20 ON THE USE OF LIME, MARL, AND SHELL-SAND. 



and, of course, all the impurities which are contained originally 

 in the quicklime employed in the gas-works. 



The relative proportions of . the chief constituents of gas-lime 

 will be recognised by throwing a glance at the subjoined 

 Table, which gives the composition of gas-lime when dried at 

 212 Fahr. : 



Composition of refuse Gas-lime. 



Water of combination and a little organic matter . . . . 7*24 



Oxides of iron and alumina 2*49 



Sulphate of lime 4'64 



Sulphite of lime 15'19 



Carbonate of lime 49-40 



Caustic lime 18'23 



Magnesia and alkalies 2'53 



Insoluble silicious matter '28 



100-00 



It need hardly be stated that the composition of gas-lime, for 

 obvious reasons, must be subject to great variations. In fresh 

 gas-lime the proportion of water amounts from 30 to 40 per cent. 



Gas-lime, it will be seen, contains a good deal of sulphite of 

 lime, i. e. a combination of lime with sulphurous acid. This 

 combination is readily decomposed even by carbonic acid ; and 

 as sulphurous acid, which is readily liberated by the carbonic 

 acid in the soil, is injurious to vegetation, gas-lime should not 

 be applied to the land in too fresh a state, for in such a state it 

 is decidedly mischievous even when used in moderate quantities. 

 The safest plan of using gas-lime appears to me to employ it in 

 the shape of a compost. Mixed with refuse matters, whether of 

 animal or vegetable origin, gas-lime, on keeping for five, or 

 better for six, months, becomes transformed into a very useful 

 fertiliser for all soils naturally poor in lime. In fact, gas-lime 

 may be used with equal advantage as quicklime in making lime- 

 compost heaps. 



2. Chalk. The localities in the West where chalk occurs are 

 confined to part of Dorsetshire, Wiltshire, and Somersetshire ; 

 its agricultural use is naturally confined to the immediate neigh- 

 bourhood where it is found. 



Chalk is a soft kind of carbonate of lime, which often consti- 

 tutes extensive beds, separated usually by geologists into the 

 upper and lower chalk beds. Neither the upper nor the lower 

 chalk is pure carbonate of lime, but both invariably contain 

 more or less silica, often in the shape of minute silica shells of 

 infusorial animals, besides oxides of iron and alumina, magnesia, 

 and traces of phosphoric and sulphuric acids as potash or soda. 

 In some beds of chalk, oxide of iron occurs in only a minute 

 quantity, whilst in others its quantity is appreciable ; hence the 

 white colour of the former and the yellowish colour of the latter. 



