ON THE USE OF LIME, MARL, AND SHELL-SAND. 19 



become reconverted into carbonate of lime, I analysed a portion 

 of such lime. The slaked and exposed lime was made from the 

 same limestone (from Williamson) of which an analysis has been 

 given already. The results of this examination are as follows : 



Composition of Quicklime spontaneously slaked by exposure to air. 



Moisture, driven off at 212 Fahr. . . . . . . '78 



Oxide of iron and alumina .. '29 



Silica.. -14 



Magnesia "14 



Sulphate of lime '11 



Carbonate of lime 15*11 



Lime-hydrate 83'43 



100-00 



It will be seen that lime slaked spontaneously contains an appre- 

 ciable quantity of carbonate of lime. The amount of carbonate 

 in slaked lime of course is not constant, but varies with the time 

 during which lime has been exposed to the atmosphere. This 

 lime, like the stone from which it is made, is an exceedingly 

 pure and valuable material, which can scarcely be surpassed by 

 any other lime. 



This will be a fitting place to put on record an analysis which 

 I lately made of gas-lime, i. e. the refuse lime from gas-works. 

 The lime in the gas-purifier principally answers the purpose of 

 removing from the crude gas, as it issues from the retorts, sul- 

 phuretted hydrogen and carbonic acid. The slaked lime conse- 

 quently becomes more or less changed into carbonate of lime 

 and into sulphuret of calcium, a combination of sulphur with 

 calcium, the metallic base of lime. At the same time some 

 tarry matter, a little ammonia, and other volatile substances, pass 

 into the gas-purifiers, and are in part retained by the lime in a 

 mechanical way. 



When gas-lime is first exposed to the atmosphere it gives off 

 a very bad smell, which arises principally from the sulphur 

 compounds in this substance ; after some time the bad smell 

 diminishes, and at last becomes almost imperceptible. This 

 shows that the sulphurous compounds in gas-lime undergo 

 changes on keeping, which may be stated briefly to consist in the 

 gradual oxidation of sulphuret of calcium, which, passing through 

 the formation of sulphite of calcium, terminates in the produc- 

 tion of sulphate of lime or gypsum. Gas-lime kept for some 

 time thus always contains some gypsum. 



From these remarks it will be gathered that gas-lime contains 

 the following ingredients : carbonate of lime, lime hydrate, sul- 

 phuret of calcium (mere traces), sulphite of lime, sulphate of 

 lime, small quantities of tarry matters, a little ammonia, water, 



c 2 



