166 Dr. Thomson on the [March, 



iind digested on it till it ceased to take up any thing. The 

 aqueous sohitiou thus obtained being evaporated to dryness, the 

 sulphate of potash remaining weighed 18*09 grains. On dis- 

 solving the sulphate of potash in water, and pouring ammonia 

 into the solution, a slight opalescence was perceptible, indicating 

 that the salt was not absolutely free from sulphate of alumina. 

 It was not possible to collect, far less to weigh, the extremely 

 minute portion of alumina thus disengaged. But by dissolving 

 small quantities of sulphate of alumina in water, and throwing 

 down ihe alumina by means of ammonia, I was enabled to con- 

 clude that the sulphate of alumina mixed with the sulphate of 

 potash from the 100 grains of ulum amounted very nearly to 0*02 

 grain. Deducting this from the 18*09 actually found, there 

 remain 18-07 grains for the sulphate of potash really contained 

 in 100 grains of alum crystals. Now 100 : 60*875 :: 18*07 : 

 11*0001125. This last number differs so little from 11 that 

 there can be no hesitation in adopting 11 as the true quantity. 

 Had I made my experiments on b0*875 grains of alum instead 

 of 100, and obtained so near a coincidence, I would not have 

 regarded myself as at liberty to consider the weight obtained 

 as differing from 11 grains ; because the sources of error are too 

 numerous to make it at all hkely that the sixth decimal figure 

 can be depended on. 



Eleven grains then is the quantity of sulphate of potash con- 

 tained in 60*875 grains of alum. Now 1 1 sulphate of potash are 

 composed of 5 sulphuric acid -\- 6 potash. Thus exactly one- 

 fourth part of the sulphuric acid in alum is united to the potash. 

 And the weight of potash contained in 60*875 parts of alum is 

 6, or an atom of potash. 



4. Ahiniina. — From the preceding experiments, it is evident 

 that three-fourths of the sulphuric acid in alum are united to the 

 alumina. This in 100 grains of alum amounts to 24*64 grains. 

 I calculated the weight of carbonate of potash, carbonate of soda, 

 and carbonate of ammonia, just sufficient to saturate 24*64 grs. 



'of sulphuric acid. Each of these quantities was added to 100 

 grains of alum previously dissolved in distilled water, and the 



.whole was well agitated till all action was at an end. By this 

 addition, the alumina was completely precipitated from the solu- 

 tion, while no excess of any -of the alkalies could be detected 

 after the precipitation in any of the residual liquids, except of 

 the ammonia, which I had added slightly in excess ; from the 

 carbonate containing rather more ammonia than I had supposed 

 it to do. To obtain the whole alumina from each of these 

 liquids, the method which I employed was this : I took three 

 pairs of double filters, each filter in every pair being exactly 

 of the same weight. The two filters constituting each pair were 

 placed the one within the other, and put into glass funnels in 

 the usual way. Into the first pair I poured the liquid contain- 

 ing the alumma separated by means of the carbonate of potash. 



