ON THE OXIDES OF IRON'. 579 



colour. When the red oxide is mixed with oil, and heated bntxvith the 

 to redness, it becomes blacky and is attracted by the mag- btcail-e'mr-'^ 

 net: but its weight is not altered. Indeed, if we repeat netic ; thuugh 

 the experiment a great number of times with the same por- ^teraUon "'^ 

 tion of oxide, the weight rather increases. When red weighr. 

 oxide is heated with charcoal, it is reduced to the metallic J^ij^^^^'"^^"^^' 

 state. 



When iron is dissolved in sulphuric acid, the solution Iron dissolved 

 evaporated to dryness and exposed to a strong heat, the '".■'"'rhu"c 

 sulphuric acid is dissipated, and red oxide of iron obtained. 

 But experiments made in this way do not lead to a satisfac- Results unsatis- 

 tory result. 100 parts of iron thus treated were converted ^e^eJ'^^^o^l'^J ^^'* 

 into 150 parts of red oxide. But it was not quite pure, still acid. 

 containing traces of sulphuric acid. This was the case 

 even when the oxide had been exposed to a heat sufficient to 

 calcine carbonate of lime. The results were not more 

 satisfactory, when the iron was precipitated from sulphuric 

 acid by an alkali. The oxide obtained, though carefully 

 edulcorated, still contained sulphuric acid. For when 

 dissolved in muriatic acid, and mixed with muriate of 

 barytes, a white insoluble precipitate fell. 



II. To ascertain the proportion of oxigen in the black Diflicuk to as- 

 oxide of iron is a more difficult task. I shall relate the ex. p^,!^",'!;^ ^'™' 

 periraents which I made in order todetenrjine the point. black oxide. 



1. When 100 grains of iron are dissolved in diluted sul- Iron dissolved 

 phuric acid, the hidrogen gas produced amounts to 163-4 J[j^J/^l"'^^'.j'" 

 cubic inches, at the temperature of 60^, and when the 

 barometer stands at 30 inches. Two experiments were 

 made, each of which gave exactly the same result. Now it 

 is well known, that when iron is dissolved in this manner, it 

 is converted into black oxide. Water is decomposed, the 

 hidrogen of which escapes- in the form of gas, while the 

 oxigen unites with the iron. It has been established, that 

 the constituents of water, reduced to the gaseous state, 

 consist of 2 parts by bulk of hidrogen and I part of oxigen. 

 Ilcnce in this case the oxigen, which combined with the 

 100 grains of iron, and converted it into black oxide, is 

 eqii-ivalent to 81-7 cubic inches. Now 81-7 cubic inches of 

 oxigen gas weigh, according to thecxperiracnts of Lavoisier 

 and Davy, 27-93 grains j according to those of Allen and 

 3 Pejiys 



