﻿Geology and Natural History. 319 



by the second column of figures. That the common salt is com- 

 bined and not simply mixed with the double carbonate appears 

 not only from the definite proportion, but also from the fact that 

 the material is so slowly acted on by water. Evidently we have 

 here a definite crystalline product of a very interesting constitu- 

 tion not unlike that of several well defined mineral species in 

 which an alkaline chloride appears to be in molecular union with 

 heterogeneous materials having a function not unlike that of 

 water of crystallization. 



Mr. Theodore W. Richards has made a very extended series 

 of observations of the heat produced by the reaction of argentic 

 nitrate on aqueous solutions of metallic chlorides and found that 

 the quantity of heat evolved was directly proportional to the 

 amount of silver nitrate used. As the investigation was a question 

 of comparison rather than of absolute measures it was possible by 

 keeping the conditions as uniform as possible to obtain results 

 which were strictly comparable. The method is described and 

 the results are given in detail in the papers above referred to and 

 it appears that all of these results are identical within the limit 

 of error of the process. We may, therefore, draw the conclusion, 

 that the amount of heat evolved by the reaction thus represented, 

 (-B m Cl„ + AgN0 3 + Aq) = AgCl + (- # m (NO,) n + - Aq), 



is constant, no matter what 7£, m, or n may be. 



Hence, also, the difference betioeen the heats of formation of 

 equivalent amounts of nitrate and chlorides in aqueous solution is 

 the same for any metal or basic radical. 



Mr. Irving W. Fay made a similar investigation on the heat of 

 formation of chlorides and sulphates in aqueous solution involving 

 the general factors 



,T^(S0 4 ) n + BaCl 2 .2H 2 0, 



but the same constancy in the heat of the reaction was not found. 

 From the table of results which is given in the original paper 

 it appears that the difference between the heats of formation of 

 of chlorides and sulphates is not the same for all metals, as it was 

 found to be in the case of the chlorides and nitrates. It appears, 

 however, that with sulphates of allied bases the agreement is as 

 close as before, but the sesquioxide salts give a larger amount of 

 heat than the protoxide, and the double salts a still larger quan- 

 tity, and sulphuric acid much the largest of all. It is a singular 

 fact, that in the case of common alum the heat evolved is greater 

 than it is with either of its constituent salts. j. p. c. 



II. Geology akd Natural History. 



1. Brief Notices of papers read before the Geological Section 

 of the American Association. Communicated to this Journal by 

 Professor W. M. Davis. — 



Testing the Durability of Building Stones. — A paper by A. A. 

 Julien treated of methods of testing building stones for absorp- 



