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(39.) It has been stated that the increase or diminution of temperature is accompanied by an increase or diminution of volume. Related to this, there is another phenomenon too remarkable to pass unnoticed, although this is not the proper place to dwell upon it: it is the converse of the former; viz. that an increase or diminution of bulk is accompanied by a diminution or increase of temperature. As the application of heat from some foreign source produces an increase of dimensions, so if the dimensions be increased from any other cause, a corresponding portion of the heat which the body had before the enlargement, will be absorbed in the process, and the temperature will be thereby diminished. In the same way, since the abstraction of heat causes a diminution of volume, so if that diminution be caused by any other means, the body will give out the heat which in the other case was abstracted, and will rise in its temperature.

Numerous and well-known facts illustrate these observations. A smith by hammering a piece of bar iron, and thereby compressing it, will render it red hot. When air is violently compressed, it becomes so hot as to ignite cotton and other substances. An ingenious instrument for producing a light for domestic uses has been constructed, consisting of a small cylinder, in which a solid piston moves air-tight: a little tinder, or dry sponge, is attached to the bottom of the piston, which is then violently forced into the cylinder: the air between the bottom of the cylinder and the piston becomes intensely compressed, and evolves so much heat as to light the tinder.