1. From the space occupied by any quantity of gas under an observed degree of pressure, to infer what its volume would be under the mean height of the barometer, taking this at 30 inches

This is done by the rule of proportion; for, as the mean height is to the observed height, so is the observed volume to the volume required. For example, if we wish to know what space would be filled, under a pressure of 30 inches of mercury, by a quantity of gas, which fills 100 inches, when the barometer is at 29 inches.

30 : 29 : : 100 : 9666.

The 100 inches would, therefore, be reduced to 96.66.

2. To estimate what would be the volume of a portion of gas, if brought to the temperature of 60° Fahrenheit.

Divide the whole quantity of gas by 480; the quotient will show the amount of its expansion or contraction by each degree of Fahrenheit's thermometer. Multiply this by the number of degrees which the gas exceeds, or falls below 60°. If the temperature of the gas be above 60°, subtract, or if below 60°, add the product to the absolute quantity of gas; and the remainder in the first case, or sum in the second, will be the answer. Thus, to find what space 100 cubic inches of gas at 50° would occupy if raised to 60°, divide 100 by 480; the quotient 0.208 multiplied by JO gives 2.08, which added to 100 gives 102.08, the answer required. If the temperature had been 70°, and we had wished to know the volume, which the gas would have occupied at 60°, the same number 2.08 must have been subtracted from 100, and 97.92 would have been the answer.

3. In some cases it is necessary to make a double correction, or to bring the gas to a mean both of the barometer and thermometer.

We must then first correct the temperature, and afterwards the pressure. Thus to know what space 100 inches of gas at 70° Fah., 29 inches barometer, would fill at 60° Fah., and 30 inches barometer, we first reduce 100 inches, by the second process, to 97.92. Then, by the first

30 : 29 : : 97.92 : 94.63.

Or 100 inches, thus corrected, would be only 94.63.

4. To ascertain what would be the absolute weight of a given column of gas at a mean temperature, from the known weight of an equal volume at any other temperature.

First find by the second process what would be its bulk at a mean temperature; and then say, as the corrected bulk is to the actual weight, so is the observed bulk to the number required. Thus, if we have 100 cubic inches of gas weighing 50 grains at 50° Fah., if the temperature were raised to 60°, they would expand to 102.08. And

102.08 : 50 : : 100 : 49.

Therefore 100 inches of the same gas at 60° would weigh 49 grains.

5. To learn the absolute weight of a given volume of gas under a mean pressure, from its known weight under an observed pressure, say, as the observed pressure is to the mean pressure, so is the observed weight to the corrected weight. For example, having 100 inches of gas which weigh 50 grains under a pressure of 29 inches, to know what 100 inches of the same gas would weigh, the barometer being 30 inches.

29 : 30 : : 59 : 51.72. Then 100 inches of the same gas, under 30 inches pressure, would weigh 51.72 grains.

6. In some cases it is necessary to combine the two last calculations. Thus, if 100 inches of gas at 50° Fah., and under 29 inches pressure, weigh 50 grains, to find what would be the weight of 100 inches at 60° Fah., and under 30 inches of the barometer, first correct the temperature, which reduces the weight to 49 grains. Then,

29 : 30 : : 49 : 50.7. 100 inches, therefore, would weigh 50.7 grains,

1 Vide Henry's Elements of Experimental Chymistry, vol. ii. p. 497.