166 THE REALITIES OF MODERN SCIENCE 



volume V constant and vary the average kinetic energy 

 of the molecules the pressure varies proportionately, 

 that is, directly. Similarly, the equation shows that, 

 if we keep the pressure constant, the volume varies 

 directly as the average kinetic energy of the mole- 

 cules. Alterations in this kinetic energy are made by 

 adding or subtracting energy, that is, by either heat- 

 ing or cooling the body under consideration. If any 

 body, when placed in contact with the gas (or with 

 the thin walls of its container), does not cause a 

 change in the product of gaseous pressure and volume 

 there is no net change in the average kinetic energy 

 of the molecules and we say that the body has the same 

 temperature as the gas. 



It was upon exactly this principle that the first 

 thermometer operated. This was made by Galileo 

 in 1597, over two hundred years before the 

 kinetic theory of gases was first formulated. The 

 instrument was essentially of the form shown 

 in Fig. 14. You see at once how it may be 

 used by the substitution method to indicate 

 when two bodies are at the same temperature. 

 If first one and then the other is brought into 

 contact with the bulb, then, the fact of an equal- 

 ity of temperature will be indicated by the same 

 FIG. 14. position l in both cases for the liquid in the stem. 



A possible source of error in such use of a thermometer may be 

 mentioned. Suppose the two bodies, A and B, are very different 

 in mass, e.g. that A comprises a smaller number of molecules than 

 B. It requires a greater addition of energy to B than to A to pro- 

 duce the same increase in the average kinetic energy. Body B has 

 the greater "heat capacity." On the other hand, suppose the 

 atomic structure of A is less complex than that of B. Because of 



