370 AIM AND PROGRESS OF PHYSICAL SCIENCE. 



the specific heat of one small fragment of a new metal is 

 important because we have no grounds for doubting that 

 any other pieces of the same metal subjected to the same 

 treatment will yield the same result. 



To find the law by which they are regulated is to 

 understand phenomena. For law is nothing more than 

 the general conception in which a series of similarly 

 recurring natural processes may be embraced. Just as 

 we include in the conception ' mammal ' all that is common 

 to the man, the ape, the dog, the lion, the hare, the horse, 

 the whale, &c., so we comprehend in the law of refraction 

 that which we observe to regularly recur when a ray of 

 light of any colour passes in any direction through the 

 common boundary of any two transparent media. 



A law of nature, however, is not a mere logical con- 

 ception that we have adopted as a kind of memoria 

 technica to enable us to more readily remember facts. 

 We of the present day have already sufficient insight to 

 know that the laws of nature are not things which we can 

 evolve by any speculative method. On the contrary, we 

 have to discover them in the facts ; we have to test them 

 by repeated observation or experiment, in constantly new 

 cases, under ever-varying circumstances ; and in propor- 

 tion only as they hold good under a constantly increasing 

 change of conditions, in a constantly increasing number 

 of cases and with greater delicacy in the means of ob- 

 servation, does our confidence in their trustworthiness 

 rise. 



Thus the laws of nature occupy the position of a power 

 with which we are not familiar, not to be arbitrarily 

 selected and determined in our minds, as one might 

 devise various systems of animals and plants one after 

 another, so long as the object is only one of classification. 

 Before we can say that our knowledge of any one law 

 of nature is complete, we must see that it holds good 



