EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 167 



nature, which explains and harmonizes the motions of all 

 the heavenly bodies, that is, shows that there is a similar 

 force which governs all those motions, or the explanation 

 may involve nothing more than a single identity, as when 

 we explain the appearance of shooting stars by showing 

 that they are identical with portions of a comet. Wherever 

 we detect resemblance, there is a more or less satisfactory 

 explanation. The mind is always somewhat disquieted 

 when it meets a novel phenomena, one which is sui 

 generis; it seeks at once for any parallels which may be 

 found in the memory of past sensations. The so-called 

 sulphurous smell which attends a stroke of lightning long 

 excited the attention and fears of men, and it was not ex- 

 plained, until the exact similarity of the smell to that of 

 ozone, or allotropic oxygen, was pointed out. The marks 

 upon a flagstone are explained when they are shown 

 to correspond with the feet of an extinct animal, whose 

 bones are elsewhere found. Explanation, in fact, generally 

 commences by the discovery of some very simple re- 

 semblance ; the theory of the rainbow began as soon as 

 Antonio de Dominis pointed out the resemblance be- 

 tween its colours &id those presented by a ray of sun- 

 light passing through a glass globe full of water. 



The nature and limits of explanation can only be fully 

 considered, after we have entered upon the subject of 

 generalization and analogy. It must suffice to remark, in 

 this place, that the most important process of explanation 

 consists in showing that an observed fact is only one case 

 of a general law or tendency. Iron is always found com- 

 bined with sulphur, when it is in contact with or included 

 in coal, whereas in other parts of the coal strata it always 

 occurs as a carbonate. We explain this empirical fact as 

 being due to the ordinary reducing powers of carbon and 

 hydrogen, which prevent the iron from combining with 

 oxygen, and leave it open to the affinity of the sulphur. 



