of Processes in the Fine and Useful Arts. 359 



arseniate of potash, prepared with eight parts of oxide of arsenic, instead 

 of six, was used, and the result was very successful. M. Braconnot thinks 

 that probably a slight variation of the proportions he has given may be 

 found advantageous ; but, in the meantime, considers it right to give the 

 best process he is able for the preparation of a colour so beautiful, and 

 which may be very useful in the arts. 



6. Account of the Process for making Ultramarine. 



M. Gruinet of Thoulouse has succeeded in forming this valuable pig- 

 ment ; and our able correspondent. Professor Gmelin of Tubingen, has also 

 discovered a process for making it, which is given in the Ann, de Chim- 

 for April. 



Prepare hydrate of silica and hydrate of alumina; the former is obtained by 

 fusing well powdered quartz with four times its weight of carbonate of potash, 

 dissolving the fused mass in water, and precipitating by muriatic acid 

 Hydrate of alumina is procured by precipitating a solution of alum with 

 ammonia. These two earths are to be carefully washed with distilled wa- 

 ter. After this, the quantity of dry earth remaining is to be ascertained, 

 by heating to redness a certain quantity of the moist precipitates. The 

 hydrate of silica which I employed in my experiments, contained in 100 

 parts 56, and the hydrate of alumina 3.21 parts of anhydrous earth. 



Dissolve afterwards, with the assistance of heat, as much of this hydrate 

 of silica as a solution of caustic soda is capable of taking up, and deter- 

 mine the quantity dissolved. Take then for 72 parts of the latter (anhy- 

 drous silica) a quantity of hydrate of alumina, which contains 70 of anhy- 

 drous alumina : it is to be added to the solution of silica, and the mixture is 

 to be evaporated, with constant stirring, until a moist powder only remains. 



This combination of silica, alumina, and soda, is the base of the ultra- 

 marine, which is to be coloured by sulphuret of sodium, and this is eflPect- 

 ed in the following manner: — Put into a Hessian crucible, provided with 

 a good cover, a mixture of two parts of sulphur, and one part of anhydrous 

 carbonate of soda ; it is to be gradually heated, until, at a moderate red 

 heat, the naass is well fused. This mixture is then to be projected, in very 

 small quantities at a time, into the middle of the fused mass. As soon as the 

 effervescence occasioned by the vapour of water ceases, a fresh portion is to 

 be thrown in. Having kept the crucible moderately red-hot for an hour, it 

 is to be taken from the fire and permitted to cool. It now contains ultra- 

 marine, mixed with sulphuret in excess, which is to be separated by water. 

 If there be sulphur in excess, it is to be expelled by a moderate heat. If 

 the whole of the ultramarine be not equally coloured, the finer parts may 

 be separated, after having reduced them to a very fine powder, by washing 

 with water. 



7. Inventions for Sharpening Blades of Knives, 



In 1827, Mr Felton took a patent for a method of sharpening edge- 

 tools by means of two ground steel cylinders, which acted like files. Their 

 edges were drawn backward and forward in the angle formed between the 

 two cylindrical files. The working surfaces are a succession of small 

 cylinders with openings between (bosses and recesses,) and the surfaces of 

 the bosses are exposed? or cut, or scribed round circularly, like the files 



