ON THE USE OF LIME, MARL, AND SHELL-SAND. 17 



neighbourhood of Cirencester are made of the most shelly lime- 

 stones. I have moreover taken the trouble of examining for 

 phosphoric acids several limestones full of fossil shells, as 

 well as some fossil remains extracted from the oolitic lime- 

 stones in our neighbourhood, and find neither the shelly lime- 

 stones richer in phosphoric acid than non-fossiliferous limestones, 

 nor a larger amount of phosphoric acid in the shells than in the 

 matrix in which they are embedded. By far the most valuable 

 property of a limestone is its power to burn in the kiln, so as to 

 produce a solid stone that admits of being removed from the kiln 

 without too much waste, and that in slaking swells greatly, pro- 

 ducing a light floury powder. Generally speaking, limestones 

 rich in carbonate of lime possess this valuable property in a 

 high degree ; and within certain limits, the amount of pure car- 

 bonate of lime in a limestone is a measure of its agricultural 

 value. In a minor degree the proportions of phosphoric and 

 sulphuric acid affect the value of limestones for agricultural pur- 

 poses ; but generally the amount of these fertilising constituents 

 is too inconsiderable to deserve any notice. Should there be 

 much phosphoric acid in a limestone, it is, of course, all the more 

 valuable ; and, on the other hand, if there is much sand or oxide 

 of iron, alumina, and other foreign impurities in a limestone, it 

 will yield a quicklime which is deteriorated in value in a corre- 

 sponding degree. 



On the changes Lime undergoes in burning and slaking. The 

 chief constituent of all limestones, it will be remembered, is car- 

 bonate of lime, i. e. a combination of lime with carbonic acid. 

 When burned in the kiln the carbonic acid is driven out by the 

 heat, and the lime, mixed with the foreign matter of the lime- 

 stone, remains behind as caustic, burned, or quicklime. One ton 

 of good limestone, on an average, yields in round numbers 

 nearly 11 cwt. of quicklime or lime-shells. The weight of 

 quicklime per bushel varies greatly with the kind of limestone, 

 and the heat to which it has been exposed ; when burnt very high 

 it becomes more compact than at a more moderate heat, and 

 then, of course, is heavier per bushel. Too great a heat should 

 be avoided, especially if the limestone contains much clay or 

 other silicious matters, since such limestones are apt, when 

 burned too highly, to yield a lime which does not readily slake. 

 Such lime is said to be over-burned. 



On exposure to a moist atmosphere burned lime slowly attracts 

 water ; or by pouring upon it a certain quantity of water, it 

 absorbs the latter at once, heats, and in either case falls to a fine 

 powder. The process is known as slaking, and the product is 

 slaked lime. 



The most economical and approved mode of reducing quick- 



