DIVISIBILITY. 



27 



FIG. 3. 



100 



212 



melting ice, and the point to which the mercury sinks is marked F. P. 

 (freezing-point). The distance between the boiling- and freezing- 

 points is then divided into 100 degrees in the so-called centigrade 

 thermometer, or into 180 degrees in the 

 Fahrenheit thermometer. The inventor of 

 the latter instrument, Fahrenheit, com- 

 menced counting not from the freezing- 

 point, but 32 below it, which causes the 

 freezing-point to be at 32, the boiling- 

 point at 180 above, it, or at 212. (Fig. 3.) 



Molecular motion. Heat is but one of 

 the results of molecular motioijL ; other results 

 are light, actinism, electricity, and magnjet- 

 igjn. 



When a body is heated the molecules vibrate 

 quicker, and this molecular motion gives rise to 

 heat waves in the assumed surrounding and all- 

 pervading medium called ether; if the heating be 

 continued to a higher degree, the body begins to 

 give out light, which is due to ether waves of 

 shorter length ; and if heated yet higher, it gives 

 out not only dark heat waves and light waves, but 

 also waves of still shorter length, which make 

 no direct impression on our senses, but which are 

 capable of producing chemical changes in certain 

 substances, and are known as actinic waves. Of 

 the character of the molecular motion causing 

 electricity and magnetism we know little, and 

 the various theories which have been advanced 



in order to explain electrical phenomena are inadequate and insufficient to do 

 go satisfactorily. 



Specific heat. Equal weights of different substances require dif- 

 ferent quantities of heat to raise them to the same temperature. For 

 instance : The same quantity of heat which is sufficient to raise one 

 pound of water from 60 to 70 will raise the temperature of one 

 pound of olive oil from 60 to 80, or two pounds of olive oil from 

 60 to 70. Olive oil consequently requires only one-half of the heat 

 necessary to raise an equal weight of water the same number of 

 degrees, {f Specific heat is therefore the heat required to raise a certain 

 weight of a substance a certain number of degrees, compared with^C 

 the heat required to raise an equal weight of water the same number 

 of degrees. | 



Centigrade. Fahrenheit. 

 Thermometric scales. 



