CH.XLIII.] CLASSIFICATION OF THE SCIENCES. 199 



When we seek for points of connexion between the 

 sciences to serve as a basis for their classification, we come 

 on this rather embarrassing fact, tliat there are subjects Subjects 

 whicli belong to more than one science. I do not mean ^^ niore"^ 

 cases like those which occur in organic classification, where tl\an ouo 

 it is sometimes doubtful on which side of a boundary-line 

 we ought to place some particular form. I mean cases 

 where there is no doubt whatever that they belong equally 

 to two sciences. Whether, for instance, does spectrum Spectrum 

 analysis belong to optics or to chemistry ? Whether do ^"%.®i.^ '^ 

 the polarizing properties of crystals belong to optics or to crystals T 

 crystallography ? Whether do the facts of electro-chemical cheniis'try. 

 decomposition and the theory of the voltaic battery belong 

 to chemistry or to electricity ? The only possible answer 

 is, that they belong equally to both. 



This, then, is one kind of connexion between the sciences 

 — namely, that one and the same subject may belong to 

 more than one science. Another kind is when one science One 

 gives suggestions to another — such as, to mention the best science 

 instance that occurs to me, the instructive and valuable sugges- 

 idea of the physiological division of labour which the another 

 science of human society has furnished to biology. A third 

 kind of connexion is when one science furnishes materials or furnish- 

 to another, as in the case of biology furnishing to che- rifis^to'*^" 

 mistry a variety of new substances, which constitute the another, 

 subject of the branch of the latter science called organic 

 chemistry. A fourth way is when one science becomes, in or sujiply- 

 the literal sense, instrumental to another : I mean, when 2i|it"\^™' 

 one science supplies another with instruments. Thus, another. 

 optics has supplied astronomy with the telescope, thereby The 

 enabling it to explore the depths of the heavens ; and has ^''l^scope 

 supplied biology with the microscope, thereby enabling it micro- 

 to examine the minutest structure of the tissues. scope. 



It may perhaps be mentally added by the reader that Connexion 

 mathematics has become instrumental to dynamics and ^a.™cs ^^' 

 astronomy, by supplying them with methods and formulte. ^^itli .. 

 This is true, and a most important truth ; but it is one 

 of a different kind from the last mentioned. The con- 

 nexion of optics with astronomy and with biology, through 



