HEAT APPARATUS. 135 



mental verification to the elevation of the melting points of sub- 

 stances which expand on becoming liquid. 



Connected with this, and probably to be completely explained 

 by it, are the curious facts of so-called Regelation, &c., and the 

 whole theory of the motion of glaciers. [Forbes, Faraday, Dolfuss- 

 Ausset, J. Thomson, &c.] 



EVAPORATION OF LIQUIDS. 



The pressure remaining the same, there is a definite boiling point 

 for the free surface of every liquid ; and (provided the mass be 

 stirred), however much heat be applied, the temperature of the whole 

 remains at the boiling-point till the last particle is evaporated. 



The effect of pressure upon the boiling point can be calculated, 

 as was that of pressure upon the melting point ; but as we do not 

 know of a substance which (at the same temperature and pressure) 

 occupies less bulk in the form of vapour than in that of liquid, 

 we may assert that the effect of pressure is in all cases to raise the 

 boiling point. The boiling point of water has been employed to 

 supersede the barometer, as in the Hypsometric Thermometer 

 (Wollaston). 



Other instruments depending on these principles are Papin's 

 Digester, The Cryophorus, Daniell's and Regnault's Hygro- 

 meters, &c. 



Joint Effects of Pressure and Temperature on Fluids. 



Liquefaction of gases by Cold or Pressure, or by Cold and 

 Pressure. [Faraday, Thilorier, Natterer, &c.] 



The Cagniard-de-la-Tour State. 



Andrews' discovery of the Critical Temperature, and of the 

 continuity of the Liquid and Gaseous States. 



Explanation of the non-condensibility of Hydrogen, Oxygen, 

 Nitrogen, Carbonic Oxide, and Marsh-gas. 



Modification of the Critical Temperature by admixture of a non- 

 liquifiable gas. [Andrews.] 



