82 Dr. Thormon on Arsenic. [Feb. 



in water, we have not the means of obtaining them in regular 

 crystals, the only method in our power of preventing mechanical 

 mixtures from being substituted for chemical compounds. 

 There are, however, two arseniates which can be procured in 

 regular crystals with great ease ; namely, arseniate of potash and 

 arseniate of soda. The first of these has been long known to 

 chemists by the name of Macquer's arsenical salt. But I have 

 not hitherto met with an accurate account of the arseniate of 

 soda in chemical books. I shall, therefore, take the opportunity 

 of describing it here. 



I. Arseniate of Soda. 



To form this salt, I took a considerable quantity of arsenic 

 acid, which I had made by boiling nitromuriatic acid on arse- 

 nious acid till the whole was dissolved, and then distilling off the 

 nitromuriatic acid. Into this acid I dropped solution of carbon- 

 ate of soda till all effervescence was at an end, and till the liquid 

 ceased to redden litmus paper. The mixture was now evapor- 

 ated to the requisite consistency on a sand-bath, and set aside 

 for crystallization. But though the weather was peculiarly 

 favourable, not a single crystal could be obtained. The liquid 

 rendered cudbear paper violet, and gave other unequivalent 

 symptoms of containing an excess of alkali. Arsenic acid was, 

 therefore, dropped into it till all effervescence ceased. The 

 liquid, which now reddened litmus paper, being set aside, 

 yielded abundance of crystals of arseniate of soda, and by 

 repeated evaporations, I succeeded in extracting the whole of 

 the salt in tolerably regular crystals to the very last drop. All 

 the crystals thus obtained possessed the very same properties ; 

 therefore, though the liquid reddened litmus paper, it contained 

 no uncombined arsenic acid. 



By dissolving the arseniate of soda thus formed in hot water, 

 and setting it aside in a cool place, I obtained it in large regular 

 transparent crystals, consisting of rhomboidal prisms, the faces of 

 which were inchned to each other at angles of 64° and 116°. 

 The bases of these prisms were rhombs, which, as far as I could 

 make out, had likewise angles of 64° and 116°. 



These crystals remained unchanged though exposed to the 

 open air for a week in my laboratoiy ; but when I carried them 

 into my library, they speedily effloresced on the surface, and 

 became white and opaque ; but did not lose their form or 

 fall to powder after a month's exposure. 



The taste is cooling, and bears some resemblance to carbonate 

 of soda, but is not so strong. It is very singular that though 

 the liquid from which these crystals were deposited reddened 

 litmus paper, the ciystals themselves render cudbear paper 

 fetronglypwrp/e, and even perceptibly affect Htnius paper rendered 

 ted by acetic acid. Now these effects are usually produced by 

 tlkaline bodies. "^ 



