SOURCES OF HEAT. 77 



Difference in temperature by touching the two cloths. The white cloth feels 

 cold in comparison with the black cloth. 



It is hardly necessary to point out experiments on the expansion of 

 bodies. They can be performed in a number of different ways; by placing, 

 water in a narrow-necked bottle, and warming it over the fire, we can 

 ascertain the expansion of liquids under the influence of heat. We may in 

 this way construct a complete thermometer. 



We may now consider the Sources of Heat, or causes of its develop- 

 ment, which are various, and in many cases apparent. The first great 

 source is the Sun, and it has been calculated that the heat received by 

 the earth in one year is sufficient to melt an envelope of ice surrounding 

 it one hundred and five feet thick. Of course the heat at the surface of 

 the sun is enormously greater than this, about one-half being absorbed in 

 the atmosphere before it reaches us at all. In fact, it is impossible to give 

 you an idea of the enormous heat given out by the sun to the earth (which 

 is a very small fraction indeed of the whole), stars, and planets, all of which 

 give out heat. We know that heat is stored in the earth, and that it is 

 in a very active condition we can perceive from the hot springs, lava, and 

 flame which are continually erupting from the earth in various places. 

 These sources of heat are beyond our control. 



But apart from the extra and intra-terrestrial sources of heat there 

 are mechanical causes for its generation upon our globe, such as friction, 

 percussion, or compression. The savage or the woodman can procure heat 

 and fire by rubbing a pointed stick in a grooved log. The wooden "breaks" 

 of a locomotive are often set on fire by friction of the wheels, so they 

 require grease, and the wheels on the rails will develop heat and sparks. 

 Our matches, and many other common instances of the generation of heat 

 (and fire) by friction, will occur to every reader. Water may be heated 

 by shaking it in a bottle, taking care to wrap something round it to keep 

 the warmth of the hand from the glass. By percussion, such as hammering 

 a nail or piece of iron, the solid bar may be made " red-hot " ; and when 

 cannon are bored at Woolwich the shavings of steel are too hot to 

 hold even if soap-and-water has been playing upon the boring- 

 machine. 



The production of heat by chemical action is termed combustion, and 

 this is the means by which all artificial heat for our daily wants is supplied. 

 We can also produce heat by electricity. A familiar and not . always 

 pleasant instance of this is seen in the flash of lightning which will fuse 

 metals, and experiment may do the same upon a smaller scale. These are, 

 in brief, the Sources of Heat, and we may speak of its effects. 



We may take it for granted that no matter from what source heat 

 is derived, it exhibits the same phenomena in its relation to objects. One 

 of the most usual of these phenomena is expansion. Let us take water, 

 and see the effect of heat upon it. 



We know that a certain weight of water under the same conditions 



