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in nature as such ; they are artificial products obtained by the 

 action of heat, or of heat followed by the addition of water, on 

 Calcium Carbonate. This, as the various limestones, occurs iri 

 immense quantities in the earth's crust as rocks of various degrees 

 of hardness, and,' in a state of solution, in many natural waters, 

 waters of the ocean, of lakes, rivers and springs. It is with 

 Calcium Carbonate that we may most conveniently begin our 

 study of Lime in the Service of Man. I shall speak first of thos£ 

 uses and applications which depend on mechanical and physical 

 properties and do not involve any chemical change. Calcium 

 carbonate is, for our present purpose, best regarded as a com- 

 pound of the second order, i.e., as a compound of two compounds, 

 Lime and Carbon Dioxide. The latter is, perhaps, better known 

 as carbonic acid gas. In ioolbs. of pure chalk or marble there are 

 4olbs. of calcium, i2lbs. of carbon and 481bs. of oxygen. One 

 third of this oxygen may be regarded as the more' peculiar 

 possession of the calcium, the remaining two thirds as belonging 

 to the carbon. Although calcium carbonate is almost 

 insoluble in pure water it can be dissolved by water 

 containing, as even rain water does contain, some car- 

 bonic acid. Partly by this solvent effect of water thus 

 charged and partly by the mechanical friction due to sand or stones 

 carried by water in motion, limestone rocks are specially liable to 

 contain hollows, caves and caverns, sometimes of vast extent. 

 In some cases caves of this kind were used by mankind in early 

 ages as shelters or habitations. A little more than a year ago Mr. 

 Balch lectured before this Society on " The Caves and Cave- 

 dwellers of the Mendips," and I need not say more concerning 

 this early and widespread use of limestone. From such primitive 

 habitations, for whose possession man must often have contended 

 with mighty and ferocious beasts, we naturally pass to buildings 

 properly so called. Amongst building materials those of cal- 

 careous origin occupy a most important place. Chalk itself has 

 been used as a building-stone although it is usually too soft for the 

 purpose. According to the Encyclopaedia Britannica chalk com- 

 pact enough to be used by the builder is quarried in Devonshire 

 and known as Beer stone. But although chalk has but 

 a very limited application in this particular respect the ' oolitic 

 and other lime-stones furnish many most useful and valu- 

 able building-stones. Bath stone and Portland stone may be 

 mentioned as examples. Some of these limestones consist, to the 

 extent of 95 per cent, or more, of calcium carbonate, others con- 

 tain considerable quantities of magnesium carbonate and are 

 known as dolomites. The ancient castle of Conisborough, familiar 

 to all readers of Scott's Ivanhoe, is built of dolomite, as is also 

 much of York Minster. The Houses of Parliament furnish a 

 modern example of a great building of the same material. St. 

 Paul's Cathedral is of Portland stone. Marble, where obtainable 

 with sufficient ease, is an admirable building-stone, but in most 

 countries it is too costly for other than decorative uses. For 



