168 CARNEGIE INSTITUTION OF WASHINGTON. 



ing solutions. The close resemblance of the surface etching phenomena thus 

 produced to the surface of moldavites and other tektites is emphasized; also 

 the mechanics of the etching process by which such extraordmary forms are 

 obtained. The distribution of strain within the moldavites is considered 

 briefly. 1'he conclusion is reached that neither the external form of the 

 moldavites nor the distribution of strain within them can be considered to be 

 an indication of their extraterrestrial origin, as has been stated by Suess. 

 This conclusion is identical with that recently advanced by Merrill, and the 

 above evidence sei-ves to strengthen the position taken by him. 



(22) The solubility-product constant of calcium and magnesium carbonates. John John- 

 ston. J. Am. Chem. Soc, 37, 2001-2020 (191o). 



In connection with certain calculations undertaken with the object of ascer- 

 taining the nature and character of the soHd deposited when solutions con- 

 taining both calcium and magnesium carbonates are evaporated down, it was 

 necessary to have trustworthy values of the appropriate solubility-product 

 constants. Owing to lack of the proper experimental data, all of the constants 

 required for the solution of the problem could not be evaluated; but the data 

 proved sufficient to yield satisfactory results for the solubility-product con- 

 stant of calcium carbonate (calcite) and of magnesium carbonate trihydrate 

 (nesquehonite). The experimental determinations of the solubility of calcite 

 were made with partial pressures of carbon dioxide ranging from about 0.0005 

 atm. (which is slightly greater than the amount ordinarily present in the air) 

 to 6 atm.; yet, in spite of this ten thousand fold variation of one of the factors 

 in the equilibrium, the calculated solubility product remains constant, a fact 

 which demonstrates the substantial accuracy of the assumptions involved in 

 the calculations. 



The results are: Kc = [Ca++] [CO3"]=0.98X10-s at 16°, when the solution 

 is saturated mth respect to calcite; KN = [Mg++] [C03"] = 1.93X10"* at 12°, 

 the solution being saturated with respect to MgCOs-SHgO, while for any 

 temperature (t) up to 50° C, Kn is given by the formula Kn = 2315/(^-1-273) — 

 11.870. These results enable one to calculate the solubility of either of these 

 substances at any partial pressure of carbon dioxide. The results of such 

 calculations are tabulated in the original. In this connection it may be 

 pointed out that there can be no real equilibrium in aqueous solutions of car- 

 bonates except in presence of a definite partial pressure of CO2 in the atmos- 

 phere in contact with the solution — in other words, that, strictly speaking, 

 we are dealing with a ternary system, namely, base-C02-H20 ; consequently 

 any carbonate solution through which a stream of gas absolutely free from 

 CO2 is passed would gradually lose its carbonate and ultimately would contain 

 only hydroxide. 



With a knowledge of these constants and of the solubility-product constant 

 of magnesium hydroxide, one can show that calcium carbonate precipitated 

 from solutions containing magnesium is likely to be contaminated with small 

 quantities of magnesium hydroxide which could be removed only slowly by 

 re-precipitations as ordinarily carried out; that mixtures of magnesium car- 

 bonate and hydroxide will in general be obtained in the precipitation of 

 magnesium by a carbonate and that the basic carbonates thus produced are 

 merely indefinite mixtures of carbonate and hydroxide; and that both calcium 

 and magnesium carbonates can be obtained free from contamination by keep- 

 ing the partial pressure of carbon dioxide above a certain limiting value, the 

 magnitude of which depends upon the conditions and in all probability need 

 not be greater than 1 atm. by a suitable choice of mode of operation. 



