earths, thoria and cerium, which have the property, when highly heated, of emitting a most powerful light.

This mantle is placed over an atmospheric, or Bunsen, burner, with the result that a magnificent white light is produced with the consumption of an almost infinitesimal amount of gas, a light of 70 candles being obtainable from 3 cubic feet of ordinary 16-candle gas; together with the further advantage that it only yields from onehalf to one-fifth the amount of heat which attends other systems of gas-lighting. In this burner gas companies possess the most powerful weapon they have ever had placed in their hands with which to fight the electric light, as the light yielded by the incandescent burner as applied to domestic purposes is quite as powerful, and the cost is very much less.

:

CHAPTER XIX.

INFLUENCE OF TEMPERATURE AND PRESSURE UPON THE VOLUME OF GAS.

THE

`HE distinguishing property of gases is their power of indefinite expansion when subjected to changes of temperature and pressure, with a corresponding change in volume; it is consequently necessary to make corrections for such changes in volume by reducing the volume of a gas under a given temperature and pressure to some common standard, which, in the case of coal-gas, is the temperature of 60° F., and a barometrical pressure of 30 inches of mercury.

These corrections are based on the following physical laws:

1st, for pressure.

By the law of Boyle or Mariotte, "the volume of a given mass of any gas varies inversely, as the pressure; the temperature being constant, calling volume V, and pressure P, the volume of V P is constant.'

Thus, if V is the volume, when the pressure is P the volume of the gas becomes:

Or, expressed in words, doubling the pressure reduces the

volume to one-half, and, conversely, reducing the pressure one-half doubles the volume.

Now, supposing we measured 1,000 cubic feet of gas under a barometrical pressure of 306 inches, and we wished to know what it would measure at 30 inches; as the pressure under which the gas is measured is greater than the standard pressure (30 inches), it is plain that under the latter pressure the volume would be greater, we therefore say:

As 30 306 :: 1,000 x,

x = 1,019.3 cubic feet.

Or, supposing that we measured 1,000 cubic feet of gas under a pressure of 29.5 inches, and we wished to know the volume at the standard pressure, in this case the gas is measured under a lesser pressure than the standard; consequently, when reduced to the latter pressure, the volume would be reduced; in this case, therefore, we say :

As 30 : 29.5 :: 1,000 : x,

x = 983.3 cubic feet.

It will be noticed that in each case the standard (30 inches) is in the first term.

2nd, for temperature.

pressure

As a general rule, bodies tend to expand when their temperatures are raised, and to contract when the temperature is lowered, gases being the most expansible of all bodies.

The law of change of volume with change of temperature is known as that of Charles and Gay Lussac, and may be stated thus:

66

The volume of a gas under constant pressure expands when raised from the freezing to the boiling temperature by the same fraction of itself, whatever be the nature of the gas."

It has been found by able experimenters that a volume of air at constant pressure expands from 1 to 1·3665

between 0° C. and 100° C., and all perfect gases expand in the same proportion.

Consequently, for every increase in temperature of 1° C., a gas expands by of its volume at 0° C., and for every rise of 1° F., a gas expands by 4 of its volume at 32° F. This fraction is called the co-efficient of expansion, and represents the increment or decrement which occurs in a measured volume of gas for every change of temperature of 1° F., provided the pressure remains unchanged.

2

Now, supposing we measure 1,000 cubic feet of gas at a temperature of 80° F., and we wish to correct it to the standard temperature of 60° F. (the pressure remaining constant), 492 volumes at 32° F. become 492 + (60 − 32 28) 520 volumes at 60° F., and 492 + (80

=

48)

=

=

540 volumes at 80° F.

32

=

The volume, therefore, of

any gas at 80° F. would bear the same ratio to the volume which it would occupy at 60° F., as 540 does to 520, consequently,

As 540 520 :: 1,000 : ∞,

x = 962.9 cubic feet.

If the gas, instead of being measured at 80° F., had been measured at 50° F., then, as before, 492 volumes at 32° F. would become 520 volumes at 60° F., and 492 volumes at 32° F. would become 492 + (50 32 510 volumes at 50° F. Then the ratio to the volume at 60° F. would be obtained as follows:

=

18)

=

:

As 510 520 :: 1,000: x,

x = 1,019.6 cubic feet.

It will be noticed that 520 always occupies the second. term in the proportion. Now, in practice, the volume of a gas is always corrected for temperature and pressure at one operation, by combining the two corrections together, but the method of performing the necessary. calculations will be better understood by a brief description of the "air thermometer," which has played a very important part in "thermo-dynamics, and has very much simplified the formula for adjusting volumes of gas under various conditions of temperature and pressure to standard.

66

The air thermometer is a tube of uniform bore, sealed at one end, and containing a column of dry air disconnected from the outer atmosphere by a short column of mercury, which moves freely with the changes of the contained air.

We will suppose the pressure on the column of air to remain constant during the following changes.

"Conceive the air thermometer immersed in a mixture maintained at 32° F., and the volume of the contained air to measure 1; then conceive the thermometer immersed in steam from water boiling under standard conditions, and, according to Regnault, the volume should now measure 13665, but probably 1.366. I adopt this figure in preference to the former. The problem now is:-Taking freezing point at 32° F. (1), and boiling point at 212° F. (1.366), find the temperature expressed at the bottom of the tube by continuing the Fahrenheit scale to that point. A simple calculation will show that it would be 460 F. Let us shift the zero to the bottom of the tube and scale upwards on the Fahrenheit system, and the freezing point will consequently be on the new scale, + 492.

"Temperature expressed on this scale is termed by scientific men absolute temperature.

"The rule, then, for converting ordinary readings on the