446 Mr. H. C. Sorby on Analysis of Animal and Vegetable [Apr. 11, 



the same time it is certainly a very common fact. "When the maximum 

 point of transparancy occurs between the different bands, there may be, as 

 it were, a double interval ; but then, sometimes, even this missing band 

 may be seen under favourable conditions. A difference in the general ab- 

 sorption may also somewhat alter the apparent position of a band, if it is 

 strongly shaded on one side, and not on the other ; and the presence of 

 impurities may also modify the results, so that absolute accuracy cannot 

 be expected in all cases ; and occasionally very narrow bands occur which 

 appear to belong to a second system. It must be borne in mind that the 

 bands are equidistant, only when measured by means of the interference- 

 spectrum. Thus, in the case of Alkanet-root, when measured with a mi- 

 crometer, instead of the intervals atoh and c to 6? being equal, they are related 

 to one another about as 1^ and 2, and thus the general law is entirely ob- 

 scured. If subsequent research should prove that the bands are normally 

 at equal intervals, it will be a fact of great value in deciding whether cer- 

 tain spectra are or are not due to a mixture of colours ; since, if a band 

 occurred at a perfectly unequal interval, it would show that there must be 

 at least two substances. Even in the present state of our knowledge, any 

 inequality should make us carefully search for some satisfactory reason for 

 such a divergence from a common rule. My meaning will be better 

 understood from the following examples. 



If a little of the colour of Brazil-wood be added to the solution of Alkanet- 

 root, the bands are not altered, and are seen at 4^, 5|, and 7^ ; but a 

 Uttle bicarbonate of ammonia developes a well-marked band, whose centre is 

 at 5i, and therefore at an interval of 1, instead of 1 j. The same is also 

 well seen in the case of a mixed solution of Brazil-wood and blue Lobelia. 

 I therefore argue that if an unknown substance gave rise to similar spectra, 

 with bands at unequal intervals, we ought strongly to suspect that it was 

 either naturally composite, or that some new compound had been formed 

 by decomposition. As a very good illustration I may refer to the product 

 of the action of acids on chlorophyll. The band in the red is not at an 

 equal interval ; but, on careful examination, it is seen to be made up of two 

 bands, the upper of which is at an equal interval. J was not aware that 

 these were due to two different substances, but was led to think it very pro- 

 bable ; and Professor Stokes informs me that he has proved it to be the case. 

 As an illustration of another kind of exception, I refer to the colouring- 

 matter of the pink Stock (Matthiola annua). The aqueous solution shows 

 two bands, whose centres are at about 3| and 5| ; and on adding ammonia 

 the upper is removed, and the lower depressed to 3|. In spirit of wine 

 they are at 4 and 5f, and ammonia developes a third at 3, which are not 

 equal intervals. However, if absolute alcohol be used, the bands are at 2|, 

 4g, and 5^, which are equal intervals ; and thus we see that the abnormal 

 inequality is due to the presence of water, which causes the spectrum to 

 be as if due to a mixture of two colours, when in reality it is the same 

 colour dissolved in two solvents. 



