A section of this calorimeter, with all the parts in place, is shown in [Fig. 120].
In the figure the steel cylinder A, about 12.5 centimeters deep and 6.2 in diameter, represents the chamber in which the combustion takes place. Its walls are about half a centimeter thick and it weighs about three kilograms. It is closed, when all the parts are ready and the sample in place, by the collar C, which is secured gas tight by means of a powerful spanner. The cover is provided with a neck D carrying a screw E and a valve screw F. In the neck D, where the bottom of the cylinder screw E rests, is a shoulder fitted with a lead washer. Through G the oxygen used for combustion is introduced. The upper edge of the cylinder A is beveled and fits into a groove in the cover B, carrying a soft metal washer. To facilitate the screwing on of the cover, ball bearings KK, made of hard steel, are introduced between the collar and the cover. The platinum wires H and I support the platinum crucible holding the combustible bodies which are ignited by raising the spiral iron wire connecting them to the temperature of fusion by an electric current. The combustion apparatus when charged is immersed in a metal cylinder M, containing water and resting on small cylinders of cork. The water is stirred by the apparatus LL. The cylinder M is contained in two large concentric cylinders, N, O, made of non-conducting materials and covered with disks of hard rubber. The space between O and N may be filled with water. The temperature is measured by the thermometer P, graduated to hundredths of a degree and the reading is best accomplished by means of a cathetometer.
Fig. 120. Hempel and Atwater’s
Calorimeter.
563. The Williams Calorimeter.—The calorimeter bomb has been improved by Williams by making it of aluminum bronze of a spheroidal shape. The interior of the bomb is plated with gold. By an ingenious arrangement of contacts the firing is secured by means of a permanently insulated electrode fixed in the side of the bomb. The calorimetric water, as well as that in the insulating vessel, is stirred by means of an electrical screw so regulated as to produce no appreciable degree of heat mechanically. The combustion is started by fusing a fine platinum wire of definite length and thickness by means of an electric current. The heat value of this fusion is determined and the calories produced deducted from the total calories of the combustion. The valve admitting the oxygen is sealed automatically on breaking connection with the oxygen cylinder. The effluent gases, at the end of the combustion, may be withdrawn through an alkaline solution and any nitric acid therein thus be fixed and determined.[579]
564. Manipulation and Calculation.—The material to be burned is conveniently prepared by pressing it into tablets. The oxygen is supplied from cylinders, of which two should be used, one at a pressure of more than twenty atmospheres. By this arrangement a pump is not required.
In practical use, a known weight of the substance to be burned is placed in the platinum capsule, the cover of the bomb screwed on, after all adjustments have been made, and the apparatus immersed in the water contained in M, which should be about 2° below room temperature. All the covers are placed in position and the temperature, of the water in M begins to rise. Readings of the thermometer are taken at intervals of about one minute for six minutes, at which time the temperature of the bomb and calorimetric water may be regarded as sensibly the same. The electric current is turned on, the iron wire at once melts, ignites the substance and the combustion rapidly takes place. In the case of bodies which do not burn readily Atwater adds to them some naphthalene, the thermal value of which is previously determined. The calories due to the combustion of the added naphthalene are deducted from the total calories obtained.
The temperature of the water in M rises rapidly at first, and readings are made at intervals of one minute for five minutes, and then again after ten minutes. The first of the initial readings, the one at the moment of turning on the current, and the last one mentioned above are the data from which the correction, made necessary by the influence of the temperature of the room, is calculated by the following formulas.[580]
The preliminary readings of the thermometer at one minute intervals are represented by t₁, t₂, t₃ ... tₙ₁. The last observation tₙ₁ is taken as the beginning temperature of the combustion and is represented in the formulas for calculations by Θ₁. The readings after combustion are also made at intervals of one minute, and are designated by Θ₂, Θ₃ ... Θₙ. The readings are continued until there is no observed change between the last two. Generally this is secured by five or six readings.
The third period of observations begins with the last reading Θₙ, which in the next series is represented by tʹ₁, tʹ₂ ... tʹₙ₂.