4S1 
ON THE DISCRIMINATION OF ORGANIC BODIES BY THEIR OPTICAL 
PROPERTIES. 
Delivered at the Royal Institution of Great Britain, on Friday, March 4, 1864. 
BY PROFESSOR G. G. STOKES, M.A., D.C.L., SEC. R.S. 
The chemist who deals with the chemistry of inorganic substances has ordinarily 
under his hands bodies endowed with very definite reactions, and possessing great sta¬ 
bility, so as to permit of the employment of energetic reagents. Accordingly he may 
afford to dispense with the aids supplied by the optical properties of bodies, though even 
to him they might be of material assistance. The properties alluded to are such as can 
be applied to the scrutiny of organic substances; and therefore the examination of the 
bright lines in flames and incandescent vapours is not considered. This application of 
optical observation, though not new in principle (for it was clearly enunciated by Mr. 
Fox Talbot more than thirty years ago), was hardly followed out in relation to chemistry, 
and remained almost unknown to chemists until the publication of the researches of 
Professors Bunsen and Kirchhoff, in consequence of which it has now become universal. 
But while the chemist who attends to inorganic compounds may confine himself with¬ 
out much loss to the generally-recognized modes of research, it is to his cost that the 
organic chemist, especially one who occupies himself with proximate analysis, neglects 
the immense assistance which in many cases might be afforded him by optical examina¬ 
tion of the substances under his hands. It is true that the method is of limited applica¬ 
tion, for a great number of substances possess no marked optical characters; but when 
such substances do present themselves, their optical characters afford facilities for their 
chemical study of which chemists generally have at present little conception. 
Tw r o distinct objects may be had in view in seeking for such information as optics 
can supply relative to the characters of a chemical substance. Among the vast number 
of substances which chemists have now succeeded in isolating or preparing, and which 
in many cases have been but little studied, it often becomes a question whether two 
substances, obtained in different ways, are or are not identical. In such cases an optical 
comparison of the bodies will either add to the evidence of their identity, the force of the 
additional evidence being greater or less according as their optical characters are more 
or less marked, or will establish a difference between substances which might otherwise 
erroneously have been supposed to be identical. 
The second object is that of enabling us to follow a particular substance through mix¬ 
tures containing it, and thereby to determine its principal reactions before it has been 
isolated, or even when there is small hope of being able to isolate it; and to demonstrate 
the existence of a common proximate element in mixtures obtained from two different 
sources. Under this head should be classed the detection of mixtures in what were sup¬ 
posed 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 attainment 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 relation that is available for the object in view. 
Refractive power, for instance, though constituting, like specific gravity, etc., one of the 
characters of any particular pure substance, is useless for the purpose of following a sub¬ 
stance in a mixture containing it. The same may be said of dispersive power. The pro¬ 
perties 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 infonnation respecting that property on which the colour 
depends; for the same tint may be made up in an infinite number of ways from the con¬ 
stituents of white light. In order to observe what it is that the body does to each con- 
* The detection of mixtures by the microscopic examination of intermingled 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. 
