Page:Elementary Text-book of Physics (Anthony, 1897).djvu/238

224 that if the volume diminish from $$OC$$ to $$OG,$$ the pressure will become greater than $$GD;$$ suppose it to be $$GM.$$ If a number of such points as $$M$$ be found, and a line be drawn through them, it will represent the relation between volume and pressure when no heat enters or escapes. It is called an adiabatic line. It evidently makes a greater angle with the horizontal than the isothermal.

The tangents to these lines at the point of intersection, being the ratios of the changes of pressure to the same changes of volume under the conditions represented by those lines are proportional to the elasticity at constant temperature, or the isothermal elasticity $$E_{t},$$ and to the elasticity when no heat is allowed to enter or escape, or the adiabatic elasticity $$E_{h},$$ respectively.

214. Specific Heats of Gases.—The amount of heat necessary to raise the temperature of unit mass of a gas one degree, while the volume remains unchanged, is called the specific heat of the gas at constant volume. The amount of heat necessary to raise the temperature of unit mass of a gas one degree when expansion takes place without change of pressure, is called the specific heat of the gas at constant pressure.

The determination of the relation of these two quantities is a very important problem.

The specific heat of a gas at constant pressure may be found by passing a current of warmed gas through a tube coiled in a calorimeter. There are great difficulties in the way of an accurate determination, because of the small density of the gas, and the time required to pass enough of it through the calorimeter to obtain a reasonable rise of temperature. The various sources of error produce effects which are sometimes as great as, or even greater than, the quantity to be measured. It is beyond the scope of this work