166 ILLUMINATION. [ VI. 



pitch, or burned to make lampblack ; the aqueous solutions 

 contain ammonia, and are sometimes used to procure it ; the 

 gas is a mixture of carbohydrogens, carbonic oxide and acid, 

 &c. The gas is freed by cooling and by lime from sulphuretted 

 hydrogen, carbonic acid, suspended tarry matter, cyanogen, 

 &c. Since the proportion of carbohydrogen determines the 

 illuminating power of gas, and some coals yield too much car- 

 bonic oxide, &c., it has been proposed to pass the gas through 

 volatile, liquid, and solid carbohydrogens, of which it will take 

 up a small quantity and increase its luminosity : this is termed 

 naphthalizing. Quite recently, hydrogen has been used, in a 

 naphthalized condition, but we may doubt its success. 



Coal Gas. — On the comparative value of the different kinds 

 of coal used for illumination, and on methods for ascertaining 

 the value of the gases, see an article by Dr. Fyfe, in Edin. 

 Phil. Journ. xlv. and in Amer. Journ. 2d ser. vii. 77-86, 

 157-167. 



Rosin Cras. — A variation of the manufacture of gas from 

 rosin is patented by Robertson (Lond. Journ. 1849, 37), in 

 which the rosin, mixed with sawdust and alkali (lime, &c.), 

 is charged into iron cases, which are put into a gas-retort and 

 heated as usual. The products of distillation are passed into 

 a second retort filled with lumps of coke, brick, &c., and heated 

 to cherry redness. To make gas alone, these products pass 

 through a third and fourth retort, filled with brick, coke, &c., 

 and are then washed and purified by lime. To obtain partly 

 gas and partly oily matters, the vapors issuing from the first 

 retort, containing coke, &c., are passed through a tank con- 

 taining water, where oily matter deposits, and then through 

 the washer and purifier. The oily products are made into a 

 grease for machinery by mixing it with lime and finely gra- 

 nulated zinc. Or, the oily product may be first distilled with 

 water, yielding a spirit, which, after several distillations with 

 a little lime, becomes colorless and thin, and is used for illu- 

 mination or for a varnish. 



On water and rosin gas, see an article by Prof. Fyfe, in the 

 Journ. Fr. Inst. (3) xx. 271, 319. 



