232 Mr. T. Preston on the Continuity of Isothermal 



alters only slightly as the pressure is varied. When the 

 pressure is gradually reduced, however (the temperature 

 being maintained constant), a point B is reached at which the 

 liquid begins to boil, and the whole mass may be transformed 

 into the gaseous state under constant pressure, if heat be 

 supplied to keep the temperature constant while the volume 

 is allowed to increase from B to D. The part BD of the 

 isothermal is consequently a right line parallel to the axis of 

 volume, and at D the whole mass is in the condition of 

 saturated vapour. Beyond D the curve DE is approximately 

 a rectangular hyperbola as it represents the isothermal of a 

 gaseous substance which approximately obeys Boyle's law. 



Very shortly after Andrews 5 celebrated experiments on the 

 isothermals of carbon dioxide, and on the continuous trans- 

 formation of matter from the gaseous to the liquid state, 

 Professor James Thomson, in an ingenious speculation (sug- 

 gested by the shape of the isothermals immediately above the 

 critical temperature), proposed an isothermal curve of the 

 form represented in fig. 2, which embraces the idea of conti- 



Fig. 2. 



O V 



nuity of transformation, so much insisted on by Andrews. 

 Here, in passing from B to D, the substance is supposed to 

 be homogeneous throughout, and not to be partly liquid and 

 partly vapour as in the corresponding part BD of the iso- 

 thermal of fig. 1. The word homogeneous must here, how- 

 ever, be taken with some reservation, for although the mass, 

 as a whole, may be apparently homogeneous — that is, one 

 cubic centimetre may be on the whole the same as another, — 

 yet when considered in very small portions the mass may be 

 intensely heterogeneous. For example, small portions may 



