MATTER FORMED OF SMALL PARTICLES 113 



an increase in the volume should cause a fall of temperature. 

 In the former case the speed of the particles which come 

 in contact with the moving boundary is increased, in the 

 latter it is diminished. The temperature- changes stated are 

 matters of direct observation, whether the hypothesis be correct 

 or not. 



The ordinary change of temperature, consequent on con- 

 tact with a body of different temperature, must be regarded, 

 in the same way, as a change in the rate of motion of the 

 particles through collision with particles moving at different 

 rates. The particles from which the change is derived are 

 not necessarily those of a gas. This explanation must be 

 looked upon as partial only ; after further investigations it 

 may be made more complete. 



It must be distinctly understood that, although all obser- 

 vations point to the probability of solids and liquids being 

 made up of minute particles similar to those constituting 

 gases, there is no reason yet adduced to suppose that Avo- 

 gadro's hypothesis can be directly applied to them. The wide 

 differences in the behaviour of various solids and liquids for 

 the same temperature and pressure changes, would rather show 

 that much wider investigations must be made before any 

 theory as to their structure can be proposed. Whatever their 

 structure may be, it is undoubtedly unlike that of gases in 

 many respects. ~ This is shown by the scarcely perceptible 

 compressibility which distinguishes most solids and liquids 

 from gases, and by the very slight trace of cohesion existing 

 between the particles of gases. At the same time, it is important 

 to remember that most kinds of matter may be made to pass 

 from one state to the others by alteration of temperature, and 

 under certain circumstances the change of state may be gradual 

 and even imperceptible. 



Additional Exercises and Questions. 



1. Connect a porous earthen war vessel, such as is used for electric 

 cells, with a thick glass tube by means of an indiarubber cork ; fill the 

 tube and vessel with coal-gas, and place it over water or mercury. Note 

 that the quantity of gas inside diminishes until a limit is reached. 



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