Gypfum. 119 the cauftic volatile; and the filtered folution affords tartar vitriolate, or Glauber’s falt. ^ When pure, it does not effervefce with any acid, and is fcarcely foluble in any of them, ex- cept the concentrated vitriolic, which, when heated, and particularly when boiling, takes a large proportion of it*. Per Morveau, 18 Roz. 3°2. Artificial gypfurn, fo far dried as to retain lts glaffy appearance, contains 34 per ct. of mere calx, 48 of the ftrongeft vitriolic acid, and i 8 of water. Mr. Lavoifier found 9 parts natural gypfum to afford by diftillation 2 of water f, which agrees very nearly with my determination of that contained in the artificial: but fome forts of gypfum contain a mixture of mild calx, which is foreign to their conftitution, and therefore effervefce with acids. This fort of gypfum feems to have been formed by precipitation from fome other acids, and its proportion of ingredients is different from that found in the former, as I have elfewhere fhewn. Moft gypfums alfo contain about 2 per ct. of iron, and feveral, more; be- fides a fmall proportion of filex and argill. The eafieft method of decompofing gypfum is to boil it in a folution of mild alkali. In a clay crucible gypfum melts at about 130° Wedgewood, but it is infufible on coal or in chalk crucibles. Ey a blow-pipe it may be re- duced to a globular form, if the blue flame plays on the edges of the lamellte, becaufe the acid is * Yet Beaume fays that dilute vitriolic acid difFolves of its weight of gypfum, 1 Beaume, 273; and that dilute nitrous acid, and marine acid alfo. and vinegar diffolve it as water. f Mem. Scav. Etrang. I 4 then
