SEAT. 45 



temperature noted in boreholes sunk into the earth's crust ; mechan- 

 ical actions, as friction and compression ; electric currents ; and, finally, 

 chemical action, as witnessed in the ordinary processes of combustion 

 and of animal life. 



Up to within a few years ago the principal method for generating heat de- 

 pended on combustion. Heat generated by electricity is now at our command. 

 The introduction of the electric furnace has provided means for obtaining much 

 higher temperatures than formerly. It is through these modern means that 

 temperatures are produced sufficiently high for the liquefaction or volatilization 

 of all substances. 



On the other hand, during the last few years methods have been invented 

 for producing very much lower temperatures than formerly. Several gases 

 which were believed to be permanent can now be liquefied, and even solidified. 



Heat Effects. The most familiar changes resulting from the 

 application of heat, that is, from the absorption of heat energy, are 

 those affecting the volume, the temperature, and the molecular arrange- 

 ment or physical state of matter. These changes do not take place 

 independently, but accompany one another. Chemical changes are 

 also often produced by application of heat, but these are considered 

 in chemistry. 



Increase of volume by heat. As a general rule^ the volume of 

 any mass increases with increase in its temperature, but this increase is 

 not alike for all matter. Gases expand more than liquids, liquids 

 more than solids, and of the latter the metals more than most other 

 solid substances. While the expansion of any two or more different 

 solids or liquids is not alike, gases show a fixed regularity in this 

 respect, namely, all gases without exception expand or contract 

 alike when the temperature is raised or lowered an equal number of 

 degrees. 



This expansion or contraction of gases is 0.3665 per cent., or ^ of their 

 volume at C. for every degree centigrade ; thus 100 volumes of air become 

 100.3665 volumes when heated from to 1 C., or 136.65 when heated from 

 to 100 C. This regularity in the expansion and contraction of gases is 

 expressed in the Law of Charles, which says : If the pressure remain constant, the 

 volume of a gas increases regularly as the temperature increases, and decreases as 

 the temperature decreases. If heat be applied to a gas confined in a closed ves- 

 sel and be thus prevented from expanding, the increase of heat will manifest 

 itself as pressure, which rises with the same regularity as shown for expansion, 

 viz., 0.3665 per cent, for every degree centigrade. 



It often becomes necessary to reduce the volume of a gas measured at any 

 temperature and at any pressure to the volume it would occupy at C. and 

 760 mm. pressure, which have been adopted as normal temperature and press- 



