the Particles of Alumina. 359 



iron, even when kept moist, have the power of cohering so as 

 to prevent chemical action. 



2. That carbonate of alumina may be formed, though de- 

 composable at a comparatively low temperature. 



3. That owing to the presence of carbonic acid or magnesia, 

 the particles of alumina are prevented from cohering. 



It will also, I think, be admitted, that the cohering property 

 which I have now described may have led to errors in analysis, 

 in which precipitated alumina may have been kept without any 

 such intervening substances as to render it soluble. I may 

 remark, in conclusion, that the alumina insoluble in dilute 

 acids was not taken up by soda, at any rate when used cold. 



Although not connected with the present subject, I will 

 take the opportunity of mentioning an unexpected case which 

 lately occurred to me. In examining a soil, I treated one 

 portion of it simply with muriatic acid and another withnitro- 

 muriatic acid. On the addition of ammonia to both solutions, 

 I found the muriatic solution yield a bulky precipitate, evi- 

 dently containing, if not consisting of protoxide of iron, 

 whereas the nitro-muriatic solution gave a comparatively small 

 quantity of precipitate evidently of sesquioxide of iron. 



On investigation, I found that the protoxide of iron had 

 carried down with it a considerable quantity of magnesia ; 

 whether in a state of mixture or combination remains to be 

 proved. 



To try the experiment on a considerable scale, I mixed an 

 equivalent of sulphate of magnesia with half an equivalent of 

 protosulphate of iron, adding an equivalent of sal-ammoniac 

 to prevent the partial precipitation of the magnesia by the 

 ammonia, which I added in excess to precipitate all that it was 

 capable of throwing down. Having carefully washed the preci- 

 pitate, I redissolved it in nitro-muriatic acid, so as to peroxidize 

 the iron, and I again added sal-ammoniac and ammonia. By 

 this peroxide of iron was thrown down ; and on adding phos- 

 phate of soda to the filtered solution, I obtained so abundant a 

 precipitate of the ammonio-magnesian phosphate, as to render 

 it probable that at least one-fourth of the magnesia had been 

 precipitated with the protoxide of iron. 



I may also notice another circumstance connected with the 

 precipitation of the magnesia with the protoxide of iron. It 

 is well known that excess of ammonia dissolves a portion of 

 protoxide of iron, which by oxidizemcnt of the air is precipi- 

 tated. This solution however does not happen in the case 

 above alluded to, the whole of the protoxide of iron being at 

 once precipitated with the magnesia. 



