xxx.] CLASSIFICATION. 685 



by Lord Rosse, Mr. Huggins, and others, will probably lead 

 at some future time to the discovery of important empirical 

 laws concerning the constitution of the universe. The 

 minute examination and classification of meteorites, as 

 carried on by Mr. Sorby and others, seems likely to afford 

 us an insight into the formation of the heavenly bodies. 



We should never fail to remember the slightest and most 

 inexplicable correlations, for they may prove of importance 

 in the future. Discoveries begin when we are least ex- 

 pecting them. It is a significant fact, for instance, that 

 the greater number of variable stars are of a reddish 

 colour. Not all variable stars are red, nor all red stars 

 variable ; but considering that only a small fraction of the 

 observed stars are known to be variable, and only a small 

 fraction are red, the number which fall into both classes is 

 too great to be accidental. 1 It is also remarkable that the 

 greater number of stars possessing great proper motion are 

 double stars, the star 61 Cygni being especially noticeable 

 in this respect. 2 The correlation in these cases is not 

 without exception, but the preponderance is so great as 

 to point to some natural connexion, the exact nature of 

 which must be a matter for future investigation. Herschel 

 remarked that the two double stars 61 Cygni and a Centauri 

 of which the orbits were well ascertained, evidently be- 

 longed to the same family or genus. 8 



Classification in Crystallography. 



Perhaps the most perfect and instructive instance of 

 classification which we can find is furnished by the science 

 of crystallography (p. 133). The system of arrangement 

 now generally adopted is conspicuously natural, and is even 

 mathematically perfect. A crystal consists in every part 

 of similar molecules similarly related to the adjoining 

 molecules, and connected with them by forces the nature 

 of which we can only learn by their apparent effects. But 

 these forces are exerted in space of three dimensions, so 

 that there is a limited number of suppositions which can 

 be entertained as to the relations of these forces. In one 



1 Humboldt, Cosmos (Bohn), vol. iii. p. 224. 



2 Baily, British Association Catalogue, p. 48. 



3 Outlines of Astronomy, 850, 4th edit. p. 578. 



