PYRONOMICS. 89 
even do not expand uniformly between 0° and 100°C. The elaboration 
of these, as well as of the actual absolute expansions, consequently always 
presents difficulties. 
The density of a body must always be connected with its expansion, for 
an increase of volume always implies a decrease in density. Water, 
however, forms an exception to this law, for, although according to a 
proposition previously given, water should be of greatest density at 
0° R., or 32° F., that is, at the freezing point, accurate experiment has 
shown, that when heated at this point it contracts, and continues to do so 
-until the temperature has risen to 4° R.,or 39°.1 F., when it is in its state 
of maximum density. Above this degree it expands according to the usual 
law. The vast importance and almost absolute necessity of this peculiarity 
of water will be referred to hereafter. 
It has been before mentioned that mercury is most desirable for filling 
thermometer tubes, owing to its uniform expansion between 32° and 212° 
F’. ; to show the difference produced by irregular expansion, we have given 
in fig. 16 the rate of expansion of mercury, water, and alcohol, at tempera- 
tures between 0° and 100° C. ‘The lowest curve represents the expansion 
of mercury, and appears a straight line, owing to the uniformity of 
expansion. The middle curve is the expansion of water. It exhibits first 
a contraction (to 4° C.), at 8° C.is as at 0°, and expands then in a very 
progressive ratio, so that at 70° C. the ratio between W and q is almost 
2:1. The upper curve exhibits the expansion of alcohol. To A, or 50°C., 
the expansion is uniform, and consequently the curve is a straight line ; 
then, however, the curvature increases more and more. The figure shows 
that water is not applicable to the filling of thermometers, and that for any 
other liquid than mercury, a great length of tube would be required. 
We have seen that the expansive effect of heat on solids and liquids is 
different according to their force of cohesion, being inversely as this 
cohesion. In gaseous bodies. therefore, in which the cohesive force is zero, 
no obstacle is presented to the expansive force of heat. This must there- 
fore be the same for all gaseous bodies, and proportional to the increment 
of temperature ; experiments instituted for the purpose have verified this 
conclusion. An air thermometer, therefore, may be constructed by 
employing air perfectly free from moisture, which may be done by passing 
it over chloride of calcium. For this purpose a thermometer tube is pre- 
pared, on which is accurately marked the ratio between the contents of the 
bulb and the volume of the divisions on the tube itself, produced by the 
graduations. The tube is now filled like a thermometer tube, the mercury 
boiled, and the tube placed in a vertical position with a tube open below, and 
filled with chloride of calcium, fastened to its open extremity. The mercury 
will escape from the tube, and in its place there will enter a quantity of air, 
purified from moisture, by passing through the chloride of calcium. The 
further entrance of air must cease while thezve is yet a small quantity of 
mercury in the tube, which must remain for two purposes, to prevent the 
escape of the air, and to serve as an index. The point at which the 
mercury stands when the tube is placed in melting ice, gives the volume of 
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