ABSTRACT
In discussing a possible explanation for the change of state of a body by displacement (p. 93) we saw that two ways are open to us. We can suppose that the arrangement or the motion of the parts of the body was modified—this is the mechanical explanation—but we can also claim that to the substance of the body another substance is added, invisible but preexisting elsewhere. If we apply this method of explanation to the phenomenon of heat, we shall arrive at the theory of heat- fluid. In general, we shall build up theories of a class different from that of mechanical theories and which may be designated by the generic term of “theories of quality.”
