120 REPORT — 1861. 



Another mode of attaining this object, and one in many cases to be pre- 

 ferred, is by using sulphuric acid and boiling down the solution of sulphate 

 of copper so as to obtain crystals, or still further, viz. to dryness. This is 

 then heated in a furnace having a plate, or floor, of brickwork or tiles, the 

 fire being applied beneath, and not passing over the salt of copper : the 

 sulphate is decomposed, and sulphuric acid passes off. But the decomposi- 

 tion is more effectual when carbon is added ; in this way sulphurous acid is 

 driven off, and it is then led into a chamber, and being treated with nitrous 

 fumes in the usual way, sulphuric acid is formed, which is again used for the 

 solution of the copper in the ore. If the ore contains suboxide of copper, it 

 is previously roasted for oxidation. Phosphates, arseniates, carbonates, and 

 oxides may be treated by this process. 



For sulphides of copper Mr. Henderson roasts with common salt, having 

 previously reduced the ore to fine powder. The chloride of copper is vola- 

 tilized and condensed in a Gossage coke tower. The sulphate of soda re- 

 maining may be washed out of the non-volatile portion, and the copper pre- 

 cipitated from the solution flowing from the tower. He separates by this 

 means the metals whose chlorides have a different rate of volatilization : 

 chlorides such as chloride of silver are obtained in the flue close to the fur- 

 nace. 



We do not allude to the other inventions contained in Mr. Henderson's 

 patents, as we are not aware of any being in use in this district. 



XVII. Nitric Acid. 



About 48 tons of nitrate of soda per week are used in this district for 

 making nitric acid. The salt yields its own weight of acid of sp. gr. ] -40. 

 Nitric acid is used here for making the nitrates of copper, lead, alumina, 

 and iron, for oxidizing tin, for etching, and also for making aniline from 

 benzole. 



XVIII. Oxalic Acid. 



One of the most important and most interesting of the new manufacturing 

 processes which we have to describe in this Report is one for the preparation 

 of oxalic acid, invented and patented by Messrs. Roberts, Dale and Co., 

 gentlemen to whom we owe a number of highly ingenious and useful prac- 

 tical processes. The method of preparing oxalic acid hitherto employed 

 consists, as is well known, in acting on organic substances, such as sugar or 

 starch, with nitric acid. This process has now been superseded by that of 

 Messrs. Roberts, Dale and Co., which depends on the action exerted by 

 caustic alkalies on various organic substances at a high temperature. That 

 oxalic acid is one of the products formed by this action is a fact well known 

 to chemists, but one that has not until recently been turned to any practical 

 use. In the year 1829, Gay-Lussac published a short memoir*, in which 

 he announced that he had succeeded in obtaining oxalic acid by heating 

 cotton, sawdust, sugar, starch, gum, tartaric acid, and other organic acids 

 with caustic potash in a platinum crucible. Since that time the subject 

 has not been attended to either by scientific chemists or by practical men, so 

 far as we know. Messrs. Roberts, Dale and Co. are, we believe, the first 

 persons who have succeeded in carrying out the process in practice on a 

 large scale. In their attempt to do so they were met by a number of 

 serious obstacles, chiefly of a practical nature. These, however, they have, 

 by dint of uncommon ingenuity, and by the application of an amount of 

 perseverance of which, perhaps, but few men are capable, succeeded in 



* Annales de Chim. et de Phys. t. xli. p. 398. 



