BRIDGMAN. — MERCURY UNDER PRESSURE. 349 



The experimental material of the paper, with respect to both subject 

 matter and experimental method, falls naturally into two parts. The 

 first is concerned with the p-v-t surface of the liquid. To map this the 

 isothermal compressibility of the liquid was found at two temperatures, 

 0° and 22°, and combined with the known dilatation at atmospheric 

 pressure. This is sufficient to cover the region in question, because 

 the change of dilatation with temperature is slight. The second part is 

 concerned with the changes taking place on the freezing curve. Ther- 

 modynamically, the data required are the change of volume on passing 

 from liquid to solid and the variation of freezing temperature with 

 pressure. From these the latent heat may be calculated by Clapey- 

 ron's equation. Two independent methods were used ; one which gave 

 both the change of volume and the relation between temperature and 

 pressure on the freezing curve, and another which gave only the freez- 

 ing curve. In addition still another method was used in determining 

 the change of volume on freezing at atmospheric pressure. 



In the conclusion, the data collected are discussed from the point of 

 view of their bearing on present theories of the liquid state, and the 

 change solid-liquid. These data for a single substance cannot j ustify 

 by any means an attempt at a new theory. This is simply a first step, 

 indicating in a general way the nature of the effects to be expected 

 at high pressures. From this point of view there are considerations 

 both for and against the use of mercury. On the one hand, the com- 

 pressibility and thermal dilatation of mercury are comparatively small, 

 so that the change in these quantities under the pressure range em- 

 ployed is not nearly as great as it would be for many organic liquids ; 

 but on the other hand, the internal structure of mercury is probably 

 at least as simple as that of any other known substance, so that in the 

 liquid state the results are not complicated by entrance of polymeriza- 

 tion, and on the freezing curve the results are not complicated by the 

 entrance of allotropic forms, as they are in the case of water, for example. 



The discussion of the methods and the possibilities of error has nec- 

 essarily been somewhat minute and painstaking. It is hoped that this 

 will be pardoned when it is realized that the field in which these meas- 

 urements are made has been hitherto practically unworked, and is one 

 where there are several unusual sources of error to be guarded against. 

 At the same time it is a field in which, by the exercise of proper pre- 

 cautions, an accuracy equal to that of the greater number of physi- 

 cal measurements can be reached. The accuracy of nearly all of the 

 following measurements is of the order of 1/10 per cent. 



