$32 OXIDATIONS OF IRON. 



which amounts to the same thing, that the thickness of thi 

 columns of fluid separating the particles is -^ of a line in 

 the concentrated solution, aud a line in that diluted with 

 water; what will happen, if a drop of alkali fall on any 

 point of the concentrated solution ? The alkali will deter- 

 mine the precipitation of a certain number of particles of 

 oxide, which will be at first r '^ of a line from each other, 

 as when they were combined with the acid : but their spe- 

 cific gravity, assisted by the pressure of the atmosphere 

 and of the solution, will be capable of overcoming the re- 

 sistance opposed to their approximation by the little column 

 of liquid that separates them. This is the reason why the 

 precipitate is blackish, retains its magnetism, and at the 

 expiration of a few minutes is insoluble in cold muriatic 

 acid. 



In the solution greatly diluted with water, though the 

 alkali determines the precipitation of an equal number of 

 particles, and though their specific gravity and pressure act 

 in the same manner, as the resistance opposed to them by 

 the columns of fluid is ten times as great, their approxima- 

 tion cannot be so complete. Hence the difference of co« 

 lour, absence of magnetism, and facility of solution in 

 acids. 

 The case 53 the Besides, when extremely fine filings of iron are calcined, 

 ides^Tcalcina- anc * divided by trituration in the course of the operation, 

 tion, till they have taken up 15 per cent of oxigen, we obtain a 



Tery fine red powder, much less magnetic than oxides with 

 30 or 40 per cent, obtained by the common process ; that 

 is to say, with common iron filings not triturated during 

 the course of the process. 

 and red oxid«« Finally, the red precipitates of solutions of iron, and 



vhen rendered most f ^hc aperitive saffrons, after they have been well 



snagneUc by 



hes*. dried, exhibit no signs of magnetism. But if they be ex- 



posed to a strong fire for some time, their bulk diminishes, 

 their colour is heightened, and they are decidedly magnetic. 

 Kow we cannot say here, that the magnetism is owing to a 

 Joss of oxigen, since the experiments of Proust, and more 

 recently those of Berthollet, have proved that these oxides, 

 exposed to the strongest heat of our furnaces, do not give 

 * out an atom of oxigen. To the same approximation is to 



bf 



