128 



Mr. J. G. Grrenfell on 



[June 15^ 



in any part of the room. The normal salt and the anhydrous salt are 

 without action on the solution. It crystallizes in a test-tube in fine 

 stellate masses, with projecting points on all sides, as alum sometimes 

 does ; these ultimately coalesce. These crystals are composed of very fine 

 parallel fibres like ferns, and are opaque white. It sometimes sets in 

 long fibres, radiating from different points like aluminium sulphate. 

 Owing to the fineness of the fibres it would be very difiicult to free them 

 from the mother-liquor. 



My reason for believing them to be a hydrate is this : — In a beaker 

 this solution gradually deposits clear crystals, varying from very fine 

 needles to rhombic plates, prisms, and short, nearly globular, highly 

 modified forms. These are formed near the top, and may perhaps be 

 different hydrates. They are formed, however, at the same time, and at 

 present I believe them all to be the hyperacid salt. Similar ones are 

 formed by putting the normal salt in the 6 to 1 solution, and this 

 remains liquid, sometimes dissolving the crystals. An opaque amorphous 

 mass is formed at the same time, which appears to be hydrated, but it 

 also is inactive. A mixture of two parts of acid to one of salt in a 

 flask, when boiled to get rid of all water, sets firmly in a clear mass, in 

 which the opaque variety makes no change. Then if a little water is 

 added the salt turns opaque white wherever the water reaches ; and this 

 is entirely absorbed, the cake remaining quite dry. 



If this is again melted it deposits clear prisms, leaving a little mother- 

 liquor ; but the opaque variety when introduced from the 6 to 1 solution 

 causes the whole mass to set firmly opaque white and become quite 

 dry. The opacity spreads slowly, and a kind of beard of fine crystals 

 can sometimes be seen growing round the prisms at the edge. Lovely 

 foliated films are often formed at the same time. The clear crystals are 

 inactive in the 6 to 1 solution, while the opaque is active ; and this is a 

 clear proof of their identity. Solutions of intermediate strength between 

 2 to 1 and 6 to 1 often deposit in flasks the whole excess in clear 

 crystals, which are sometimes inactive in the 6 to 1 in a test-tube. It 

 is almost impossible to obtain these solutions supersaturated in flasks, 

 though it may be done with the utmost facility in test-tubes. Out of 

 many trials with one flask I only succeeded once by leaving it to cool on 

 the sand-bath. In a test-tube they give the same forms as the 6 to 1. 

 The variety of the forms in which these solutions crystallize is truly 

 astonishing, according to the proportion of acid and salt, amount of 

 water, and the temperature. A flask once gave the most exquisite little, 

 flat, open flakes closely resembling snow-flakes ; but I have not been 

 able to reproduce them. In short the relations of these two substances 

 to each other want working out thoroughly. A certain amount of acid 

 added to the salt which is in excess gives a thin liquid, which will not 

 crystallize, and a little fine white powder, the anhydrous salt. Two 

 drops of acid in a test-tube half-full of solution cause drops to evaporate 



