THE EXPLANATION OF LAWS 81 



WHAT IS A THEORY ? 



How then does science explain laws ? It explains 

 them by means of " theories," which are not laws, 

 although closely related to laws. 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 

 1 1 Kin 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 

 Two of these laws state how the pressure, exerted 

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

 the volume of the vessel, and with the temperature 

 "f the gas. Boyle's Law states that the pressure is 

 ; sely proportional to the volume, so that if the volume 

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

 at a constant volume, the pressure increases propor- 

 :lly 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 tho ^,is :md its power of conducting heat and so on. 

 AS are " explained " by a doctrine known as 



Theory of Gases, which was proposed e 

 in the last and is accepted universally to-day. 



According to this theory, a gas consists of an imn 

 number of very small particles, called molccul 

 about in all < 5, colliding with each otl \vith 



<>ntaining vessel ; the speed of the ili^ht 

 ^creases with tin: temperature ; ' 



ild IHJ \\. -it the 



acre used in a strictly technical sense of 

 meaning is about to be CN ; 



ulras .-wsoo.urd with th. word in ordinary language-. In Chaplci Ylll 



ory " in centra-distinction 

 to " i 



