Page:Outlines of Physical Chemistry - 1899.djvu/183

 HEAT OF FOBMATION

��oxygen. The other appendages are thick platinum wires ; one of them (b) is in metallic connection with the body of the bomb and serves as a support for the substance to be burnt ; the other (c) is insulated by ivory or caoutchouc joints and can be connected with a pole of a battery.

If the substance to be burnt is solid, it is used in the form of a small compressed cylinder. If it is liquid, it is contained in a small plati- num capsule ; and if it is easily volatile, the contain- ing vessel may be provided with a cover of collodion. The material is placed on the platinum support, and above it the two thick wires are connected by a thin spiral of iron wire. The bomb is closed and oxygen introduced through a, until there is a pressure inside of 25 atmospheres. The screw valve is then tightened and the bomb immersed in the water of a calorimeter (provided with a mechanical stirrer).

After a preliminary period, which lasts only a few minutes, the prin- cipal period is introduced by starting the combustion of the iron spiral and the organic substance by means of an electric current. The regular descent of the ther- mometer, that is the final period, generally commences after four or five minutes. The whole operation lasts only about a quarter of an hour, during which the temperature is read off at the commencement of each minute. The

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