i84 On definite Proportions. 



green precipitate upon the addition of caustic ammonia 

 in great abundance. This is Thenard's supersj^lt of sul- 

 phuric acid and black protoxide of iron. If, on the con- 

 trary,' we add the ammonia by little and little, the salt falls 

 tlown at first yellow, and alterwards green. This uncry- 

 Slallizable salt is again a triple combination of the two 

 salts of iron, and it is to be presumed that it contains oive 

 of its constituent parts in twice or four times as great a 

 proportion as the emerald green salt. If we boil this salt 

 with nitric acid, it is converted into a salt of the oxide, 

 •which, after the expulsion of the nitric acid, leaves a por- 

 tion of the subsalt undissolved. The solution of the neu- 

 tral sulphate of the protoxide is orange, but becqjues light 

 yellow when it is diluted with water and decomposed by 

 an acid, as the vellovv salts of iron lose a great part of their 

 colour by the addition of an excess of acid ; but I do not 

 consider this circumstance as a proof of the existence of 

 an acid sulphate of the oxide of iron, as Thenard maintains. 

 Without doubt M. Thenard holds a distinguished rank 

 among the chemists of our time, and his results may be 

 coiisidered as possessing considerable authority. So much 

 the more unfortunate is it when such a person ventures tQ 

 ground on such experiments, as have certainly been made, 

 and considered as unsatist'actorv, by oiher chen)ists, a sy- 

 stematic essav, which discourages others from investigatinp: 

 the subject, as they entertain no doubt ot the accuracy 

 of the facts that are advanced in it. 



Since I have not hitherto been able to discover anv su- 

 persalt of the sulphuric acid and the protoxide of iron, 

 whatever probability there may be of the existence pf such 

 a compouiHl, thpre is hitherto no known saline compound 

 corresponding to the natural pyrites. May not the non- 

 existence of such a salt be the reason that this pyrites re- 

 mains so little altered, notwithstanding; its ejjposure to 

 moisture in our mines? So also its insolubility in dilute 

 sulphuric and muriatic acid depends on the inability of the 

 hydrogen of the water to coml)ine with the sulphur in 

 more than one proportion ; while the pyrites contains pre- 

 cisely a double portion, as we shall sec hereafter. 

 [To be continued,! 



XLVI. ne^ 



