AIM AND PROGRESS OF PHYSICAL SCIENCE. 325 



is an aid to the knowledge of the conformable regularity in 

 bodily structure of an entire species of organisms. In like 

 manner, the knowledge of the specific heat of one small frag- 

 ment 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 under- 

 stand 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 concep- 

 tion ' mammal ' all that is common to the man, the ape, the 

 dog, the lion, the hare, the horse, the whale, &c., so we com- 

 prehend 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 conception 

 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 

 proportion 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 observation, 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 without exception, and make this the test of 

 its correctness. If we can be assured that the conditions under 

 which the law operates have presented themselves, the result 



