(>N WATEB AND ITS COMPOUNDS 109 



is expelled fnmi it, and there remains a \\liitc stony mass, whi-h U 

 dense, compact, and rather tenacious. Lime is usually sold in t In- 

 form, and hears the name of 'quick' or 'unslaked' lime. If water be 

 poured over such lime, a great rise in temperature is remarked either 

 directly, or after a certain time. The whole mass becomes hot, part of 

 the water is evaporated, the stony mass in absorbing water crumbles into 

 ponder, and if the water be taken in sufficient quantity and the lime 

 be pure and well burnt, not a particle of the original stony mass is left 

 it all crumbles into powder. If the water be in excess, then naturally 

 a portion of it remains and forms a solution. This process is called 

 ' slaking ' lime. Slaked lime is used in practice in intermixture with 

 sand as mortar. Slaked lime is a definite hydrate of lime. If it is 

 dried at 100 it retains 24-3 p.c. of water. This water can only be 

 expelled at a temperature above 400, and then quicklime is re-obtained. 

 The heat evolved in the combination of lime with water is so intense 

 that it can set fire to wood, sulphur, gunpowder, &c. Even on mixing 

 lime with ice the temperature rises to 100. If lime be melted with a 

 small quantity of water in the dark, a luminous effect is observed. But, 

 nevertheless, water may still be separated from this hydrate. 71 If 

 phosphorus be burnt in dry air, a white substance called ' phosphoric 

 anhydride ' is obtained. It combines with water with such energy, that 

 the experiment must be conducted with great caution. A red heat is 

 produced in the formation of the compound, and it is impossible to 

 separate the water from the resultant hydrate at any temperature. 

 The hydrate formed by phosphoric anhydride is a substance which is 

 totally undecomposable into its original component parts by this action 

 of heat. Almost as energetic a combination occurs when sulphuric 

 anhydride, SO 3 , combines with water, forming its hydrate, sulphuric 

 acid, H 2 SO,. In both cases definite compounds are produced, but 

 the latter substance, as a liquid, and capable of decomposition by heat, 

 giving off the vapour of its volatile anhydride even at the ordinary 

 temperature, forms an evident link with solutions, and, with an 

 excess of water, it gives, as a soluble substance, a true solution. 

 If 80 parts of sulphuric anhydride retain 18 parts of water, this 

 water cannot be separated from the anhydride, even at a tempera- 

 ture of 300. It is only by the addition of phosphoric anhy- 

 dride, or by a series of chemical transformations, that this water can be 

 separated from its compound with sulphuric anhydride. Oil of vitriol, 



71 In combining with water one part by weight of lime evolves 245 units of heat. A 

 high temperature is obtained, because the specific heat of the resulting product is small. 

 Sodium oxide, NaoO, in reacting on water, H 2 O, and forming caustic soda (sodium 

 hydroxide), NaHO, evolves 552 units of heat for each part by weight of sodium oxide. 



