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their chemical composition, not with the diversity of their modes 

 of origin. To mark this unity we require a name that shall 

 embrace them all, and we will use the name given in systematic 

 chemistry, that is to say, Calcium Carbonate, or Carbonate of 

 Calcium. Incidentally I may observe that such a name ; while 

 giving to those who have even a smattering of chemical know- 

 ledge, valuable information as to the chemical relationships of the 

 substance ; has a disadvantage as compared with a unitary name 

 such as water. A double-barelled name like calcium carbonate 

 is apt to mislead those who have not studied chemistry, since it 

 fails to suggest that it is the name of a single substance and not 

 of a mixture. Let me remind you, then, that by calcium car- 

 bonate I mean a perfectly definite compound, capable of being 

 built up from and of being resolved into other substances simpler 

 than itself, but, nevertheless, a single substance and not a 

 mixture. 



Calcium carbonate is only very slightly soluble in pure water,. 

 14 parts in one million, or about one grain in one gallon, or 1 lb. in 

 7,000 gallons. If the water contain, as many natural waters do, 

 carbon dioxide, or carbonic acid gas, its power of dissolving 

 calcium carbonate is considerably increased. The solubility in- 

 creases with the amount of carbonic acid up to a maximum about 

 82 times as great as that in perfectly pure water. The increased 

 solubility probably depends on the formation of a new compound, 

 Calcium Bicarbonate, related to the carbonate just as the sodium 

 bicarbonate used in medicine and cooking is related to common 

 washing soda. When calcium carbonate is heated it begins to 

 break up into two substances entirely different from itself, each 

 of which is simpler in composition than the parent substance, but 

 neither of which is an element. These substances are Lime and 

 Carbon Dioxide. The decomposition, or, as it is often called, 

 dissociation, has been very carefully studied. It is scarcely per- 

 ceptible below i,ooo° F. and at this temperature it proceeds but 

 a little way unless the disengaged carbon dioxide is swept away 

 as fast as it is liberated by a current of some indifferent gas. At 

 about 1,700° F. the decomposition is complete if the heating be 

 performed in an open vessel, but not otherwise. 



On the large scale lime-burning, as the process of decom- 

 posing calcium carbonate by heat is called, is carried out in special 

 furnaces called kilns. There are two main types of these. In the 

 older forms large lumps of limestone are built up into an arch over 

 the fire grate, the kiln filled up with smaller pieces, the fire is 

 kindled and kept up for about two days. When the kiln is cold 

 the lime is removed and the process begun again. In the other 

 and more modern form the action is continuous, coal and limestone 

 being fed into the upper part and the lime withdrawn below. 



I take leave once again to emphasise the distinction between 

 limestone, native calcium carbonate, and lime, the result of 

 heating the limestone and expelling the carbon dioxide. In making 

 lime 44 per cent, of carbon dioxide escapes and 56 per cent, of lime 



