88 WHAT IS SCIENCE? 



laws are predicted which had not been discovered at 

 the time and would never have been suspected apart 

 from the theory. Thus, it is easy to see that, if the 

 molecules have a definite size, the behaviour of a gas, 

 when the number of molecules contained in a given vessel 

 is so great that the space actually occupied by the mole- 

 cules is nearly the whole of the space in the vessel, will 

 be very different from its behaviour when there are 

 so few molecules that practically all that space is unoccu- 

 pied. This expectation, a direct result of the theory, is 

 definitely confirmed by experiments which show a change 

 in the laws of a gas when it is highly compressed, and all 

 its molecules forced into a small volume. 



This test of predicting new and true laws is always 

 applied to any theory when it is proposed. The first 

 thing we do when anyone proposes a theory to explain 

 laws, is to try to deduce from the theory, or from some 

 slight but very natural development of it, new laws, 

 which were not taken into consideration in the formulation 

 of the theory. If we can find such laws and prove by 

 experiment that they are true, then we feel much more 

 confidence in the theory ; if they are not true, we know 

 that the theory is not true ; but we may still believe that 

 a relatively slight modification will restore its value. It 

 is in this way that most new laws are actually suggested 

 for the purposes discussed in the previous chapter. At 

 the present time, in the more highly developed sciences, 

 it is very unusual for a new law to be discovered or sug- 

 gested simply by making experiments and observations 

 and examining the results (although cases of this character 

 occur from time to time) ; almost all advances in the 

 formulation of new laws follow on the invention of theories 

 to explain the old laws. Indeed it has been urged that 

 the only use of theories is thus to suggest laws among 

 which some will be found to be true. This opinion has 

 been much favoured by philosophers and mathematicians 



