Page:Outlines of Physical Chemistry - 1899.djvu/153

 The Mechanical Equivalent of Heat l

From the work of a number of scientists (Joule, Mayer, Fame, Hirn, and others) we know that the large calorie is

The small calorie is therefore equal to 427 gram-metres, 42,700 gram-centimetres, or

42,700 x 980-96 ergs.

Bowland has recently calculated the relationship :

at 15° 1 small calorie = 41,890,000 ergs, at 18° „ „ „ = 41,830,000 ergs.

We may take this last figure as the mean equivalent of one small calorie.

Thermo-chemical Notation

In thermo-chemistry, the symbols of the elements represent the atomic weight in grams. Thus the equation

C + 2 = C0 2 + 97-65 Cal.

expresses that, by the combustion of 12 grams of carbon (amorphous) by 82 grams of oxygen, 97*65 large calories are evolved.

��1 The principle of the conservation of energy and the idea of the mechanical equivalent of heat are to be found in all physics text- books, and form part of the scientific training of every student. — The principle, according to which the entropy of a system of bodies tends continually to increase, can only be appreciated from a thermo- dynamical study. I may, however, here indicate that it is by virtue of this principle that a certain quantity of heat cannot be wholly transformed into mechanical energy ; when, in a change of state of a system, n calories are involved, a certain number (x) of these may do a certain mechanical work, but the remainder (n—x) is retained in the form of degraded caloric energy (i.e. at a lower temperature).

to an air-thermometer and at the mean temperature of the laboratory. At this temperature each degree of the air-thermometer is equal to about 1*007 degree of the mercury-thermometer.
 * 427 kilogram-metres if the calorie is reported with reference

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