264 Common Science 



When the part of the earth where they are found was 

 hot enough to melt stone, the liquid ruby or sapphire or 

 emerald, or whatever the stone was to be, happened to 

 be near some coloring matter that dissolved in it and gave 

 it color. Several of these stones are made of exactly 

 the same kind of material, but different kinds of coloring 

 matter dissolved in them when they were melted. 



Many articles are much used chiefly because they are 

 good emulsifiers or good solvents (dissolve things well). 

 Soap is a first-rate emulsifier ; water is the best solvent 

 in the world ; but it will not dissolve oil and gummy 

 things sufficiently to be of use when we want them dis- 

 solved. Turpentine, alcohol, and gasoline find one of 

 their chief uses as solvents for gums and oils. Almost 

 all cleaning is simply a process of dissolving or emulsify- 

 ing the dirt you want to get rid of, and washing it away 

 with the liquid. Do not forget that heat helps to dis- 

 solve most things. 



Application 63. Explain why clothes are washed in hot 

 suds ; why sugar disappears in hot coffee or tea ; why it 

 does not disappear as quickly in cold lemonade; why you 

 cannot see through milk as you can through water. 



Inference Exercise 

 Explain the following : 



381. A kind of lamp bracket is made with a rubber cup. When 



you press this cup against the wall or against a piece of 

 furniture and exhaust the air from the cup, the cup sticks 

 fast to the wall and supports the lamp bracket. 



382. You can take a vaseline stain out with kerosene. 



383. If the two poles of an electric battery are connected with 



a copper wire, the battery soon becomes discharged. 



384. Electric bells have iron bars wound around and around with 



insulated copper wire. 



