164 



The last circumstance I have often verified, as well as Dr. 

 C.'s correction of Guyton Morveau, that the white powder 

 is carbonate, not hydrated oxide. But I have not found 

 the exclusion of air, or even its separation by boiling, so 

 effectual as it appeared to Dr. Christison. I find, like him, 

 that any of the neutral salts usually contained in natural 

 waters, lessen or prevent the action described. I resume his 

 account, much curtailed, of the action of particular classes 

 of salts. 



The preserving power depends entirely on the acid of the 

 salt, not on the base, and those salts are preservative in 

 the highest degree whose acid forms, with oxide of lead, salts 

 of the least solubility, either in wateif alone, or water con- 

 taining carbonic acid. When the corresponding salt of lead 

 is insoluble, or very nearly so, an extremely minute propor- 

 tion of any alkaline or earthy salt acts as a preservative. 



Acetates and nitrates are not completely preservative, even 

 in the proportion of one per cent, to the water, or 700 grains 

 in the gallon ; phosphate of soda is almost effectually so in 

 the proportion of one to thirty thousand, or about two grains 

 in the gallon. None of the above salts being commonly 

 present in natural waters, they are not of immediate practical 

 importance; but they are material in a scientific point of view, 

 as proving the defensive power to vary according to the solu- 

 bility of the salt of lead denoted by each acid, the acetate 

 and nitrate being among the more soluble of salts of lead, 

 phosphate of lead one of the most insoluble of neutral salts. 

 Common salt and other chlorides, gypsum and other sul- 

 phates, hold a middle place. Dr. Christison seems to have 

 found them defensive in proportions ranging from l-2000th 

 to 1 -4000th, the sulphates being the most effectual. 



In those solutions of muriates, sulphates, and carbonates 

 of lime, soda and magnesia, which constitute the greater 

 part of spring waters, I have observed, I think, quite as, great 



