, from this order of things, if we remove :* °* 
or aeriform state, that many of the most important ope- from the surface of a liquid the whole or a of the “5 
rations, both in art and nature, are brought about, The 
force which different bodies exercise when they are re- 
duced into the gaseous state, and are at the same time 
subjected to strong pressure, is almost inconceivable ; 
it is, in short, unlimited in its extent, and may be so 
managed as to produce any assignable degree of effect. 
Notwithstanding, however, the expansive power which 
bodies exercise during their conversion from the fluid 
to the gaseous form ; yet when they are finally brought 
into this state, they are compressible. Thus, although 
the conversion of water into an aeriform fluid is capable 
of generating an almost immense degree of mechanical 
power, in consequence of the greater space which it 
then occupies, a power of which we take advantage in 
the steam engine, yet the vapour, when once produced, 
may be easily compressed into a less bulk, by the 
weight of a comparatively small quantity of mercury. 
Farther increments of heat, when added to the gas, aialy 
weight of the atmosphere, by placing it under the re- 
ceiver of an air-pump, or by. ascending a high moun- 
tain, it boils ata lower temperature than under ordina~ 
ry circumstances. Thus water, which boils at 212°, 
when the barometer stands at 30 inches, boils at about 
70° in vacuo; and on the top of Mount Blane, it was 
found by Saussure to boil atabout 187°. As the tem 
perature at which different species of elastic fluids are 
formed, Sepentp so much upon the pressure which is 
applied to them; so, after the gaseous state has been as- 
sumed, the elastic force of the vapour varies according 
to the 
of experiments, in which he determines the height of 
the mercurial column which the ee of water-wiil 
support at different temperatures. Thus if the vapour 
of water be taken at the freezing and boiling points, and 
at 122°, which is intermediate between them, he found 
1 
eof pressure under which it exists. Mr Dalton : 
Dalton has made this the subject of an extensive range Pe™e 
