456 



Mr. H. C. Sorby on [June 1 9 



substances have their centres nearly at equal intervals. All three kinds 

 are so rapidly decomposed by strong direct sunlight, that the study of 

 their fluorescence is a matter of some difficulty. Whilst thus alluding to 

 this subject, I would take the opportunity of describing a method which 

 I have found extremely convenient in deciding whether a substance pos- 

 sesses genuine fluorescence, or whether it is spurious and due to the 

 presence of solid particles so minute that the solution may appear quite 

 clear and transparent. I illuminate by means of light which has passed 

 through a solution of sulphate of didymium. If, then, there be only a 

 spurious fluorescence, the spectrum shows all the absorption-bands of the 

 didymium — in fact we obtain merely a spectrum of that body modified 

 by reflection ; whereas if it be a true and pure fluorescence, no trace of 

 the bands of didymium can be seen. If there be a genuine fluorescence 

 along with some reflected light, a more or less faint spectrum of didy- 

 mium will be visible due to this spurious light, with a brighter part due 

 to the genuine fluorescence. 



Action of Acids on the Chlorophyll Group. 



As previously mentioned, all the above-named members of the chloro- 

 phyll group are decomposed with greater or less facility by acids. Each 

 gives rise to an independent product ; so that, though they have many 

 generic peculiarities in common, they must be regarded as well-marked 

 species, and not in any way as mere accidental modifications of a single 

 substance. 



Xanthophyll Group. 



A considerable number of different colouring-matters might be classed 

 with this group, but on the present occasion I purpose to consider only 

 the six most commonly met with in leaves or fronds or in fungi. They 

 are characterized by being insoluble in water, but soluble in bisulphide 

 of carbon and fixed oils, and by giving spectra with two more or less 

 well-marked absorption-bands, varying in position with the particular 

 substance, and very considerably raised towards the blue end when a 

 liquid of great band-raising power is used as a solvent. When the solu- 

 tion is strong, the whole of the blue end is absorbed up to these bands. 

 Sometimes three absorption-bands may be seen ; but in that case it is 

 generally easy to prove, either by chemical or photochemical methods, 

 that the third band is due to a second substance. Weak acids or alkalies 

 have no immediate effect in altering the position of the bands ; but when 

 dissolved in absolute alcohol all the members of the group are slowly 

 decomposed by a little hydrochloric acid, gradually fading to a colourless 

 solution, even in the dark. The rate at which this takes place and the 

 production of intermediate coloured products depend to some small ex- 

 tent on the presence of air, but still more on the presence of minute 

 quantities of sensitizing substances. The nature of this action is still 



