358 Mr. R. Phillips on the Spontaneous Cohesion of 



carbonate of soda or the sesquicarbonate of ammonia, the 

 result as to the solubility of the precipitated alumina is totally 

 different; the precipitate in the latter case is more dense, and 

 instead of being semitransparent and gelatinous, it is opake 

 and pulverulent, these differences evidently denoting some 

 variation of constitution. Accordingly, I find that whatever 

 may have been the length of time which I have kept this pre- 

 cipitate moist, I have always found it totally and immediately 

 soluble to cold dilute sulphuric acid. 



If it were not almost universally admitted that carbonic acid 

 does not combine with alumina, the most obvious conclusion 

 to arrive at would be that carbonate of alumina is actually 

 formed, and that the carbonic acid preventing the cohesion 

 of the particles of alumina, it remained soluble in acid. 



I shall now, however, state some experiments which seem 

 to me to prove that carbonate of alumina may be formed. I 

 precipitated 200 grains of alum by excess of sesquicarbonate 

 of ammonia, and washed the precipitate long after it ceased to 

 render the water alkaline. After the alumina had been pre- 

 cipitated about fourteen days, I dissolved it in a counterpoised 

 vessel in dilute sulphuric acid ; solution took place with effer- 

 vescence, and twenty grains of carbonic acid gas were evolved. 

 It follows therefore, I think, that carbonate of alumina was 

 formed, and that the carbonic acid interposed to prevent the 

 cohesion of the alumina. 



A very moderate heat seems, however, sufficient to decom- 

 pose carbonate of alumina. A quantity of precipitate, pre- 

 pared as above stated by sesquicarbonate of ammonia, was 

 dried by the heat of steam ; a portion of this was added to 

 dilute sulphuric acid, and although effervescence occurred and 

 some alumina was dissolved, the greater portion had lost its 

 carbonic acid and become insoluble. 



I have further found, that when hot solutions of alum and 

 carbonate of soda are mixed, the precipitate formed is not 

 totally and immediately soluble in dilute sulphuric acid. 



That other substances have the power of intervening to pre- 

 vent the cohesion of precipitated alumina is shown by the fol- 

 lowing experiment : — To a solution of alum I added one of 

 sulphate of magnesia, and by ammonia precipitated a mixture 

 of the two earths ; and on keeping the mixed precipitate for 

 such a length of time as would have rendered only one-fifth of 

 alumina precipitated per se soluble, I found that the mixed 

 earths were totally and almost immediately dissolved by cold 

 dilute .sulphuric acid. 



The experiments which I have now detailed prove, I think, 

 the following facts : — 



1. That the particles of alumina, like those of peroxide of 



