Sentence:  Since the increase of <WSD>disgregation</WSD> is the action by means of which heat performs work, it follows that the quantity of work must bear a definite ratio to the quantity by which the <WSD>disgregation</WSD> is increased; we will therefore fix the still arbitrary determination of the magnitude of <WSD>disgregation</WSD> so that, at any given temperature, the increase of <WSD>disgregation</WSD> shall be proportional to the work which the heat can thereby perform. 
Meaning of the word marked with <WSD> tags:  A measure of the separation of the particles of a system; "an early formulation of entropy".