THE VALENCY AND SPECIFIC HEAT OF THE METALS 597 



which is unaffected by heat, 39 and may therefore serve as a fire- 

 resisting material, and was employed by Deville for the construction 

 of furnaces in which platinum was melted, and silver volatilised by the 

 action of the heat evolved by the combustion of detonating gas. The 

 hydrated lime, slaked lime, or calcium hydroxide, CaH 2 O 2 (sp. gr. 

 '2 '07) is a most common alkaline substance, which is employed largely 

 in building for making mortars or cements, in which case its bind- 

 ing property is mainly due to the absorption of carbonic anhydride. 40 

 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, which is known as slaked lime, and in a 

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



burnt and unaltered by air. It attracts moisture from the air and then disintegrates into 

 a powder ; if left long exposed in the air it also attracts carbonic anhydride and 

 increases in volume ; it does not entirely pass into carbonate, but forms a compound of 

 the latter with caustic lime. 



59 Lime, when heated to a white heat in the vapour of potassium, gives calcium, and 

 in chlorine it disengages oxygen. Sulphur, phosphorus, &c., when heated with lime, are 

 absorbed by it. 



40 The greater quantity of lime is used in making mortar for binding bricks or stones 

 together, in the form of lime or cement, or the so-called slaked lime. For this purpose 

 the lime is mixed with water and sand, which serves to separate the particles of lime 

 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 cohesion. 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 progress of time. This is the reason (and not, as is 

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

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

 cements will be described in the chapter on silicon. 



