OBJECTS.] INTRODUCTORY. 79 



steam is formed, has a greater resistance to overcome 

 when salt is dissolved in the water. And just as 

 the presence of alcohol lowers the freezing point of 

 the water with which it is mixed, so does the presence 

 of salt lower the freezing point of water. Sea water, 

 which is a weak brine, begins to freeze at about 27; 

 and the ice which is formed is quite pure, while the 

 remainder of the sea water becomes richer in salt. 



If we mean by attraction that which opposes any 

 force which tends to separate bodies, then we may say 

 that the molecules of salt and those of water attract 

 one another. And such attraction between molecules 

 of matter of different kinds is called chemical at- 

 traction. 



54. Quicklime and Water : Plaster of Paris 

 and Water : Combination. 



Quicklime is a substance obtained by heating chalk or 

 limestone to redness. When pure, it is a white hard solid 

 which can be made to pass into the liquid and gaseous 

 states only at enormously high temperatures. If a 

 lump of fresh quicklime is placed in a saucer and about 

 one-third of its weight of water poured upon it, there will 

 be a great turmoil, heat will be evolved, the water will 

 disappear, and the lime will crumble down into a soft 

 white powder. This operation is what bricklayers call 

 slaking lime. And if no more water has been added 

 than the proportion mentioned, the pure white powder 

 which results will be solid and dry, the water having 

 to all appearance, vanished. 



In the solution of salt we saw a solid become 

 fluid under the influence of water ; in the slaking of 

 lime the fluid water enters into the structure of a solid. 



