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LV. On the Spojitaneoug Cohesion of the Particles of 

 Alumina. By Richard Phillips, F.U.S.* 



IT has been observed by M. Wittstein (Chemical Gazette, 

 April 184*7), that the precipitate which is obtained from 

 the persulphate or perchloride of iron, if kept for a great length 

 of time in water, loses almost entirely the property of being 

 soluble in acetic acid. This statement recalled an observation 

 which I made several years since with respect to the similar 

 cohering power of alumina ; the two cases differing, however, 

 in these respects, that whereas the sesquioxide of iron requires 

 one or probably two years for the production of the effect, 

 alumina undergoes the change partially in a very short time : 

 the precipitated alumina does not, however, assume a crystal- 

 line appearance, as stated by M. Wittstein to be the case with 

 the cohering sesquioxide of iron. 



It is well known, both with respect to recently precipitated 

 sesquioxide of iron and alumina, that they are soluble even in 

 acetic acid, and consequently in the more powerful acids ; but 

 if the precipitated alumina be kept for two days even moist, 

 and in contact with the solution from which it was precipitated, 

 I find that sulphuric acid does not immediately dissolve it. 



The following experiment will perhaps elucidate the fact 

 which I have now stated : I precipitated the alumina from a 

 large quantity of alum by excess of ammonia, and after keep- 

 ing it moist for fourteen days, I washed it to get rid of the 

 excess of ammonia and the salts existing in solution ; to a 

 quantity of this precipitated alumina I added dilute sulphuric 

 acid in excess, and almost immediately separated by the filter 

 the insoluble from the dissolved portion ; the latter, being 

 washed and ignited, weighed twenty and a half grains. The 

 sulphuric solution of the soluble portion was decomposed by 

 ammonia, and weighed, after washing and igniting, eight 

 grains ; so that in about a fortnight, nearly 72 per cent, of the 

 particles of the alumina had so cohered as to resist the im- 

 mediate chemical action of the sulphuric acid. I did not try 

 whether solution would have been effected by long cold digestion 

 in this case, but I have observed that time greatly increases the 

 solvent power of the acid ; I found, however, in the present 

 instance, that the alumina which cold dilute acid would not im- 

 mediately dissolve was entirely soluble in it when boiling ; but 

 if my recollection serves, I have found that, by long keeping, 

 the alumina has become insoluble even in boiling sulphuric 

 acid. 



When, however, a solution of alum is decomposed by the 



* Read before the British Association held at Swansea, August 9, 1848, 

 and communicated by the Author. 



