MAXIMUM DENSITY OF WATER. 113 



been formed and empirical formula? constructed for deter- 

 mining the relation between the temperature and the elastic 

 force of vapor, at the saturating density, for certain ranges 

 of temperature. If, therefore, the dew-point be ascertained, 

 we can at once determine the pressure of the vapor in the 

 air by means of these tables. For, if t' be the dew-point, 

 and p' the corresponding pressure, then at any other tem- 

 perature t of the air above t', we have, for the required 

 pressure, 



65. Effect of Compression or Dilatation on the 

 Temperature of a Gas. It is an experimental fact that, 

 if a quantity of air be suddenly compressed, its temperature 

 is raised ; and that, if the compression be of small amount, 

 the relative increase of temperature is proportional to the 

 condensation. Thus, if the density be changed from p to 

 p', the increase of temperature is proportional to 



ff-p 



P 



If the air be allowed to dilate, its temperature is dimin- 

 ished according to the same law. A. stream of compressed 

 air when issuing from a closed vessel is sensibly chilled. The 

 reason that the compression or dilatation must be sudden, 

 is that no heat should be allowed to escape, or to be admit- 

 ted. If the experiment be performed in a non-conducting 

 vessel, there is no necessity for rapidity of action. 



66. Expansion of Bodies Maximum Density of 

 Water. In general, all solid and liquid bodies expand 

 under the action of heat, and contract when heat is with- 

 drawn. The expansion of mercury is proportional to the 

 increase of temperature, within certain limits; this is also 

 the case with solid bodies, such as glass and steel. For 



