EXPERIMENTAL KNOWLEDGE OF THE rROPEETIES OF MATTER. 517 



constant. These lines are obviously the same as Ramsay and Young have 

 investigated ; for if the density of a given mass is constant, its volume is 

 constant. Isopyknic lines are therefore isochoric lines, and Wi'oblevvski 

 is attacking precisely the same problem as Ramsay and Young have 

 apparently brought to a successful conclusion for several widely different 

 substances by means of their own laborious and skilful experiments. 

 Wroblewski calculates their data by means of Sarrau's equation, which, 

 however well it may apply to portions of the data for CO2, is not to be 

 taken as a rigorously proved theorem which can enable one to dispense 

 ■with direct experiment. A criticism of Wroblewski's method and con- 

 clusion is given by Ramsay and Young in ' Philosophical Magazine,' 23, 



1887, p. 547. 



Covipressihility of Liquids. 



Araagat, in the course of his examination of the effect of pressure on 

 gases, found ' for the gases he examined, including CO2 for low tempera- 

 tures as for high, when the pressure is very great, the following relation 

 p (v — a) = cpnstant ; this relation holds good very approximately at 

 ordinary temperatures, whether at these temperatures the compression 

 condenses the gas to a liquid or not — whether or not the temperature is 

 above or below the critical point. The higher the temperature the 

 higher the pressure required beyond which the above relation holds; but 

 at ordinary temperatures the pressure need not exceed from 400 to 

 500 atmos. ; the constant a for all pressures above a certain number 

 of atmos. and for the volumes corresponding varies with the gas or liquid 

 and with the temperature. 



Amagat's results, however, were obtained for gases mostly, and for 

 CO2 at temperatures not very much below their critical points. The 

 problem is not the same when we take the case of water, mercury, or 

 other liquids, the compressibility of which is slight, and for which the 

 compression is usually measured by a special instrument, a piezometer. 



In a previous paper ^ Amagat made a special investigation into the 

 cSect of variation of temperature on the compression of a variety of 

 liquids, among them ethyl — chloride, bromide, oxide ; methyl — and ethyl- 

 acetates, methyl alcohol, ethyl alcohol, amyl alcohol, some paraffins ; 

 benzene, chloroform, carbon bisulphide, and acetone; the pressures not 

 being greater than 37 atmos., the minimum temperature for each body 

 being about 11°-13°, and the maximum about 100°. 



Again, Amagat has made recently ^ a special investigation into the 

 compressibility of water, and in particular into the effect of compression 

 on the point of maximum density. In some respects water behaves quite 

 differently from all other liquids. Thus, as the pressure increases the co- 

 efficient of expansion increases, more and more slowly at higher pressures, 

 but, at pressures up to and beyond 200 atmos., rather rapidly ; at about 3,000 

 atmos; the coefficient of expansion ceases to increase, and, probably, at 

 higher pressures, it diminishes, as is the case with other liquids. Between 

 two given pressures the compressibility diminishes as the temperature 

 rises, contrary to the behaviour of other liquids. 



Meanwhile, as pressure is continuously increassed, the point of maxi- 

 mum density is lowered till at 200 atmos. it is nearly 0° ; at 700 atmos. it 



' Ann. Chini. Ph. 22, 1881, pp. 319-397. 2 Ihid. 5, 11, 1877, p. 520. 



» a R. 104, 1887, p. 1159 ; 105, 1887, p. 1120; and C. S. J. Ahs. 1887, p. 695, and 



1888, p. 215. 



