Since the final expression contains only the ratio between

the initial and final volumes or pressures, it is immaterial in which system of units these values are measured. By replacing the natural logarithms by the ordinary (Brigg's) logarithms we obtain —

A = 4:'5ST log '^ cal. = 191*6riog -|' megergs.

In order, therefore, to expand a gram-molecule of a gas at 0 so far that its pressure sinks from 760 mm. to 76 mm., a work of 191-6 X 273 = 52,300 megergs = 1251 cal. must be done. This work (or quantity of heat) is taken from the expanding gas, and since the temperature is kept constant at O'^ during the process, heat must be introduced from outside in order that no cooling may take place. Expansion at constant temperature is called isothennal.

If we use n gram-molecules instead of 1, then the work done is n times that indicated by the preceding formulae, but there is no other change in the result.

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