On the Fluorescence of Platino-cyanide of Potassium. 95 



1 have certainly not observed the disengagement of carbonic 

 acid during the formation of indiretine, but the evolution of gas 

 might easily elude observation during tlie long-continued boiling 

 which is necessary for the production of this substance. 



The want of material has for the present interrupted the 

 further prosecution of my experiments. As soon as I shall have 

 obtained an additional quantity of the fresh plant^ it is my inten- 

 tion to continue them. The action of acids on indican requires 

 still further examination, and the eflfects produced by ferments 

 and other agents remain to be investigated. I also propose to 

 examine other indigo-bearing plants, in order to ascertain whether 

 they contain indican or not. 



The results of the present investigation may be summed up in 

 the following propositions : — 



1. The Isatis tinctoria does not contain indigo-bluc ready 

 formed, either in the blue or colourless state. 



3. The formation of the blue colouring matter in watery ex- 

 tracts of the plant is neither caused nor promoted by the action 

 of oxygen or of alkalies. 



3. Indigo-blue cannot be said to exist in any state of combi- 

 nation in the juices of the plant : it is merely contained in them 

 potentially. 



XI. On the alleged Fluorescence of a solution of Platino-crjanide 

 of Potassium. By Professor G. G. Stokes, F.R.S. 

 To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



I was surprised, some time since, by reading in Poggendorff^s 

 Annalcn the statement of Dr. Bottger, which has since been 

 republished in the Philosophical Magazine, that a solution of 

 platino-cyanide of potassium is highly fluorescent. My own 

 observations had led me to the conclusion, that " it is only in 

 the solid state that the platino-cyanides are sensitive, — their solu- 

 tions look like mere water*." In consequence of Dr. Bottger's 

 statement, I have just examined again a solution of platino- 

 cyanide of potassium, with the advantage of bright sunshine and 

 a complete quartz train ; and so far from its being eminently 

 fluorescent, I find it to be not fluorescent at all. It is useless to 

 conjecture the cause of the discrepancy, which will probably be 

 cleared up in time ; my only object in writing is to prevent the 

 spread of an erroneous statement. 



I am, Gentlemen, 



Your faithful Servant, 

 Pembroke College, Cambridge, G. G. Stokes. 



July 5, 1855. 



* Phil. Tiaus. for 1853, p. 396. 



