366 INDUCTION. 



This supposition by no means requires that the effect should not, during 

 its progress, undergo many modifications besides those of quantity, or that 

 it should not sometimes appear to undergo a very marked change of char- 

 acter. This may be either because the unknown cause consists of several 

 component elements or agents, whose effects, accumulating according to 

 different laws, are compounded in different proportions at different periods 

 in the existence of the organized being ; or because, at certain points in its 

 progress, fresh causes or agencies come in, or are evolved, which intermix 

 their laws with those of the prime agent. 



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 xchy 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 dei'iva- 

 tion 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 

 shonld also know what are its limits ; under what conditions it would cease 

 to be fulfilled. 



The periodical return of eclipses, as originally ascertained by the perse- 

 vering 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 ai'e 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 uni- 

 versal 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 

 atoms of acid required to neutralize one atom of any base is equal to the 

 number of atoms of oxygen in the base ; that the solubility of substances 

 in one another depends,* at least in some degree, on the similarity of 

 their elements. 



An empirical law, then, is an observed uniformity, presumed to be re- 



* Thus water, of which eight-ninths in weight are oxygen, dissolves most bodies which 

 contain a high proportion of oxygen, such as all the nitrates (which have more oxygen than 

 any others of the common salts), most of the sulphates, manj' of the carbonates, etc. Again, 

 bodies largely composed of combustible elements, like hydrogen and carbon, are soluble in 

 bodies of similar composition ; resin, for instance, will dissolve in alcohol, tar in oil of turpen- 

 tine. This empirical generalization is far from being universally true ; no doubt because it is 

 a remote, and therefore easily defeated, result of general laws too deep for us at present to 

 penetrate ; but it will probably in time suggest processes of inquiry, leading to the discovery 

 of those laws. 



