282 OPERATIONS SUBSIDIARY TO INDUCTION. 



rous plant is a silica, if it is four times as long as it is 

 broad, but if it be shorter than this it is a silicula. 

 Such terms being established, the form of the very 

 complex leaf or frond of a fern* is exactly conveyed 

 by the following phrase : ' fronds rigid pinnate, 

 pinnse recurved subunilateral pinnatifid, the segments 

 linear undivided or bifid spinuloso-serrate.' 



" Other characters, as well as form, are conveyed 

 with the like precision : Colour by means of a classi- 

 fied scale of colours. . . . This was done with most 

 precision by Werner, and his scale of colours is still 

 the most usual standard of naturalists. Werner also 

 introduced a more exact terminology with regard to 

 other characters which are important in mineralogy, 

 as lustre, hardness. But Mohs improved upon this 

 step by giving a numerical scale of hardness, in 

 which talc is 1, gypsum 2, calc spar 3, and so on. 

 . . . Some properties, as specific gravity, by their 

 definition give at once a numerical measure ; and 

 others, as crystalline form, require a very consider- 

 able array of mathematical calculation and reasoning, 

 to point out their relations and gradations." 



$ 3. Thus far of Descriptive Terminology, or of 

 the language requisite for placing upon record our 

 observation of individual instances. But when we 

 proceed from this to Induction, or rather to that 

 comparison of observed instances which is the pre- 

 paratory step towards it, we stand in need of an 

 additional and a different sort of general names. 



Whenever, for purposes of Induction, we find it 

 necessary to introduce (in Mr. WhewelFs phraseology) 

 some new general conception; that is, whenever the 



* " Hymenophyllutn Wilsoni.' 



