To determine the volume of gas at the temperature of explosion, we simply apply the law of Charles.[A] Thus-- V : V' :: T : T' or V' = VT'/T in which V represents the original volume. V' " new volume. T " original temperature on the absolute scale. T' " new temperature of the same scale In the present case T' = 6001 deg.. Therefore substituting, we have V' = 161.82x6001/273 = 3557 litres or at the temperature of explosion 1 gram-molecule of nitro-glycerine produces 3,557 litres of permanent gas. [Footnote A: According to the law of Charles, the volume of any gas varies directly as its temperature on the absolute scale, provided the pressure remains constant. Knowing the temperature on the centigrade scale, the corresponding temperature on the absolute scale is obtained by adding 273 to the degrees centigrade.] ~Pressure or Crusher Gauge.~--There are many forms of this instrument. As long ago as 1792 Count Rumford used a pressure gauge. The so-called crusher gauge was, however, first used by Captain Sir Andrew Noble in his researches on powder. Other forms are the Rodman[A] punch Uchatius Eprouvette, and the crusher gauge of the English Commission on Explosives. They are all based either upon the size of an indent made upon a copper disc by a steel punch fitted to a piston, acted upon by the gases of the explosive, or upon the crushing or flattening of copper or lead cylinders. [Footnote A: Invented by General Rodman, United States Engineers.] [Illustration: FIG. 55.--PRESSURE GAUGE.] Berthelot uses a cylinder of copper, as also did the English Commission, but in the simpler form of apparatus mostly used by manufacturers lead cylinders are used. This form of apparatus (Fig. 55) consists of a base of iron to which four uprights _a_ are fixed, set round the circumference of a 4-inch circle; the lead plug rests upon the steel base let into the solid iron block. A ring _c_ holds the uprights _d_ together at the top. The piston _b_, which rests upon the lead plug, is a cylinder of tempered steel 4 inches in diameter and 5 inches in length; it is turned away at the sides to lighten it as much as possible. It should move freely between the uprights _d_. In the top of this cylinder is a cavity to hold the charge of explosive. The weight of this piston is 12-1/4 lbs. The shot _e_ is of tempered steel, and 4 inches in diameter and 10 inches in length, and weighs 34-1/2 lbs. It is bored through its axis to receive a capped fuse.