108 ELEMENTS OF LABORATORY WORK 



temperature in a gas would not be easy to observe, on account 

 of the speedy adjustment of its own temperature with that 

 of the vessel ; hence all comparisons and measurements of the 

 changes become extremely difficult. 



It is easy to show that the diminution in the pressure on a 

 gas, with its consequent expansion, takes place together with a 

 fall in temperature. If a thermometer is placed under the 

 cover of an air-pump, and a portion of the air removed by 

 means of the pump, the thermometer may easily be made to 

 mark a fall of 8 or 10. In this case we have a diminution 

 of pressure on the small quantity of air remaining which 

 enables it to expand and fill the given space. In" so doing 

 it becomes considerably colder. The same result may be 

 shown by exhausting the air from a vessel by means of a 

 filter-pump. 



If the pressure upon a gas be increased that is, if its 

 volume be diminished the temperature increases. This may 

 be easily shown by using a force-pump to compress a large 

 quantity of air within a given space containing a 'thermometer. 

 The temperature will be found to rise just as distinctly as it 

 fell during the converse process. 



Many modifications of these experiments may be made, and 

 they may be extended to other gases with the same results. 

 It is of great importance to consider these results side by side 

 with the observations that an increase in the temperature of a 

 gas, from contact with a hot body, causes its volume to 

 increase, provided the pressure remains the same ; while a 

 decrease in temperature causes a contraction in volume. These 

 apparently divergent facts are reconciled if we take into con- 

 sideration certain external changes, which must be considered 

 at a later stage. 



Similar results, with certain modifications, are found in 

 dealing with liquids and solids. For example, the stretching 

 of a wire cools it, while examples of compression causing a rise 

 of temperature are common. Friction may be looked upon as 

 a special case of compression. But the greater simplicity, to 

 be subsequently demonstrated, in the structure of gases makes 

 them easier to investigate than solids or Jiquids. 



