134 EXPERIMENTAL GENERAL SCIENCE 



muth, and cadmium. Alloys containing mercury are called 

 amalgams, and are frequently used by dentists. Our silver 

 and gold coins always contain a certain amount of copper for 

 the purpose of making them harder. Pure gold is said to be 

 24 carats fine. This is much too soft for use in ornaments and 

 the like. Eighteen-carat or 14-carat gold is commonly used. 

 Zinc, tin, and aluminum are insoluble in most liquids, which 

 explains their use in cooking utensils and other vessels. 



117. Solution and Change of State. Practically all liquids 

 have definite temperature points at which, when pure, they 

 assume gaseous and solid conditions. When a solid is dis- 

 solved in a liquid, however, it usually affects both its boiling 

 and freezing points, spreading them apart as it were, by raising 

 the boiling point and lowering the freezing point. Boiling 

 syrup may be made much hotter than boiling water, while 

 brine, made of table salt (sodium chloride) and water, may be 

 cooled to 22 below zero Centigrade before it becomes solid. 

 By the use of another salt, calcium chloride, a brine may be 

 made which does not freeze until the temperature reaches 55 

 below zero Centigrade. When a liquid is boiling, dissolving 

 a solid in it will reduce its temperature for the reason that, in 

 dissolving, the molecules of the solid are spread much farther 

 apart, and the heat necessary for this is absorbed from the 

 liquid. Salt in contact with ice dissolves in the water from 

 the melting ice and, absorbing heat in the process, forms a 

 brine that does not freeze until a much lower temperature is 

 reached. The heat given out by the brine, as its temperature 

 lowers, goes to melt more ice. This explains the custom of 

 putting salt on icy sidewalks and car tracks in winter. In the 

 ice cream freezer, the cold brine absorbs heat from the cream. 

 In making a freezing mixture of this kind, one part salt_and 

 three parts ice is about the right proportion. 



118. Emulsions. Emulsions are not true solutions, but 

 since they consist of substances in close association, they may 



