14 



HEAT. 



The bulb with the long tube a, full of 

 air, has its open end plunged in the 

 jar of water b ; the heat of the spirit 

 lamp c being applied, the air will be 

 expanded by the heat, and a portion of 

 it will be expelled ; it will rise through 

 the water of the jar and escape : the 

 lamp being removed, as soon as the 

 remaining air cools, it will contract to 

 its original bulk, and the pressure of 

 the air on the surface of the water in 

 the jar will force water up into the 

 bulb with great velocity, and the quan- 

 tity of water that enters the bulb will be 

 equal in bulk to the air expelled. 



Convincing demonstrations of the 

 power, wisdom, and goodness of Pro- 

 vidence are obtained by watching the 

 silent, and often unobserved, but irresis- 

 tible agency of heat, in the distribution 

 of temperature over the globe. Some 

 examples of this may be adduced. 



The surface of the earth in many 

 parts of the world would be excessively 

 and injuriously heated by the sun's rays, 

 if means were not provided for with- 

 drawing portions of this heat. The 

 transparent air not being heated in the 

 same degree by the influence of the sun, 

 and there being a strong tendency to an 

 equilibrium in heat, the stratum of air 

 nearest to the earth receives a portion 

 of its excessive heat ; in consequence of 

 which it is expanded, and rendered 

 lighter than the less heated air above, 

 and must therefore ascend, since it is a 

 law of nature that light fluids ascend 

 through heavier ; another portion of 

 colder air descends to the surface of the 

 earth, and is warmed, and made to as- 

 cend. Thus the earth is cooled, and 

 refreshed by the agency of air, the heated 

 particles of which are, by the principle 

 of expansion, enabled to ascend into the 

 higher regions of the atmosphere ; from 

 whence they are wafted away to colder 

 climates, to mitigate the extremes of the 

 seasons there. 



Again, cold air flowing over the sur- 

 face of the ocean, from polar regions 

 towards the equator, is veiy much 

 warmed in its progress ; if it were not, 

 the climate that we inhabit would be 

 less genial than it is. As the water of 

 the ocean does not freeze, except in very 

 high latitudes, it is considerably warmer 

 than the air which passes over it. On 

 account of the strong tendency to an 

 equilibrium, before mentioned, the air 

 which is nearest to the water receives a 

 portion of the heat which the water con- 

 tains, above that of the air ; the portions 



of air so heated ascend, while the por- 

 tions of water, which are lowered in 

 temperature, contract in bulk, become 

 heavier, and descend; other warmer 

 portions of water ascend to the surface, 

 and other colder portions of air descend ; 

 and this process goes on as long as the 

 water is warmer than the air. 



But, perhaps, some readers may find 

 difficulty in believing that such small 

 differences in the relative weights or 

 specific gravities of different portions of 

 the same fluid, should enable some to 

 ascend, and others to descend, pro- 

 ducing such important effects in the 

 economy of nature. A few experiments 

 may tend to impress conviction on their 

 minds. 



Ex. Fill a tall glass with hot water, 

 and take up in a dropping tube a little 

 cold water, slightly coloured with litmus, 

 or any other dye, that it may be better 

 seen, and let it fall gently into the hot 

 water, the end of the dropping tube 

 being held below the surface, and the 

 coloured water will fall to the bottom, 

 because being cold its specific gravity is 

 rather greater than that of the hot water. 



Perhaps it may be objected to this 

 experiment, that the colouring matter 

 added to the cold water must tend to 

 increase its weight. Any error arising 

 from this cause may easily be guarded 

 against by making the two fluids, while 

 cold, of the same specific gravity : the 

 difference between cold and hot will still 

 be found to produce the same effect. 



Ex. Fill a small glass bulb, having a 

 nairow neck, with port wine, or with a 

 mixture of water with a small quantity 

 of spirit of wine, coloured with tincture 

 of litmus, and put the bulb so filled into 

 a tall, narrow, glass jar, which must then 

 be filled up with cold water; imme- 

 diately an ascending current will be seen 

 proceeding from the bulb, the coloured 

 fluid will accumulate on the surface of 

 the water in the jar, while colourless 

 water will be seen accumulating at the 

 bottom of the bulb. By close inspec- 

 tion, the descending current may also be 

 observed ; and it will be obvious that the 

 coloured and the colourless liquids pass 

 each other in the narrow neck of the 

 bulb, without mingling. In a short time 

 the whole of the coloured liquid will 

 have ascended, and the bulb will be 

 entirely filled with clear water. 



It is proved by this experiment that 

 a very small difference in the specific 

 gravities, in two portions of a fluid, 

 enables the light portion to ascend, 



