222 Prof. G. F. Fitzgerald. Clausius's Formula [Mar. 31, 



features of this class of curve. It is the particular case of the 

 ■quartic — 



(L + aM) 2 (M 2 - LN) = Z?LM 3 , 



where L, M, and "N are lines in which M is the line infinity, and L 

 and N are at right angles. In the general case this quartic is a 

 continuous curve with a cusp at the intersection of L and M and 

 Xi -j- aM. as the cuspidal tangent, while L is a tangent to another branch 

 that passes through the same point. Its general features are some- 

 thing like this — 



Fig. 2. 



General Feature Diagram. 



There is generally a double inflexion in the part of the curve 

 outside the triangle L, M, N, and the particular one of a series of 

 such curves for which the two inflexional tangents coincide is what 

 corresponds to the critical isothermal in a gas. It is only what 

 corresponds to the part outside the triangle that is of physical 

 interest. 



In the particular case of the curves representing the isothermals of 

 alcohol, the negative parts of the curve lie at a very great distance 



