26 PRACTICAL LESSONS IN SCIENCE. 



some solid in which they are inclosed. In a gaseous body the 

 repellant forces prevail and the molecules are driven away from 

 each other to the greatest possible distance; consequently gases 

 have no independent form or volume. In the firmest solid the 

 repellant force is still active, and some degree of mobility still 

 exists, even in the hardest body. But in the case of gases the 

 particles seem to have been separated so far as to be beyond the 

 influence of cohesive attraction, so that if they ever become 

 liquids it must be through the action of some other force than 

 cohesion. 



If we apply heat to a solid, as ice, it soon becomes a liquid; 

 and if the heat continues to increase, the liquid expands into a 

 gas. If heat is removed the pressure of the air and the force of 

 gravity bring the molecules together into a liquid form, and 

 finally the cohesive force draws them into the solid form again. 

 In common language, the gas cools and condenses into a liquid 

 and the liquid cools to a solid ; that is, cold is often spoken of as 

 an active force, while it is simply a condition of a body due to 

 the diminished action of heat ; as heat diminishes, the body be- 

 comes cooler, and the attractive forces prevail more and more 

 fully. 



By temperature is meant the warmth or intensity of heat in a 

 body as compared with some standard. Temperature is not the 

 same as quantity of heat ; that is, a pond of water at a tempera- 

 ture of 40 degrees would contain more heat than a pail of water 

 at a temperature of 100 degrees. 



We can tell by touch that one body is warmer than another if 

 the difference is considerable, but the touch does not give us an 

 accurate knowledge of the difference. In some substances there 

 is a definite amount of expansion or contraction for a definite 

 increase or diminution of the intensity of heat! This makes it 

 possible to construct an instrument called a thermometer, or 

 heat measurer, which, by the expansion or contraction of some 

 substance, enables us to get an accurate idea of the relative tem- 

 perature of bodies. In the thermometer for ordinary use the 

 temperature is indicated by the expansion or contraction of 



