Mechanical Science. 427 



always more or less oxidized. According to him, immersion when 

 hot in mercury is the only method of obtaining steel hard and unin- 

 jured by oxidation. — Jahrb.der fVien, 1828. 



11. On the dyeing off^ool with Prussian Blue. — A very favour- 

 able report has been made to the Academy of Sciences at Paris by 

 MM. Thenard, D'Arcet, and Chevreul, on a process devised for the 

 above purpose, by M. Raymond, fils. A reward of 25,000 francs was 

 offered by the French government in 1811, and has not yet been 

 claimed. 



One great difficulty in the way of dyeing wool blue by means 

 of this substance was the charging the wool with a sufficient 

 quantity of peroxide of iron previous to its immersion in the 

 ferrocyanic acid. After numerous trials, M. Raymond found that 

 this object was best attained by the use of a solution prepared by 

 mixing water, sulphuric acid, nitric acid and proto-sulphate of iron, 

 so as to convert the latter into per-sulphate, and then adding a mix- 

 ture of sulphuric acid and bitartrate of potash, which, according to 

 him, is equivalent to tartaric acid and sulphate of potash. This 

 solution, which he calls the tartro-sulphate of peroxide of iron, 

 should have a specific gravity of 1.333, and is then to be used as 

 a bath either for woollen cloth or for the unspun wool. 



Four operations are then mentioned as necessary to produce the 

 colour in the wool, 



i. Iron bath. This should be of specific gravity 1007. 

 It should be heated by steam, and when at from 86° to 104°, 

 the cloth is to be immersed and retained in motion on a wheel, 

 whilst the heat is to be raised to the boiling point. Two sets of test 

 specimens are required to guide the dyer ; one a series of pieces 

 of cloth with different quantities of oxide of iron, the other a 

 corresponding series dyed blue. From these it may be judged 

 when the cloth has received the quantity of iron required for a par- 

 ticular tint. It is then to be removed from the bath, allowed to 

 drain a short time, and then washed in river water with great care. 

 Cloths to be dyed of a clear blue are to be immersed in the bath 

 when cold, and an addition of sulphuric and cream of tartar is 

 required. A bath which has been once or twice used may be 

 refreshed by the addition of tartro-sulphate of iron ; but a period is 

 at last attained when, containing an injurious quantity of acid and 

 greasy matter, it should be thrown away. 



ii. Blue bath. This is a solution in water of ferro-prussiate of 

 potash; the latter being in the proportion of 0.085 of the weight 

 of the cloth. The cloth is to be immersed for a quarter of an hour, 

 and then taken out. A quantity of sulphuric acid, equal in weight 

 to the ferro-prussiate, is then to be diluted with 5.5 times its 

 weight of water and divided into three equal portions ; one of these 

 is to be added to the bath, and the cloth immersed for a quarter of 

 an hour : being taken out, the second portion is to be added, andr 



