114 TOWN GEOLOGY. [v. 



its water of crystallisation, that is, water which it 

 holds (as do many rocks) locked up in it unseen, and 

 only to be discovered by chemical analysis. It is then 

 anhydrous that is, waterless oxide of lime, what we 

 call quick-lime; that which figured in the comi-tragedy 

 of " Flour-bag Creek ; " and then, as you may find if 

 you get it under your nails or into your eyes, will burn 

 and blister like an acid. 



This has to be turned again into a hard and tough 

 artificial limestone, in plain words, into mortar; and 

 the first step is to slack it that is, to give it back the 

 water which it has lost, and for which it is as it were 

 thirsting. So it is slacked with water, which it drinks 

 in, heating itself and the water till it steams and 

 swells in bulk, because it takes the substance of the 

 water into its own substance. Slacked lime, as we all 

 know, is not visibly wetter than quick-lime ; it 

 crumbles to a dry white powder in spite of all the 

 water which it contains. 



Then it must be made to set, that is, to return to 

 limestone, to carbonate of lime, by drinking in the 

 carbonic acid from water and air, which some sorts of 

 lime will do instantly, setting at once, and being 

 therefore used as cements. But the lime usually 

 employed must be mixed with more or less sand to 

 make it set hard : a mysterious process, of which it 

 will be enough to tell the reader that the sand and 

 lime are said to unite gradually, not only mechanically, 

 that is, by sticking together ; but also in part chemi- 

 cally that is, by forming out of themselves a new 

 substance, which is called silicate of lime. 



Be that as it may, the mortar paste has now to do 

 two things ; first to dry, and next to take up carbonic 



