It must be observed that the true weight of any body, or that which the body would appear to have if weighed in vacuo, is greater than tho weight which it is observed to have when weighed in air, by the weight of a volume of air equal to the difference between the volume of the body and that of the object by which the weight is determined.
It should also be observed that the numbers expressing the specific gravities of substances are strictly correct only on the parallel of latitude passing through the place where the weight of the water under the unit of volume is determined ; for the force of gravity, and consequently the weight of any substance under a given volume, increases in proceeding from the equator towards either pole of the earth.
In order to determine the specific gravity of the atmosphere, or of any gas whatever, the air or gas must be weighed in a globular vessel of glass, of sufficient magnitude to prevent the unavoidable errors of the operation from sensibly affecting the results.
In taking the specific gravities of gases, correction must be made for temperature. It is known that for every degree of Fahrenheit's scale there is an expansion equal to of the bulk occupied at 32° Fakir. If, for example, cubic inches of gas at 70° were reduced to 60° the change in volume would be as follows :—Since 70-32=38, 491 parts of a gas at 32' would nt 70° have increased in bulk 3S parts or would have become equal to 529 parts. Again 60-32=28, so that a gas which at 32° occupied 491 parts, would at 60° occupy a ' space equal to 519 parts. The volume therefore of any gas at 70° would bear the same proportion to the bulk which it would occupy at GO° as 529 does to 519. Hence, 529 : 510 : : : 9'026 cubic inches. A correction is also required for pressure; but in taking the specific gravity of a gas, ltegnault has reduced the number of corrections required by counterpoising the globe in which the gas is to be weighed by a second globe of equal size. The film of moisture adhering to the glass is equal in both globes, and as the bulk of air displaced is equal in both cases, the calculation for its buoyancy is thus got rid of. A balance capable of weighing 2 lbs., and turning with tho 150th part of a grain when loaded, is placed on a chest, furnished with folding doors, within which the glass globes, each of the capacity of about 600 cubic inchm, attached to the scale pans, aro freely suspended. The counterpoise globe is hermetically sealed, the other globe is furnished with a stop-cock, the air is exhausted from it, and it is filled with the gas to be tried in a pure and dry state. The globe is again exhausted so as to get rid
of the last portions of atmospheric air, when it is again filled with the gas, which may be again pumped out and again fdled. To get rid of the correction for temperature the globe is placed in melting ice, which reduces the gas to the French standard ; more gas is introduced to equalise the pressure, the atop-cock is then closed, the globe withdrawn and wiped carefully with a damp cloth to prevent the surface from becoming electric, and it is then attached to the scale-pan. Two hours are allowed to elapse before it is weighed, in order that it may acquire'the temperature of the surrounding air and get rid of the currents about it. The weight is then accurately noted, the globe again plunged in ice, the gas removed by means of the air pump, and the elasticity of the remaining portion in the globe measured by the pump gauge. The empty globe is again weighed as before, and the difference of the two weights will give the weight of -a bulk of gas the elasticity of which is equal to that of the atmosphere as marked by the height of the barometer II' diminished by the elasticity It of the remaining gas as measured by the gauge. If the capacity of the globe has been previously determined with accuracy, the corrected weight of the gas will be obtained by the following proportion The following table has been calculated by Professor Miller from Regnault'e experiments, and reduced to the English standard tempe rature and pressure.
temperature. So also if these two bodies be removed from a warm to a cold atmosphere the oil will cool much quicker than the water. In comparing various bodies with water it will be found that they all vary in their rates of heating and cooling. Taking water as the standard of comparison, the thermal unit is the quantity of heat required to raise!1 lb. of pure water from 32° to 33°. And in general the quantity of heat required to raise 1 lb. of any other body from 32° to 33° is called its specific heat. When the quantity of heat required to raise the temperature of a body one degree is uniform throughout, or in a very large portion of the thermometric eeale, the specific heat of such body is said to be uniform. Where such is not the case, it is said to be 'variable, and is in general found to increase with the temperature.