80 Detection of Adulterations. 



ceived the chlorine, to the liquid still remaining in the retort, and 

 put into the mixture a known weight of marble. The quantity of 

 marble dissolved will indicate, as before, the quantity of free acid. 

 If this be taken from the quantity of real acid poured upon the ox- 

 ide, we have the quantity absorbed by the oxide and its impurities ; 

 and if from this difference we subtract the quantity of acid decom- 

 posed by the peroxide alone, (a quantity known by the ascertained 

 purity of the specimen) we shall know how much has combined with 

 the impurities of the mineral. The artist will consider this last quan- 

 tity as so much acid lost, and will add it to the price of the oxide. 



This process is long and difficult, and it is much to be desired that 

 some simpler process could be employed to determine the quality of 

 a substance so extensively used. On enumerating the substances to 

 be found in the native oxide, it will be observed that none of them 

 yield oxygen when treated with sulphuric acid, and therefore that 

 the whole of the oxygen afforded by the common process, proceeds 

 from the manganese. Hence, the quantity of oxygen obtained by 

 the action of sulphuric acid upon any specimen, should be in direct 

 proportion to the quantity of pure peroxide which it contains. As 

 44 grains of pure peroxide afford, when treated with sulphuric acid, 

 almost precisely 23.6 cubic inches of oxygen gas (therm. 60° and 

 bar. 30 inches) it will be easy to ascertain the purity of any natu- 

 ral oxide, and consequently the quantity of chlorine it will yield when 

 treated with hydrochloric acid. Such are the conclusions which, from 

 our theoretical views, we think it fair to draw ; and we do not know 

 any principle in opposition to them. It is true that carbonic acid gas 

 will frequently accompany the oxygen, but it will be absorbed by the 

 water of the pneumatic trough, to which, if requisite, lime might be 

 added. At some future opportunity we shall perform and publish 

 some experiments on the subject ; giving, at the same time, the mode 

 of ascertaining the quantity of acid neutralized by the impurities 

 commonly mixed with the mineral. In conclusion, we would observe 

 that chemists and mineralogists do not specify whether the iron in 

 manganese ores exists as a peroxide, or as a protoxide. In order to 

 test the practicability of the plan above suggested, it will be necessa- 

 ry to determine this point, and to know whether the oxide of iron 

 takes any oxygen from that of manganese, when an acid is poured 

 upon a mixture of the two. 



Peroxide of Mercury; Red Precipitate. — This substance is adul- 

 terated with various red powders, among which are the deutoxide 



