82 \V1-IAT IS SCIENCE? 



impacts on the walls of the vessel tends to force the walls 

 outwards and represent the pressure on them ; and by 

 their motion, heat is conveyed from one part of the gas 

 to another in the manner called conduction. 



When it is said that this theory explains the laws of 

 gases, two things are meant. The first is that if we 

 assume the theory to be true we can prove that the laws 

 that are to be explained are true. The molecules are 

 supposed to be similar to rigid particles, such as marbles 

 or grains of sand ; we know from the general laws of 

 dynamics (the science which studies how bodies move 

 under forces), what will be the effect on the motions of 

 the particles of their collisions with each other and with 

 the walls ; and we know from the same laws how great 

 will be the pressure exerted on the walls of the vessel 

 by the impacts of a given number of particles of given 

 mass moving with given speed. We can show that par- 

 ticles such as are imagined by the theory, moving with 

 the speed attributed to them, would exert the pressure 

 that the gas actually exerts, and that this pressure 

 would vary with the volume of the vessel and with the 

 temperature in the manner described in Boyle's and Gay- 

 Lussac's Laws. In other words, from the theory we can 

 deduce the laws. 



This is certainty one thing which we mean when we 

 say that the theory explains the laws ; if jthe laws 

 not be deduced from the theory, the thegi^woul 

 eXpIairTtheTaws^arrd the theory would not be true- But 

 tKrs"canfTOlrbe aH that we mean. For, if it were, clearly 

 any other theory from which the laws could be deduced, 

 would be equally an explanation and would be equally 

 true. But there are an indefinite number of " theories " 

 from which the laws could be deduced ; it is a mere 

 logical exercise to find one set of propositions from which 

 another set will follow ; and anyone could invent in a few 

 hours twenty such theories. For instance, that the two 



