Discrimination of Organic Bodies by their Optical Properties. 395 



marked characters with reference to different parts of the spectrum, 

 to determine what portion of these characters belongs to one sub- 

 stance, and what portion to another, it presents much greater diffi- 

 culties. It was with reference to this subject that the second of the 

 objects mentioned at the beginning of the discourse had been spoken 

 of as that the attainment of which was by far the more difficult. 

 The problem can, in general, be solved only by combining processes 

 of chemical separation, especially fractional separation, with optical 

 observation. When a solution has thus been sufficiently tested, 

 those characters which are found always to accompany one another, 

 in, as nearly as can be judged, a constant proportion, may, with the 

 highest probability, be regarded as belonging to one and the same 

 substance. But while a combination of chemistry and optics is in 

 general required, important information may sometimes be obtained 

 from optics alone. This is especially the case when one at least of 

 the substances present is at the same time fluorescent and peculiar 

 in its mode of absorption. 



To illustrate this the case of chlorophyll was referred to. An 

 eminent French chemist, M. Fremy, proposed to himself to examine 

 whether the green colour were due to a single substance, or to a 

 mixture of a yellow and a blue substance. By the use of merely 

 neutral bodies, he succeeded in separating chlorophyll into a yellow 

 substance, and another which was green, but inclining a little to 

 blue ; but he could not in this way get further in the direction of 

 blue. He conceived, however, that he had attained his object by 

 dissolving chlorophyll in a mechanical mixture of ether and hydro- 

 chloric acid, the acid on separation showing a fine blue colour, while 

 the ether was yellow. Now solutions of chlorophyll in neutral sol- 

 vents, such as alcohol, ether, &c, show a lively fluorescence of a 

 blood-red colour ; and when the solution is examined in a pure spec- 

 trum, the red fluorescence, very copious in parts of the red, compa- 

 ratively feeble in most of the green, is found to be very lively again 

 in the blue and violet. Now a substance of a pure yellow colour, 

 and exercising its absorption therefore, as such substances do, on the 

 more refrangible rays, would not show a pure red fluorescence. 

 Either it would be non-fluorescent, or the fluorescence of its solution 

 would contain (as experience shows) rays of refrangibilities reaching, 

 or nearly so, to the part of the spectrum at which the fluorescence, 

 and therefore the absorption, commences ; and therefore the fluor- 

 escent light could not be pure red, as that of chlorophyll is found to 

 be even in the blue and violet. The yellow substance separated by 

 M. Fremy, by the aid of neutral reagents, is, in fact, non-fluorescent. 

 Hence the powerful red fluorescence in the blue and violet can only 

 be attributed to the substance exercising the well-known powerful 

 absorption in the red, which substance must therefore powerfully 

 absorb the blue and violet. We can affirm, therefore, a priori, that 

 if this substance were isolated it would not be blue, but only a 

 somewhat bluer green. The blue solution obtained by M. Fremy owes 

 in fact its colour to a product of decomposition, which when dissolved 

 in neutral solvents is not blue at all, but of a nearly neutral tint, show- 

 ing, however, in its spectrum extremely sharp bands of absorption. 



