C_p = 1.26 C_v.
From this it follows that C_p, i.e., the specific heat at constant pressure of acetylene, should be 0.373.] It will be seen that this value for k differs considerably from the corresponding ratio in the case of air and many common gases, where it is usually 1.41; the figure approaches more closely that given for nitrous oxide. For the specific heat of calcium carbide Carlson quotes the following figures:
0° 1000° 1500° 2000° 2500° 3000° 3500°
0.247 0.271 0.296 0.325 0.344 0.363 0.381
The molecular volume of acetylene is 0.8132 (oxygen = 1).
According to the international atomic weights adopted in 1908, the molecular weight of acetylene is 26.016 if O = 16; in round numbers, as ordinarily used, it is 26. Employing the latest data for the weight of 1 litre of dry hydrogen and of dry normal air containing 0.04 per cent. of carbon dioxide at a temperature of 0° C. and a barometric pressure of 760 mm. in the latitude of London, viz., 0.089916 and 1.29395 grammes respectively (Castell-Evans), it now becomes possible to give the weight of a known volume of dry or moist acetylene as measured under stated conditions with some degree of accuracy. Using 26.016 as the molecular weight of the gas (O = 16), 1 litre of dry acetylene at 0° C. and 760 mm. weighs 1.16963 grammes, or 1 gramme measures 0.854973 litre. From this it follows that the theoretical specific gravity of the gas at 0°/0° C. is 0.9039 (air = 1), a figure which may be compared with Leduc's experimental value of 0.9056. Taking as the coefficient of expansion at constant pressure the figure already given, viz., 0.003738, the weights and measures of dry and moist acetylene observed under British conditions (60° F. and 30 inches of mercury) become approximately:
Dry. Saturated.
1 litre . . . 1.108 grm. . . 1.102 grm.
1 gramme . . . 0.902 litre. . . 0.907 litre.
1000 cubic feet . 69.18 lb. . . . 68.83 lb.
It should be remembered that unless the gas has been passed through a chemical drier, it is always saturated with aqueous vapour, the amount of water present being governed by the temperature and pressure. The 1 litre of moist acetylene which weighs 1.102 gramme at 60° F. and 30 inches of mercury, contains 0.013 gramme of water vapour; and therefore the weight of dry acetylene in the 1 litre of moist gas is 1.089 gramme. Similarly, the 68.83 pounds which constitute the weight of 1000 cubic feet of moist acetylene, as measured under British standard conditions, are composed of almost exactly 68 pounds of dry acetylene and 0.83 pound of water vapour. The data required in calculating the mass of vapour in a known volume of a saturated gas at any observed temperature and pressure, i.e., in reducing the figures to those which represent the dry gas at any other (standard) temperature and pressure, will be found in the text-books of physical chemistry. It is necessary to recollect that since coal-gas is measured wet, the factors given in the table quoted in Chapter XIV. from the "Notification of the Gas Referees" simply serve to convert the volume of a wet gas observed under stated conditions to the equivalent volume of the same wet gas at the standard conditions mentioned.
HEAT OF COMBUSTION, &C--Based on Berthelot and Matignon's value for the heat of combustion which is given on a subsequent page, viz., 315.7 large calories per molecular weight of 26.016 grammes, the calorific power of acetylene under different conditions is shown in the following table:
Dry. Dry. Saturated.
0° C. & 760 mm. 60° F & 30 ins. 60° F. & 30 ins.
1 gramme 12.14 cals. 12.14 cals. 12.0 cals.
1 litre 14.l9 " 13.45 " 13.22 "
1 cubic foot 40.19 " 380.8 " 374.4 "