400 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the compressibility of a compound contains too many possible variables 

 to form at once the certain basis of exact reasoning ; and among elements 

 only mercury, lead, copper, and iron in the uncertain form of steel, seem 

 to have been even crudely studied.* The problem is moreover compli- 

 cated by the fact that the coefficient of compressibility diminishes as the 

 pressure increases. 



The work which is needed in order to compress a given substance to a 

 given extent can only be computed accurately when the varying com- 

 pressibility through the whole range is known ; and since the pressures 

 involved in the present question are clearly many thousands of atmos- 

 pheres, the precise solution of the problem seems to be a distant matter, 

 although by no means impossible. 



By a process of approximation some light may be obtained, however. 

 If one selects a single series of compounds, such as the chlorides, it is 

 obvious that a large part of the compressibility throughout the series 

 should correspond to the compressibility of the chlorine. In those cases 

 where the compressibility of the metal is smallest, the change of volume 

 would be due almost solely to the compression of the non-metal. 



In view of these considerations, the first approximation should be 

 obtained by comparing the actual contractions taking place during the 

 formation of amounts of substance containing the same weight of chlorine 

 with the heat evolved in each case. The starting point in each case is 

 liquid chlorine, having a molecular volume of about 50 (or an atomic 

 volume of about 25) at 20°. The heat of formation of the chloride is 

 usually given in tables of data as starting from chlorine gas, under 

 atmospheric pressure ; hence the latent heat of evaporation and expan- 

 sion of tbe chlorine should be subtracted from the usual values in order 

 to institute a precise comparison. f However, these quantities cannot be 

 large in proportion to the heat of combination with the metal, and they 



* Landolt and Bornstein, Phys. Cliem. Tab., pp. 268, 278 (1894). Unless 

 otherwise stated, all data used in this paper were taken from this admirable book 

 of tables. 



t The latent heat might be approximately calculated from the data of Knietsch 

 (Landolt and Bornstein, p. 80 (1894)) as follows : — 



_ RT*dP 8.32 X (293.5)2 X 0.19 . 



Q = PdT ~ 6.62 X 1 = Joules, or 20.5 



kilojoules, between 20° and 21° C, for the evaporation of one gram-molecule. The 

 wide deviations from the gas-law exhibited by chlorine render the calculation 

 very uncertain. It is enough, however, to show that the value is relatively small. 

 The heat absorbed on expansion must also be in doubt on account of the same 

 deviations. 



