xxx.] CLASSIFICATION. 731 



inference, because so far as difference exists, inference is 

 impossible. But classification consists in detecting resem- 

 blances of all degrees of generality, and ascertaining 

 exactly how far such resemblances extend, while assigning 

 precisely the points at which difference begins. It enables 

 us, then, to generalise, and make inferences where it is 

 possible, and it saves us at the same time from going too 

 far. A full classification constitutes a complete record of 

 all our knowledge of the objects or events classified, and 

 the limits of exact knowledge are identical with the limits 

 of classification. 



It must by no means be supposed that every group 

 of natural objects will be found capable of rigorous 

 classification. There may be substances which vary by 

 insensible degrees, consisting, for instance, in varying 

 mixtures of simpler substances. Granite is a mixture 

 of quartz, felspar, and mica, but there are hardly two 

 specimens in which the proportions of these three con- 

 stituents are alike, and it would be impossible to lay 

 down definitions of distinct species of granite without 

 finding an infinite variety of intermediate species. The 

 only true classification of granites, then, would be founded 

 on the proportions of the constituents present, and a 

 chemical or microscopic analysis would be requisite, in 

 order that we might assign a specimen to its true position 

 in the series. Granites vary, again, by insensible degrees, 

 as regards the magnitude of the crystals of felspar and 

 mica. Precisely similar remarks might be made concern- 

 ing the classification of other plutonic rocks, such as 

 syenite, basalt, pumice-stone, lava. 



The nature of a ray of homogeneous light is strictly 

 defined, either by its place in the spectrum or by the cor- 

 responding wave-length, but a ray of mixed light admits 

 of no simple classification ; any of the infinitely numerous 

 rays of the continuous spectrum may be present or absent, 

 or present in various intensities, so that we can only class 

 and define a mixed colour by defining the intensity and 

 wava-length of each ray of homogeneous light which is 

 present in it. Complete spectroscopic analysis and the 

 determination of the intensity of every part of the spec- 

 trum yielded by a mixed ray is requisite for its accurate 

 classification. Nearly the same may be said of complex 



