90 PHYSICS. 
the air at zero, when the ratio between the volume of the tube included 
between any two divisions and the volume of the bulb is known. 
The instrument is now introduced into a box filled with water heated to 
a temperature ¢ ( pl. 19. fig. 14), so that the tube with the index may project 
above the side of the box. The index wil! then be driven to a certain point, 
and the increase of volume for the temperature ¢ may be determined. !n this 
manner the coefficient of expansion for dry gases is found to be 0.375, 
which Rudberg, by means of another apparatus, corrected to 0.365. This 
coefficient of expansion increases with increasing pressure. 
In referring previously to the specific gravity of bodies, the temperature 
was left out of account. This could very well be done, as the slight 
differences of temperature usually occurring during such determinations, 
exercise little influence on.the density of solids and liquids. The case is, 
however, very different with regard to gases, where the least change of 
temperature produces a material difference in the density. In investigations 
of the density of gases, a hollow ball is employed, provided with an arrange- 
ment by means of which it can be screwed on the plate of an air pump, 
there to be exhausted. A tightly-fitting stop-cock prevents the entrance of 
air when the ball is removed. The exact capacity of the ball must be 
known, which is best obtained by filling it with distilled water and then 
weighing it. The ball is then emptied, dry air admitted and weighed, and 
then again weighed after exhaustion of the air. If the experiment be ver- 
formed with a perfectly exhausted ball, at a barometric pressure of 29 
inches, and at a temperature of 0°C., or the results corrected to these condi- 
tions, the density of dry air, or its specific gravity,will be found to be 0.001299. 
Any other gas may be substituted for atmospheric air, and its density ascer- 
tained in the same manner. For this purpose, the Pneumatic apparatus 
figured in pl. 19, fig. 15, may be employed. This consists of a receiver, c, 
provided with a cock, d. This receiver is placed in a trough filled with 
mercury, a hand air-pump screwed on at d, and by the exhausting of the air, 
filled with the mercury. When entirely filled with the mercury, the cock 
is closed, and the air-pump replaced by the exhausted ball, y. The 
was, as generated, is admitted through the tubes a and bd into the receiver, 
and thence, opening the stop-cocks d and e, into the ball. 
b. Effects of Heat in Changing the State of Aggregation of Bodies. 
The state of aggregation of a body depends entirely upon heat, that is, 
whether it is to be solid, liquid, or gaseous. By heat many solids become 
liquid, and liquids gaseous ; and conversely, by withdrawal of heat, gases 
may be changed into liquids, and these into solids. Sometimes the same 
body can be made to assume all three states in succession. TEven if, in the 
case of certain bodies, this process has not been observed, we are fairly 
entitled to conclude that it is owing to the difficulty of attaining the 
extremes of temperature necessary for the purpose. It is thus certain that 
upon heat it depends whether a body shall be solid, liquid, or gaseous, 
264 
