Sulphalit of Iwi variaujly ettididt 453 



V. 



AhJiraB of a Memoir entitled " Enquiries concerning the Nature of PriiJJian Blue'* 

 By Mr. Proust *. 



I 



F iron were fufceptible, as chemifts imagine, of uniting with cxygene in all the inter- 

 mediate proportions between -^U^ and -r*^"^, which appear to be the two extreme terms 

 of its union with that principle, ought it not to afford, with any given acid, as many dif- 

 ferent combinations as it is fuppofed capable of producing oxides ? Why, for example, 

 does not this metal, which affords, with the fulphuric acid, a fait conflant in its properties 

 when oxided no farther than -5%'^, exhibit different combinations, equally conftant in their 

 Tefpe£live properties, when it contains -ji^^, _?^»_, or -^^-^ of oxygene ? 



A great number of fa£i;s prove, on the contrary, that iron does not reft indifferently at 

 all the different degrees of oxidation between the two terms abovementloned ; and, not- 

 withflanding the different degrees of oxygenation through which iron is fuppofed to pafs 

 when its fulphate is expofed to the air, we are acquainted with no more than two fulphates 

 of this metal. 



The flrfl: is the green, or cryilallizable fulphate, in which LavoilTer has proved that the 

 iron contains fW- of oxygene. This fait, when pure, is infoluble in fpirit of wine ; its for 

 lution in water is of a very pale fea green colour. It is not altered by the gallic acid, af- 

 fords no blue with alkaline prufliates, &c. 



The fecond kind of fulphate, no lefs conftant in its properties, is that red deliqucfcent 

 Incryftallizable combination which is foluble in alkohol, and is known by the name of 

 mother water of vitriol ; but is not really fuch, unlefs when it produces no alteration in 

 the oxygenated muriatic acid ; that is to fay, when its oxide contains -r-sl of oxygene. 



This fulphate is eafily obtained by treating it with nitric acid until its folution no longer 

 difengages nitrous gas, by the addition of a new quantity of acid. 



This fulphate, befideSf poffeffes exclufively the property of forming a black precipitate 

 with the gallic acid, and of affording PrufTian blue with the alkaline pruffiates, as will 

 hereafter appear. 



Between thefe two fulphates, which Mr. Prouft calls the green fulphate and the red ful- 

 phate, there is no intermediate point. If the green fulphates, by expofure to the contaft 

 of air, allume a colour which appears to belong to neither of thefe fpecies, it may be de- 

 cidedly fliown that they are limply a mixture of the two, by feparation by means of al- 

 kohol. Each fait will then exhibit all its diftin£tive properties. The green fulphate 

 will conft.mtly produce a green precipitate with the cauftic alkalis; which precipitate 

 will foon become black if preferved under water without the contaiSt of the air, bccaufe 

 its panicles continually approach each other, and render the colour more and more in- 

 tcnfe. 



The red fulphate, on the contrary, will afford, with the fame alknlis, a yellow red pre- 



■ This jbftraiCl is tranlLited from tJic Aunalts il'J Cliijliic XJiHI. S5. T fic editors do not Ciy wlicrc tlic 

 original is to be found, 



clpitate^ 



