Dr. Ure on Pnissic Acid. 313 



(juanlily of above liquid Acid. Specific Gravity. Real Acid p. Cl. 



33.3 0.9914 6.3 



30.8 0.9923 5.0 



28.6 0.9930 4.5 



25.0 0.9940 4.0 



22.2 0.9945 3.6 



20.0 0.9952 3.2 



18.2 0.9958 3.0 



16.6 0.9964 2.7 



15.4 ,.. 0.9967 2.5 



14.3 0.9970 2.3 



13.3 0.9973 2.1 



12.5 0.9974 2.0 



11.8 0.9975 1.77 



10.5 0.9978 1.68 



10.0 0.9979 1.60 



From the preceding table, it is obvious that for acid of spe- 

 cific gravity 0.996 or 0.997, such as is usually prescribed in 

 Medicine, the density is a criterion of greater nicety than can 

 be conveniently used by the majority of practitioners. In fact, 

 the liquid at 0.996 contains about double the quantity of real 

 acid, which it does at 0.998. It is therefore desirable to have 

 another test of the strength of this powerful and dangerous me- 

 dicine, which shall be easier in use, and more delicate in its in- 

 dications. Such a test is afforded by the red oxide of mercury, 

 the common red precipitate of the shops. The prime equivalent 

 of prussic acid, is exactly one-eighth of that of the mercurial 

 peroxide. But as the prussiate of mercury consists of two 

 primes of acid to one of base, or is in its dry crystalline state, 

 a bicyanide, we have the relation of one to four in the formation 

 of that salt, when we act on the peroxide with cold prussic acid. 

 Hence we derive the following simple rule of analysis. To 100 

 grains, or any other convenientquantity of the acid, contained in a 

 small phial, add in succession, small quantities of the peroxide of 

 mercury in fine powder, till it ceases to be dissolved on agitation. 

 The weight of the red precipitate taken up being divided by 

 four, gives a quotient representing the quantity of real prus- 



