FUEL BY-PRODUCTS 55 



After the first box is nearly saturated, the carbon 

 dioxide acts on the calcium sulphide already formed, 

 giving free hydrogen sulphide, according to the equation 



(4) CaS+H 2 + C0 2 =3CaC0 3 + H 2 S. 



The first box is now removed from action, so that the 

 second box becomes the first box, the third the second, and 

 so on, and a new box is put on at the end of the series ; 

 by these means economy in the use of lime is obtained. 

 The spent lime emptied from the boxes has a highly 

 objectionable odour, and has no value apart from its 

 demand for agricultural purposes. Where a small gasworks 

 is situated in an agricultural district, no difficulty occurs in 

 obtaining a sufficient sale for the gas lime, although, in any 

 case, it is not a profitable by-product ; where, however, 

 large works are situated in urban areas, it is impracticable 

 to get it all removed to the land, and large quantities are 

 taken and dumped out at sea. 



The successful application to the land of gas lime, lime- 

 stone, chalk and other bulky calcareous substances, depends 

 on a great variety of conditions ; but gas lime has a peculiar 

 action, due to the sulphides, sulphites, cyanides and other 

 poisonous compounds contained. In fresh gas lime the 

 proportion of water varies from about 30-40 %. Air 

 soon acts on fresh gas lime ; atmospheric carbon dioxide 

 brings equation (4) into play and, similarly acting on calcium 

 thio-carbonate gives rise to carbon disulphide as well ; 

 the objectionable odour begins to disappear on storage, 

 since the odorous compounds are oxidized to sulphates and 

 other inoffensive compounds. Fairly old samples of gas 

 lime contain no sulphide of lime, but only sulphite, while 

 the cyanides are converted into Prussian blue. Table 9 

 indicates the composition of a typical well-weathered gas 

 lime : 



