HEAT. 
sof sible, and mercury the least so, of any fluids, because 
they possess ively the lowest and the highest 
iling points. Recherches, t. i. p. 271, 311. 
_- We have hitherto spoken of the expansion of fluids 
y heat, and their consequent’ contraction by cold, as 
‘a property possessed by all bodies of this deseription ; 
‘but there is one very remarkable exception to the gene- 
_ ral law, that of water. Water, through a certain range 
of temperature, like other fluids, experiences the usual 
“augmentation of bulk by an addition of heat ; but when 
it is cooled down to about 40°, it seems to arrive at its 
maximum of density ; and if the cooling process be far- 
ther continued, it begins to expand, and continues its 
nsion until it arrives at its freezing point, when it 
becomes solid, By employing certain precautions, par- 
ticularly by avoiding every degree of agitation, it will 
continue fluid for several degrees below the usual point 
of congelation, and it still goes on increasing in volume, 
until its actual solidification takes place. “When a wa- 
ter thermometer, therefore, stands at 50°, it is impos- 
sible to know whether it be really at the temperature of 
50°, or at that of 30°, 18° above, or two degrees below, 
its freezing point. 
- This anomaly, with respect to the expansion of water 
-by the abstraction of heat, was one of the discoveries of 
the Florentine academicians, about the middle of the 
17th century ; but it was not much attended to, until 
it was again brought into view by Deluc. He repeat- 
ed the experiment with more accuracy, and he attempt- 
ed to ascertain the exact degree at which the expansion 
commences. This he fixed at 41°; and he farther dis- 
covered, that the expansion is nearly equal when water 
is either heated or cooled, the same number of degrees 
above or below 41°: (Recherches, t. i. p. 225.) Some 
circumstances which were not sufficiently attended to 
by Delue, particularly the expansion of the glass in 
which the water is contained, were afterwards carefully 
» noticed by Sir C. Blagden, and in consequence of these 
corrections, he fixed the point of greatest density at 39°. 
(Phil. Trans. 1788, p. 125, et seq.) Le Fevre-Gineau 
eame to the same conclusion respecting this anomaly 
in the expansive power of water, by a very different 
process. He weighed a cylinder of copper in water at 
various temperatures, and thus obtained the weight of 
a cylinder of water exactly equal to that of the metal 
which he employed.. The result of this process was to 
fix the maximum of the aac of water at the 40th de- 
gree. Dr Hope adopted another method of arriving at 
the same conclusion, which we have fully explained 
in our article Expansion. Rumford has since performed 
a set of experiments upon the same principle, and with 
nearly the same results. There have been some objec- 
tions made against these experiments, although they 
appear so correct, and were so much varied ; but upon 
' the whole, we conceive the conclusion to be are ly 
established, that the greatest density of water is at about 
8 or 9 degrees above its freezing point. The cause of 
this liarity with respect to water is involved in con- 
siderable obscurity. It appears to be connected with 
the increased space which water, as well as many other 
- substances, occupy; when they assume the crystalline 
form ; and we may conjecture, that some tendency to 
this state takes place in water before its actual occur- 
rence. See Expansion, p. 257, 258. ; 
The greater space which water occupies, when in the 
state of ice, has been long observed, and has been made 
the subject of some curious experiments, particularly 
by Major Williams, who, during an intense frost at 
Quebec, actually burst a cannon, by the ores of 
only a comparatively small quantity of water: (Zdin 
Trans, vol. ii. p. 23, et seq.) When water is converted 
into the solid state, it forms spicular crystals, which 
cross each other at a determinate angle ; and thus inter- 
stices are left, which cause it to occupy more space, 
Dr Thomson found, that the specific gravity of ice is to 
water at 60°, as 92 to 100; wnlbans we may conceive 
the great force that will be exerted, when water sud- 
denly expands itself, so as to have its gravity diminished 
in the above ratio. 
The expansion of solids, which we are now to consi- 
der, is much less than that of liquids. The same ele- 
vation of temperature, which increases the volume of a 
gas from 100 to 137 parts, increases that of a liquid 
only to about 104 or 105 parts, while it adds to a solid 
not more than y,!,~ of its bulk. A set of e i 
on the expansion of metals was performed by Smeaton, 
when he found, that, in passing from the freezing to 
the boiling points of water, platina was expanded from 
100,000 to 100,087, gold from 100,000 to 100,094 parts, 
steel to 100,112 parts, copper to 100,170 parts, silver 
to 100,189 parts, tin to 100,288 parts, lead to 100,287 
parts, and zinc to 100,296 parts: (Phil. Trans. 1754, 
p. 612.) It appears that the densest metals are gene~ 
rally the least expansible; but this proportion does not 
universally obtain, 
The expansion of glass, which is a point of great im- 
portance to ascertain, as affecting the result of many 
other experiments, was made the subject of careful exa- 
mination by Deluc. Supposing its bulk at 32° to be 
100,000, at 212° it was found equal to 100,083, an 
expansion nearly the same with that of platina. From 
this estimate we learn that glass is expanded about 
xosies of its bulk by one degree of heat, and the ex- 
pansion seems to be nearly uniform, for the same de- 
gree of heat, through the different parts of the scale. 
his equal augmentation of bulk, by equal increments 
of heat, is supposed to prevail Ferner in solids ; but, 
from the minuteness of the effect, it is less easy to as- 
certain this point, than with respect to gases and li- 
uids, 
; Several of the metals expand at the time they are 
converted from the fluid to the solid form ; and this, as 
is the ease with water, seems to depend upon the oc- 
currence of a kind of crystallization, or regular arrange- b&°™ 
ment of their particles. . This is said to be particularly 
the case with respect to iron, and is the reason why we 
are able to take such accurate casts, when the melted 
metal is poured into moulds.. It must, however, be ob- 
served, that the increase of bulk, which either water or 
melted metals acquire when they become solid, is no 
exception to the general law of expansion ; for this ap- 
plies solely to bodies as long as they retain the same 
state.» The only anomaly, therefore, is the expansion 
of water while it remains fluid. 
It is upon the expansive power which heat exercises 
over bodies, that the thermometer is constructed ; an in- 
strument, as its name imports, employed for the pur- 
pose of measuring the degree of heat, or temperature 
of substances to which it is applied, and which, consi- 
dered in all its relations, may perhaps be regarded as 
the most useful agent in philosophical researches of 
which we are in possession. It ap to have been 
invented by Santorio, the celebrated Italian physician, 
who devoted so much of his attention to-what has been 
called statical medicine. His instrument depended 
upon the expansive power of the air, and consisted of 
a tube, with a bulb of considerable size, the lower end 
of the tube being left open, and being plunged into a 
fluid, which was suffered to rise to a certain degree into 
the tube. As the air in the globe was expanded or 
679 
Effects of 
Meat. 
Expansion 
of wolide. 
Smeaton's 
experiments 
on metals, 
Deluc’s ex- 
permnents 
on glass. 
Expansion 
of metals 
when they 
ww. 
lid. 
Thermome- 
ter 
invented by 
Santorio. 
