Discrimination of Organic Bodies by their Optical Properties, 389 



tion of mixtures in what were supposed to be solutions of single 

 substances*. 



Setting aside the labour of quantitative determinations carried out 

 by well-recognized methods, the second object is that the attain- 

 ment of which is by far the more difficult. It involves the methods 

 of examination required for the first object, and more besides ; and 

 it is that which is chiefly kept in view in the present discourse. 



The optical properties of bodies, properly speaking, include every 

 relation of the bodies to light ; but it is by no means every such rela- 

 tion that is available for the object in view. Refractive power, for 

 instance, though constituting, like specific gravity, &c, one of the 

 characters of any particular pure substance, is useless for the pur- 

 pose of following a substance in a mixture containing it. The same 

 may be said of dispersive power. The properties which are of most 

 use for our object are, first absorption, and secondly fluorescence. 



Colour has long been employed as a distinctive character of bodies ; 

 as, for example, we say that the salts of oxide of copper are mostly 

 blue. The colour, however, of a body gives but very imperfect in- 

 formation respecting that property on which the colour depends ; for 

 the same tint may be made up in an infinite number of ways from 

 the constituents of white light. In order to observe what it is that 

 the body does to each constituent, we must examine it in a pure 

 spectrum. [The formation of a pure spectrum was then explained, 

 and such a spectrum was formed on a screen by aid of the electric 

 light. On holding a cell containing a salt of copper in front of the 

 screen, and moving it from the red to the violet, it was shown to cast 

 a shadow in the red as if the fluid had been ink, while in the blue 

 rays it might have been supposed to have been water. Chromate of 

 potash similarly treated gave the reverse effect, being transparent in 

 the red, and opake in the blue. Of course the transition from trans- 

 parency to opacity was not abrupt ; and for intermediate colours the 

 fluids caused a partial darkening. Indeed, to speak with mathema- 

 tical rigour, the darkening is not absolute even when it appears the 

 greatest ; but the light let through is so feeble that it eludes our 

 senses. In this way the behaviour of the substance may be examined 

 with reference to the various kinds of light one after another ; but in 

 order to see at one glance its behaviour with respect to all kinds, it 

 is merely requisite to hold the body so as to intercept the whole 

 beam which forms the spectrum — to place it, for instance, immedi- 

 ately in front of the slit.] 



To judge from the two examples just given, it might be supposed 

 that the observation of the colour would give almost as much infor- 

 mation as analysis by the prism. [To show how far this is from being 

 the case, two fluids very similar in colour, port-wine and a solution 

 of blood, were next examined. The former merely caused a general 

 absorption of the more refrangible rays; the latter exhibited two 



* The detection of mixtures by the microscopic examination of inter- 

 mingled crystals properly belongs to the first head, the question which the 

 observer proposes to himself being, in fact, whether the pure substances 

 forming the individual crystals are or are not identical. 



