THE EXPLANATION OF LAWS 81 



WHAT IS A THEORY ? 



How then does science explain la ws_ ? It explains 

 them by means of " theories, n whicrT are not laws, 

 although closely relafe^iEoTaws. We will proceed at 

 once to learn what a theory is, and how it explains laws. 1 

 For this purpose an example is necessary, even though 

 its use involves entering more into the details of science 

 than is our usual practice. A great many laws are known, 

 concerning the physical properties of all gases ; air, 

 coal-gas, hydrogen and other gases, differ in their chemical 

 properties, but resemble each other in obeying these 

 laws. Two of these laws state how the pressure, exerted 

 by a given quantity of gas on its containing vessel, varies 

 with the volume of the vessel, and with the temperature 

 of the gas. Boyle's Law states that the pressure is 

 inversely proportional to the volume, so that if the volume 

 is halved the pressure is doubled ; Gay-Lussac's states 

 that, at a constant volume, the pressure increases propor- 

 tionally to the temperature (if a certain scale of tempera- 

 ture is adopted, slightly different from that in common 

 use). Other laws state the relation between the pressure 

 of the gas and its power of conducting heat and so on. 

 All these laws are " explained " by a doctrine known as 

 the Dynamical Theory of Gases, which was proposed early 

 in the last century and is accepted universally to-day. 

 According to this theory, a gas consists of an immense 

 number of very small particles, called molecules, flying 

 about in all directions, colliding with each other and with 

 the wall of the containing vessel ; the speed of the flight 

 of these molecules increases with the temperature ; their 



1 The reader should be warned that he must remember that the 

 word " theory " is here used in a strictly technical sense of which the 

 meaning is about to be explained. He must not attach to it any of the 

 ideas associated with the word in ordinary language. In Chaptei VIII 

 some reference will be made to the use of " theory " in centra-distinction 

 to " practice." 



