34 



of endurance, while magnesium carbonate merely doubles it. It is 

 interesting that magnesium carbonate should here be more effective 

 than the corresponding salt of calcium, since in all other cases the 

 latter is the more beneficial.^ Sodium sulphate is likewise of very 

 little neutralizing value, and the soluble salts of magnesium possess 

 none so far as was ascertained. Calcium sulphate raises the limit 

 only six times, the presence or absence of an excess of calcium car- 

 bonate not affecting the value of the sulphate. This comparatively 

 slight ef&cacy of calcium sulphate in neutralizing " black alkali " is 

 rather surprising in view of the accepted ideas of students of alkali 

 soils in regard to the curative value of gypsum. ^ The comparative 

 inefficacy of calcium sulphate in this case contrasts strikingly with 

 its power to neutralize sodium in the forms of sulphate (Table VII) 

 and chloride (Table VIII). 



Calcium chloride is the only salt found to be very effective in 

 neutralizing sodium carbonate, raising the limit of endurance for the 

 latter fifty times. A mixture of solutions of the two salts causes an 

 immediate heavy precipitate of calcium carbonate, to which fact the 

 efficacy of the added salt must be largely ascribed. We should be 

 dealing in this case with a solution of sodium chloride containing a 

 large excess of calcium carbonate. Yet by direct addition of solid 

 calcium carbonate to a solution of sodium chloride, the limit of endur- 

 ance for the latter can be raised onlj^ three times, i. e., to 0.06 normal 

 (see Table VIII). Here again chemistry appears to be powerless to 

 afford an explanation of a phenomenon which, in the present state 

 of our knowledge, must be regarded as paradoxical.^ 



A ver}^ noteworthy result was obtained by experiments with sodium 

 carbonate, as well as with sodium bicarbonate, in the presence of an 

 excess of calcium sulphate and calcium carbonate. In solutions of 

 critical concentration of both of these mixtures a majority of the 

 roots of the lupine plants were completely destroyed with pronounced 



' The probability of the formation of a double carbonate, with a consequent 

 lowering of the active mass, as well as a probable change of nature of the ions, 

 suggests itself very forcibly in this connection. It is hoped that time and oppor- 

 tunity will be found in the near future to test this supposition in the laboratory. 



^Hilgard. Bui. 128, Agr. Exp. Sta., Univ. Calif., pp. 16 to 18 (1900). It should be 

 stated, however, that Hilgard recommends the application of gypsum under phys- 

 ical conditions which would not probably be considered analogous to those under 

 which the experiments here described were performed. 



^In marked contrast with this anomalous case is that of the mixture of magne- 

 sium sulphate and calcium chloride, in which a precipitate of calcium sulphate is 

 formed and which is therefore to be regarded as a solution of magnesium chloride 

 containing a solid excess of calcium sulphate. Here the limit of endurance is the 

 same as when solid calcium sulphate is added directly to a solution of magnesium 

 chloride. The same thing is true of a mixture of sodium sulphate and calcium 

 chloride in which the limit of endurance is the same as for sodium chloride plus 

 calcium sulphate. 



