Chap. I] EXPLANATIONS AND DEFINITIONS 9 



The reverse process by which a compound is split up into water 

 and an anhydrous compound is called dehydration. This process 

 is represented in the equations by the reverse arrows. 



ENERGY 



Energy is ordinarily defined as the power of doing work. As 

 examples of various types of energy might be mentioned : mechani- 

 cal energy, heat energy, electrical energy, and chemical energy. 

 These can be transformed from one form to another. To illustrate : 

 (1) electrical energy can be converted into energy of motion, as 

 evidenced in the motor ; (2) electrical energy can be converted into 

 heat energy, as in the electric stove ; (3) mechanical energy of 

 motion can be converted into electrical energy, as in the dynamo ; 

 also, (4) chemical energy can be transformed into heat energy, 

 as is true in the burning of wood. 



SOME PHYSICAL TERMS 



Specific gravity. — By specific gravity is meant the comparison 

 between the weight of a substance and the weight of an equal 

 volume of some other substance taken as a standard. The 

 standards usually referred to are air for gases, and water for liquids 

 and solids. For instance, the specific gravity (sp. gr.) of mercury 

 is 13.6, meaning that mercury is 13.6 times as heavy as an equal 

 volume, of water. Again, the specific gravity of carbon dioxide 

 (air standard) is 1.5, meaning that it is 1.5 times as heavy as an 

 equal volume of air. 



The specific gravity of solutions, as a salt solution, will neces- 

 sarily vary with the concentration. 



Diflfusion. — This term in its ordinary use has to do with the 

 tendency of two liquids or two gases of different densities to mix 

 uniformly. Diffusion can take place either when the substances 

 are simply superimposed, or when they are separated by a per- 

 meable membrane. The following illustrations may help to make 

 this clear. 



1. When the gases or liquids are not separated by a membrane. 



(a) If a bottle of hydrogen is inverted over a bottle of chlorine 

 gas, the lighter hydrogen molecules will move down among the 

 chlorine molecules, while the heavier chlorine molecules will move 



