PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 335 



the waste slags. At ^fantsel tlic slag is now run into molds of about a 

 cubic foot eacli, and distributed to workmen. Each man takes his share 

 of the blocks in an iron wiieolbarrow and wheels them home, when they 

 still contain heat enough to cook the meal for the family. After tliey arc 

 cooled these rectangular blocks are an excellent material for building walls. 



ZiNC Wasted in Galvanizing Iron. 



A large portion of the zinc nsed for coating iron is evaporated and lost. 

 PlaTis for preventing this loss are worthy of the attention of inventors. 

 The whole history of zinc is that of a waste product. It was first found 

 in chimneys where ores of other metals were being smelted, and people 

 were thus led to seek for it in its own ores.* 



Waste from Gas-Works. v 



Constant progress has been made in the utilization of the waste sub- 

 stances produced in the manufacture of illuminating gas. At one time the 

 companies paid persons for carting away the lime used for purifying the 

 gas. The lime absorbs bisulphide of carbon, sulphuretted hydrogen and 

 sulphur, coming from the distillation of the coal, and when exposed for a 

 long time to the atmosphere it absorbs oxygen and becomes the sulphate 

 of lime or plaster. This is now understood by a sufficient number of 

 farmers to make a demand for the waste lime at a moderate price. 



Mr. Cleland, the Director of the Liverpool Gas Works, states that he has 

 largely reduced the cost of purifying gas by using oxide of iron, and 

 saving the sulphur and ammonia. The material from the purifiers is heated 

 to about 1,000 deg. Fah. in a close iron retort. A portion of the sulphur 

 combines chemically with the iron, while the balance is distilled over. As 

 soon as the sulphur ceases to come over, the contents of the retort are 

 drawn and moistened, and in this state exposed to the action of the atmo- 

 sphere. The oxidation is rapid, and the mass glows until frequently wet 

 and stirred. In a few weeks a sulphate of iron is produced, containing 30 

 to 40 per cent, sulphuric acid. The salt is decomposed by passing the 

 vapor of ammonia from the waste waters of the hydraulic mains through 

 it. In this way sulphate of ammonia and an oxide of iron are obtained. 

 The oxide of iron can be used again. The sulphate of ammonia is purified 

 by crystallization. Mr. Clelland says that he has obtained 100 tons of 

 sulphur in this way. 



Preparation of Sal Ammoniac. 



About two per cent, of ammoniacal gas water goes over with the tarry 

 products and is collected at the end of the hydraulic main in cisterns. This 

 was formerly a waste product; it is now saved and the greater- portion of 

 sal ammoniac of commerce is prepared from it. In London alone 840,000 

 tons of coal are c(jnsumed every year in the manufacture of gas. This 

 yields about 37,000,000 pounds of gas water. The water is subjected to 

 distillation in two retorts, the first of which is heated directly by the fire, 

 and the second by the latent heat of the steam from the first. The steam 

 and gas are passed througii a worm to be condensed, and tlow into a large 

 leaden tank containing muriatic acid. Uncondensable gases pass out of 



