ON THE USE OF LIME, MARL, AND SHELL-SAND. 5 



The characters and composition of these different forms in 

 which lime is applied to the land will be described in the order 

 here given. 



Burnt, or quicklime, as every one knows, does not occur as a 

 natural mineral, but is produced on burning limestone in a kiln. 

 The quantities of quicklime depend in a great measure on the 

 composition of the stone from which it is obtained. 



1. On the Composition of several kinds of Limestone found in the 

 West of England, and on Quick and Slaked Lime. 



The bulk of all limestone consists of carbonate of lime, j. e. a 

 combination of carbonic acid with lime. Pure white marble 

 consists almost entirely of this combination, and in some of the 

 best descriptions of ordinary limestone the proportion of carbonate 

 of lime amounts, as we shall see presently, to 94 or even 96 per 

 cent. Usually, however, the proportion of carbonate of lime is 

 smaller, since carbonate of magnesia, oxides of iron, some alu- 

 mina, and more or less silicious matters, and a few other mineral 

 matters that need not be mentioned here in detail, enter into the 

 composition of ordinary limestones. 



The colour, degree of hardness, and general appearance of the 

 various kinds of limestones depend in a great measure on the 

 relative proportions in which the constituents just named occur in 

 them. The differences which prevail in the various kinds of 

 limestones, however, are not entirely due to variations in compo- 

 sition, but they can be traced likewise to purely physical causes 

 which have been in operation when the solid limestones were 

 formed. Frequently the composition of different limestones pre- 

 sents us with but slight variations, whilst their texture, degree of 

 hardness, and adaptation for practical purposes exhibit great 

 contrasts. According to the prevailing physical and chemical 

 properties, limestones may be conveniently classed as follows : 



a. Building stones. 



b. Hydraulic limestones. 



c. Agricultural limestones. 



a. Limestones used for building. Some of our best building- 

 stones occur in the West of England, as, for instance, the cele- 

 brated Bath stone and similar oolitic limestones which occur in 

 the neighbourhood of Bath and Bristol. Good building-stones 

 are hard and uniform in texture, and capable of resisting atmo- 

 spheric influences in a higher degree than ordinary limestones, 



