244 1-HILOSOPH1CAL TRANSACTIONS. [ANNO 1782. 



well dried, so as to lose no part of the acid, contain 42.56 of acid, 38 of earth, 

 and 19.44 of water. 



Of magnesia or muriatic earth. — This earth, perfectly dry and free from fixed 

 air, could not be dissolved in any of the acids without heat. In the temperature 

 of the atmosphere even the strongest nitrous acid did not act on it in 24 hours; 

 but in a heat of 180° these acids, diluted with 4 or 6 times their quantity of 

 water, attacked it very sensibly; but as much of the acids dissipated by heat, I 

 could not judge of the exact quantity of acid requisite to dissolve a given quan- 

 tity of it, any otherwise than by precipitating the solutions by another substance, 

 whose capacity for taking up acids was known. The substance used was a 

 tolerably caustic vegetable alkali. By this method I found, that 100 gr. of pure 

 magnesia take up 125 gr. of mere vitriolic acid, 132 of the nitrous, and 140 of 

 the marine. None of these solutions reddened vegetable blues; all of them 

 appeared to contain something gelatinous ; that in the marine acid became 

 greenish on standing for some time. 



100 gr. of perfectly dry Epsom salt contain 45.67 of mere vitriolic acid, 36.54 

 of pure earth, and 17-83 of water; but 100 gr. of crystallized Epsom lose 48 

 by drying, and consequently contain 23.75 of acid, 19 of earth, and 57.25 of 

 water. Common Epsom salt contains an excess of acid, for its solution reddens 

 vegetable blues. 100 gr. of nitrous Epsom, well dried, contain 35.64 of acid, 

 27 of pure earth, and 37.36 of water. The solution of marine Epsom cannot 

 be tolerably dried without losing much of its acid, together with the water. 

 The specific gravity of pure muriatic earth is 2.3296. 



Of earth of alum or argillaceous earth. — This earth I found to contain about 

 26 per cent, of fixed air, though I had previously kept it red-hot for half an 

 hour: this surprized me much, as most writers say it contains scarcely any. It 

 dissolved in acids with a moderate effervescence till the heat was raised to 220°, 

 after which I found the solution lighter than the quantities employed in the pro- 

 portion I mentioned. 



100 gr. of this earth, exclusive of the fixed air, require 133 of the mere 

 vitriolic acid to dissolve them. This solution I made in a very dilute spirit of 

 vitriol, whose specific gravity was I.O93, in which the proportion of acid to that 

 of water was nearly as 1 to 14. This solution contained a slight excess of acid, 

 turning vegetable blues into a brownish red; but it crystallized when cold, and 

 the crystals were of the form of alum; so that I believe this to be nearly the 

 proper proportion of its acid and earth; but there was not water enough to form 

 large crystals. As this solution contained an excess of acid, I added more earth 

 to it, but could not prevent its tinging blue paper red, till it formed an insoluble 

 salt, that is, one that required an exceeding large quantity of water to dissolve 

 it, and while part was thus become insoluble, yet another part would still retain 



