pJI THE OXIDES OF IRON. |85 



two degrees of oxidation ; one at a minimura, the other I*rOu»t, that 



-. rri, II r J.-1 r ^ • there i« no 



at a maximum*. ThenarcL known lor the accuracy of ois y^j^^ ^^^^ j^ 



experiments, asserts, that there are three sorts of oxides of these iwo pro- 



iron ; the white, the green, and the redf. Z't'l^^At 



Acoording to Berthollet, " the oxidation of metals, and proportions 



the properties of the oxides they form, depend on the f ^/^^^""®» 

 r r •' » but may be 



Strength of their affinity for oxigen, their power of cohe- more than 

 «ion, their fusibility, their volatility, the degree of oxi-^^®* 

 dation they are capable of attaining in consequence Qf Jcn]i«jeti 

 these qualities, the condensation the oxigen undergoes in 

 them, and the quantity of caloric it contains." 



Now as the affinity of oxigen for metals, as well as their 

 cohesion, fusibility, and volatility, must vary in proportion 

 to the quantities of this substance already combined with 

 theui, it follows, that ** the coynhi7iation of oxigen (in metals) 

 7nay vary in them, and even indejinitely , from the point when, 

 tlie force of cohesion losing its })reponderance, oxidation 

 becomes poflible, to that point where it ceases to be so, 

 unless the mutual affinity of the two elements be assisted by 

 some other affinity, which extends the limit of oxidation 

 farther." 



Proust deduces his two degrees of oxidation from the Argument? ^f 

 colour of the precipitates obtained in solutions of iron on 

 the addition of prussiates, alkalis, gallic acid, and hydro- 

 sulphurets. 



Thoroughly saturated prussiate of potash precipitate! 

 from its solution iron at a nhinimum in the state of white 

 precipitate, which afterward becomes green ; and it preci- 

 pitates iron at a maximum in the state of blue prussiate. 



Alkalis precipitate oxidulated iron from its solutions in a 

 green oxide, which gradually turns black : iron oxided at 

 a maximum they precipitate red : and when a solution con- 

 tains iron in both these states, the red is precipitated first, 

 and the black afterward. 



Gallic acid produces no change in solutions of oxide oF 

 iron at a minimum; that which is at a maximum it preci- 

 pitates black. 



♦ Ann. de Chim. voK XXIll, p. 85, 



jl* Ann. de Chim. yoI. LVI, p. 57: or Journal, toI. XIV, p. SS4, 



SuIphuretteiL 



