148 PHYSIOLOGY FOR NURSES 



i.e., 110 two bodies can occupy the same space at the 

 same time; compressible, depending on its porosity; and 

 containing properties by which matter may be converted 

 into energy. 



Energy is capacity for work and is made known to 

 us by changes in matter. Some of the many forms of 

 energy are mechanical energy, gravity, cohesion, ad- 

 hesion, heat, light, electricity, and chemical energy. 



Mechanical energy changes the condition of masses 

 of matter as respects motion or rest (Mallet). The use 

 of water to turn a mill wheel is an illustration of me- 

 chanical energy by that natural force which we call grav- 

 ity. It is simply the weight of the water which turns 

 the wheel. If the water is too small in quantity or the 

 wheel too heavy, no movement Avill result. If the mass 

 of water is great in proportion to the resistance of the 

 wheel, the motion will be rapid. Hence mechanical 

 energy is dependent on the mass to be moved. 



Gravity is the mutual attraction Avhich masses of 

 matter exert upon each other. It increases with the 

 masses concerned and decreases with their distance from 

 one another. The earth, the largest mass of matter with 

 which we are familiar, attracts whatever is unsupported 

 in proportion to its density and the measure of this 

 attraction is the weight of the body. Various systems 

 of measuring weight are used, the avoirdupois in Eng- 

 lish-speaking countries and the Metric in France. The 

 Metric system, however, is becoming universal in 

 science. 



Specific gravity, or specific weight means the weight 

 of a given body when compared with the weight of a 

 known standard. For liquids and solids the standard 

 is distilled water at four degrees centigrade, and for 

 gases hydrogen at zero centigrade. 



