Manchester Memoirs, Vol. lix. (191 5), No. 11. 35 



value. All the discoveries of Pasteur hardly cost a few 

 thousands of pounds : they produced hundreds of millions 

 and alleviated many sufferings. The richest parts of the 

 earth, situated in the torrid zone, have until now been 

 poisoned by maladies which render the sojourn of 

 Europeans in these parts painful and dangerous. What 

 do the few thousands of pounds, devoted each year to the 

 study of tropical diseases, signify in comparison with the 

 milliards of which we can to-day foresee the conquest? 



The study of the natural sciences teaches us, better 

 than the other sciences, to understand the meaning of 

 the expression natural law. A lazu of nature suffers no 

 exceptions. As soon as an exception is proved, it is no 

 longer a law, but a rule, which is only applicable in certain 

 circumstances, The two ideas of law and rule are often 

 confused, which gives rise to misunderstandings, and even 

 to mistakes. 



When we say, for example, that water consists of one 

 part of hydrogen and eight parts of oxygen, we have 

 enunciated a law, because up to the present no one has 

 been able to prove an exception. 



But when we say that the price of a commodity 

 increases with the decrease of the supply or the increase 

 of the demand, and vice-versa, we have not laid down a 

 law, but a rule. Here, for example, is an exception : 

 the more a book is in demand, the lower becomes the price. 

 The relations between the price, the supply and the 

 demand are often too complicated to be expressed in a 

 single law applicable in all circumstances. It is probable 

 that many so-called laivs, often quoted in the sciences of 

 sociology and history, and held to be unquestionable, are 

 really rules. 



It *is incontestable that in the domain of the natural 

 sciences, similar confusions are very often the case. But 



