122 REPORT — 1861. 



soda in this part of the process is sufficiently apparent. The supernatant 

 liquid is drawn off and evaporated to dryness, and the residual mass is heated 

 in furnaces in order to destroy the organic matter and recover the alkalies 

 which it contains, and which are employed again after being causticized for 

 acting on fresh sawdust. In consequence of the elimination of soda, the 

 relative proportion of the two alkalies recovered from the liquor is, of course, 

 different to what it was at the commencement; and before being used again 

 the quantity of each alkali contained in the mixture must be ascertained. 



The oxalate of soda, after being washed, is decomposed by boiling with 

 hydrate of lime. Oxalate of lime falls to the bottom, and caustic soda passes 

 into solution, and may be employed again for any purpose to which it is ap- 

 plicable. The resulting oxalate of lime is decomposed by means of sulphuric 

 acid, the proportions employed being three equivalents of acid to one of the 

 oxalate ; and the liquor decanted from the sulphate of lime is evaporated to 

 crystallization in leaden vessels. The crystals of oxalic acid, which are slightly 

 coloured by organic matter, are purified by recrystallization. 



From about 2 lbs. of sawdust 1 lb. of crystallized oxalic acid may be 

 obtained. There is no loss of oxalic acid. The only loss experienced is in 

 alkalies. The quantity of acid at present manufactured by Messrs. Roberts, 

 Dale and Co. amounts to 9 tons per week ; and their works are capable of 

 being extended so as to produce 15 tons, which is supposed to be the total 

 quantity consumed throughout the world. Their plant is extensive and 

 costly, and bears evidence of an uncommon spirit of enterprise on the part 

 of the proprietors. 



In order to give an idea of the effect which the introduction of this pro- 

 cess has had on the market, it may be mentioned that the selling price of the 

 aeid at this time is 8c?. to 9c?. per lb., whereas in 1851 it was 15d. to 16c?. 

 per lb. 



Oxalic acid is used extensively in calico-printing, woollen-dyeing, woollen- 

 printing, silk-dyeing with wood colours, in straw-bleaching, and for making 

 binoxalate of potash, the so-called "salt of lemons." 



XIX. Pyroligneous Acid. 



The only improvement introduced into the manufacture of this acid during 

 the last few years consists in the use of sawdust instead of wood in the 

 process of destructive distillation. The sawdust is introduced into the front 

 of the retort through a hopper, and is gradually moved to the other end by 

 means of an endless screw, worked by machinery. During its transit it 

 becomes completely carbonized, the gaseous and liquid products escape 

 through a pipe, while the charcoal is allowed to fall into a vessel of water. 

 The latter precaution is necessary, since the carbon is obtained in such a 

 minute state of division that no cooling in the air or in closed vessels would 

 be sufficient to stop the combustion. In other respects the process does 

 not differ essentially from that with wood. No more acid is obtained than 

 with wood, and less naphtha. The quantity of the former varies, however, 

 with the temperature employed. The usual temperature is that of a dull red 

 heat. From 1 ton of sawdust 100-120 gallons of liquid, containing 4 per 

 cent, of glacial acid and 15 gallons of tar, are obtained, and 100 parts of 

 the crude distillate yield 3 of naphtha. The advantage consists in the 

 cheapness of the material employed ; but, on the other hand, one of 

 the resulting products, viz. the finely divided charcoal, is comparatively 

 worthless. 



This invention forms the subject of Mr. Halliday's patent, which was taken 

 out in the year 184'8-4'9. Quite recently Mr. Bowers has patented another 



