§78 OXIDATIONS OF XROtf. 



The green cr>- to the air resume their white colour after some time, fttfrtfjf 

 exposure*to 3 ' ^ e not s * !a1t;en "> kut agitation greatly accelerates this change, 1 

 the air. which is another similarity between these precipitates and 



those produced from the red salts of iron by sulphuretted 

 hidrogen. 

 These green These green and earthy precipitates are not hidrosulphu- 



precipi.ates are retSj as ^g^t be supposed; but hidrurets, which probably 

 retain a little acid. This is proved by their being decom- 

 posed by oxigenized muriatic acid, without leaving any 

 traces of sulphur; and by retaining their green colour, and 

 other properties annexed to it, when redissolved in acids; 

 which could not be the case if they were hidrosulphurets, 

 since acids decompose these instantly. 

 The green ox- From these elucidations the cause of the green colour of 

 ides of iron are ox ^ e f \ von an< j its colouration by muriatic acid, suggested 

 the same. ,■"■•.■ • , BO 



itself as it were spontaneously. The green oxide is never 



formed, unless hidrogen be set free: a part of this therefore 

 remains engaged in the oxide, and imparts to it the green co- 

 lour, with the property of being less soluble in water, or more 

 crystallizable. This property of rendering a salt more ciys- 

 tallizable, possessed by a principle of so little density, ap- 

 pears at fiifst sight inconsistent ; but it is confirmed by the 

 superoxigenized muriate of potash, which is rendered two or 

 three times less soluble than the simple muriate by the addi- 

 tion of oxigen. 

 Oxigenized Oxigenized muriatic acid then acts on a green salt of iron, 



t'ak'^awat 01 ' 1 as ll does on sulphuretted hidrogen, phosphorus, &c. It 

 their hidrogen. deprives the oxide of the hidrogen with which it is combined, 

 as it does the sulphur and the phosphorus: which at the 

 same time proves, that in the oxide of iron it is not in the 

 same state of dilatation as that in which we are acquainted 

 with it uncorabined, for in this state it does not combine 

 with oxigenized muriatic acid at the temperature of the at- 

 mosphere. 

 The presence The hidrogen likewise betrays itself in the offensive smell 



of hidrcgen in gjy en ou t fc.y a concentrated solution of iron, when a fixed 



the, green ox- ° - 



Structions respecting it. I have left sulphuretted hidrogen in content 

 with sulphate of magnesia for an hour before it precipitated. Sometimes 

 the precipitate is green in the very act of falling down; at other times if 

 does not become green till some moments after. 



alkali 



