CATARACTS AND INUNDATIONS. 



quantity of oxyde of mercury, which is much greater in that 

 %vhich is precipitated by the water, than in that which is not de- 

 composable by the fluid. M. Fourcroy has proved this, by evapo- 

 rating equal quantities of both these solutions in an apothecary's 

 phial, to reduce them into red precipitate, and he obtained one 

 fourth more of this precipitate from the solution which is decom 

 posed by water, than from that which is not rendered turbid. 

 The specific gravity likewise appeared to be a good method of 

 ascertaining the relative quantities of oxyd of mercury contained in 

 these different fluids. He compared weights of equal masses of 

 three mercurial nitrous solutions : the one, which was not at all 

 precipitated by distilled water, and was the result of the first men- 

 tioned experiment, weighed one ounce one drachm and sixty- 

 seven grains, in a bottle which contained exactly an ounce of dis- 

 tilled water. The second solution was made by a very gentle heat, 

 and produced a slight opal colour with distilled water, and scarcely 

 an^ sensible quantity of precipitate. The same bottle contained one 

 ounce six drachm* twenty four grains. Lastly a third mercurial 

 solution considerably heated, and which precipitated a true turbith 

 mineral or' a dirty yellow, by distilled water, weighed in the same 

 bottle, one ounce seven drachms twenty five grains. A decisive ex- 

 periment remained to be made to confirm this opinion still more 

 perfectly. If the solution precipitated by water, owed this property 

 to a quantity of mercurial oxyd too large with respect to the acid, 

 it would of course lose that property by the addition of acid ; this 

 accordingly happened. Aquafortis was poured on a solution \\hich 

 was decomposed by water, and it soon acquired the property of no 

 longer being precipitated, and was absolutely in the same state as 

 that which had been made slowly at first, by the mere heat of the 

 atmosphere. Monnet has mentioned this process, as a means of 

 preventing crystals of mercurial mtrat from becoming converted 

 into oxyd by the contact of the air. It is by a contrary process, 

 and by evaporating a portion of the acid of a good solution, which 

 is not precipitated by water, that it is converted into a solution 

 much more strongly charged with mercurial oxyd, and consequently 

 capable of being decomposed by water; its original property 

 may be restored by the addition of a quantity of acid, equal to 

 that which it lost by evaporation. 



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