THE VALENCY AND SPECIFIC HEAT OF THE METALS 607 



Lime, like other alkalis, acts on many animal and vegetable sub- 

 stances, and for this reason has many practical uses for example, 

 for removing fats, and in agriculture for accelerating the decom- 

 position of organic substances in the so-called composts or accumu- 

 lations of vegetable and animal remains used for fertilising land. 

 Calcium hydroxide easily loses its water at a moderate heat (530), 

 but it does not part with water at 100. When mixed with .water, 

 lime forms a pasty mass known as slaked lime and in a more 

 dilute form as milk of lime, because when shaken up in water it 

 remains suspended in it for a long time and presents the appearance 

 of a milky liquid. But, besides this, lime is directly soluble in water, 

 not to any considerable extent, but still in such a quantity that lime 

 water is precipitated by carbonic anhydride, and has clearly dis- 

 tinguishable alkaline properties. One part of lime requires at the 

 ordinary temperature about 800 parts of water for solution. At 100 

 it requires about 1500 parts of water, and therefore lime-water 

 becomes cloudy when boiled. If lime-water be evaporated in a 

 vacuum, calcium hydroxide separates in six-sided crystals 41 If 

 lime-water be mixed with hydrogen peroxide minute crystals of 

 calcium peroxide, Ca0 2 ,8H 2 0, separate ; this compound is very un- 

 stable and, like barium peroxide, is decomposed by heat. Lime, as a 

 powerful base, combines with all acids, and in this respect presents a 

 transition from the true alkalis to magnesia. Many of the salts of 



from each other. If only lime paste were put between two bricks they would not hold 

 firmly together, because after the water had evaporated the lime would occupy a smaller 

 space than before, and therefore cracks and powder would form in its mass, so that it would 

 not at all produce that complete cementation of the bricks which it is desired to attain. 

 Pieces of stone that is, sand mixed with the lime hinder this process of disintegration, 

 because the lime binds together the individual grains of sand mixed with it, and forms 

 one concrete mass, in consequence of a process which proceeds after the desiccation or 

 removal of the water. The process of the solidification of lime, taken as slaked lime, 

 consists first in the direct evaporation of the water and crystallisation of the hydrate, so 

 that the lime binds the stones and sand mixed with it, just as glue binds two pieces of 

 wood. But this preliminary binding action of lime is feeble (as is seen by direct experi- 

 ment) unless there be further alteration of the lime leading to the formation of carbonates, 

 silicates, and other salts of calcium which are distinguished by their great cohesiveness. 

 With the progress of time the cement is partially subjected to the action of the carbonic 

 anhydride in the air, owing to which calcium carbonate is formed, but not more than half 

 the lime is thus converted into carbonate. Besides which, the lime partially acts on the 

 silica of the bricks, and it is owing to these new combinations simultaneously forming 

 in the cement that it'gradually becomes stronger and stronger. Hence the binding action 

 of the lime becomes stronger with the lapse of time. This is the reason (and not, as is 

 sometimes said, because the ancients knew how to build stronger than we do) why build- 

 ings which have stood for centuries possess a very strongly binding cement. Hydraulic 

 cements will be described later (Chapter XVIII., Note 25). 



41 Professor Glinka measured the transparent bright crystals of calcium hydroxide 

 which are formed in common hydraulic (Portland) cement. 



"14 



