46 CHEMICAL PHYSICS. 



ure. The method for making this calculation is given in the paragraph on 

 gas-analysis, for which see Index. 



In expansion of solids and liquids the heat energy applied is utilized partly 

 in forcing the molecules farther apart or overcoming cohesion, partly in raising 

 the temperature or giving the molecules greater motion, and partly in doing 

 external work, that is, work that the expanding body does in raising a load that 

 may be resting upon it, or overcoming any force exerted against it. In the 

 expansion of gases, there being no cohesion between the molecules, the heat 

 energy is utilized in raising the temperature and in doing work by the gas against 

 external force. 



Expansion is a very important thing which must be taken into account by 

 constructors, for example, of bridges, railroad beds, etc. Also in very careful 

 physical measurements, expansion of glass apparatus, solutions, barometers, 

 etc., must be carefully noted. 



Increase in temperature by heat. As was said above, the present 

 molecular theory of matter regards the molecules as in motion, and 

 the result of this energy of motion is what we call temperature, or 

 intensity or degree of heat. Anything that increases this motion of 

 the molecules, causes a rise of temperature. Our skin is possessed of 

 nerve structures by which we can judge whether a body is hot or 

 cold or whether one body is hotter than another, but these are not 

 delicate enough to make quantitative distinctions between tempera- 

 tures. For this purpose we make use of instruments called 



Thermometers. In these, use is made of the fact that bodies expand 

 while the temperature rises, and that the expansion is proportional to 

 rise of temperature. Thermometers, therefore, measure directly only 

 expansion, and indirectly degrees of heat, but the expansion is used 

 as a measure of the degree of heat. The most common thermometer 

 is the mercury thermometer. This instrument may be easily con- 

 structed by filling a glass tube, having a bulb at the lower end, with 

 mercury, and heating the tube until the mercury boils and all air has 

 been expelled, when the tube is sealed. It is then placed in steam 

 arising from water boiling actively under normal barometric pressure 

 of 760 millimeters of mercury, and the point to which the mercury 

 rises is marked B. P. (boiling-point) ; after which it is placed in 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 or Celsius 

 thermometer, into 80 degrees in the Reaumur thermometer, and into 

 180 degrees in the Fahrenheit thermometer. The inventor of the 

 latter instrument, Fahrenheit, commenced counting not from the 

 freezing-point, but 32 degrees below it, which causes the freezing- 



