CHAPTER XVI. 



OF EMPIRICAL LAWS. 



1. SCIENTIFIC inquirers give the name of Empirical 

 Laws to those uniformities which observation or experiment 

 has shown to exist, but on which they hesitate to rely 

 in cases varying much from those which have been actually 

 observed, for want of seeing any reason why such a law 

 should exist. It is implied, therefore, in the notion of an 

 empirical law, that it is not an ultimate law ; that if true at 

 all, its truth is capable of being, and requires to be, accounted 

 for. It is a derivative law, the derivation of which is not yet 

 known. To state the explanation, the why, of the empirical 

 law, would be to state the laws from which it is derived ; the 

 ultimate causes on which it is contingent. And if we knew 

 these, we should also know what are its limits j under what 

 conditions it would cease to be fulfilled. 



The periodical return of eclipses, as originally ascer- 

 tained by the persevering observation of the early eastern 

 astronomers, was an empirical law, until the general laws of 

 the celestial motions had accounted for it. The following are 

 empirical laws still waiting to be resolved into the simpler 

 laws from which they are derived. The local laws of the flux 

 and reflux of the tides in different places : the succession of 

 certain kinds of weather to certain appearances of sky : the 

 apparent exceptions to the almost universal truth that bodies 

 expand by increase of temperature : the law that breeds, both 

 animal and vegetable, are improved by crossing : that gases 

 have a strong tendency to permeate animal membranes : that 

 substances containing a very high proportion of nitrogen 

 (such as hydrocyanic acid and morphia) are powerful poisons : 

 that when different metals are fused together, the alloy is 

 harder than the various elements: that the number of 



