24-2 Intelligence and Miscellaneous Articles. 



entirely from a solution, even when very dilute, and notwithstanding 

 an excess of ammonia and the absence of ammoniacal salts, provided 

 a considerable time is allowed to elapse between that of the preci- 

 pitation and filtration. 



Lastly, alumina may be precipitated immediately and completely 

 by sulphuret of ammonium, even when the solution is very dilute 

 and no ammoniacal salts are present. 



The authors observe that nothing is more easy than to show the 

 great solubility of alumina in solution of ammonia, and the very 

 strongly marked precipitating power of sulphuret of ammonium. 

 Prepare two solutions, one of 2 grammes of ammoniacal alum in 

 50 grammes of water, and the other of 2 grammes of the same salt 

 in 200 grammes of water, and let the same volume of solution of 

 ammonia, say 15 cubic centimetres, be poured into both solutions, 

 it will be seen that the precipitate produced in the solution, the total 

 volume of which is 65 cubic centimetres, is incomparably greater 

 than that produced in the solution the volume of which is 215. It 

 is sufficient to filter the most bulky solution, to divide into two equal 

 portions, and to expose one to ebullition and the other to a current 

 of sulphuretted hydrogen, to produce in both cases a considerable 

 precipitate of alumina. 



If, instead of passing sulphuretted hydrogen into the alumino- 

 ammoniacal solution, a current of carbonic acid be sent through it, a 

 large precipitate of alumina will also be obtained ; but whereas, in 

 the first case, the whole of the alumina is precipitated, in the second 

 only a considerable portion is thrown down. In fact if the alumina 

 precipitated by the sulphuretted hydrogen be separated by the filter, 

 and the filtered liquor be even long boiled, not the slightest trace of 

 alumina will appear ; but if the solution which has been treated with 

 carbonic acid be boiled, a sensible quantity of flocculent alumina will 

 appear, when all the carbonate of ammonia has been volatilized. 

 The same is also the case if the alumino-ammoniacal liquor be treated 

 with sal-ammoniac. 



When 10 grammes of sal-ammoniac in powder are added to 250 

 cubic centimetres of alumino-ammoniacal liquor, prepared by the 

 precipitation of 2 grammes of ammoniacal alum, by 15 cubic centi- 

 metres of solution of ammonia, a considerable deposit of alumina is 

 formed : let this alumina be separated by filtration, and again add 10 

 grammes of sal-ammoniac to the filtered solution, and there will be 

 no fresh appearance of alumina; but if another 10 grammes of the 

 salt be added, more alumina, though small in quantity, is thrown 

 down. 



It is then evident that in- order to precipitate the last portions of 

 alumina immediately, they require a much larger quantity of sal-am- 

 moniac than the first, and the quantity is perhaps greater than che- 

 mists generally suppose. If also, instead of immediately filtering the 

 precipitate obtained by carbonic acid and sal-ammoniac, it be suf- 

 fered to remain for some days in close vessels, all the alumina will 

 be precipitated. — Ann. de Ch. et de Phys., Aout 184G. 



