100 Oeheimrath Professor Otto N. Witt [March 21, 



alone is sufficient to cause the formation of this colour is perfectly 

 incomprehensible if the old views on the constitution of phtaleines, 

 which are still given in the majority of text-books, be adhered to, but 

 it is exactly what we might expect to happen if we assume that the 

 ammonium salt of phenolphtaleine possesses a cycloid constitution in 

 its ethereal solution, and that it is isomerised into the quinonoid form 

 by the addition of water.* 



Thus our knowledge of the chemical causes of the physical pro- 

 perties of colouring-matters is continuously developing. Quite lately 

 we have even begun to form definite views about the connection of the 

 chemical constitution of aromatic substances with that peculiar form 

 of selective absorption of light which we call fluorescence, and which 

 has formed, from the physical point of view, the subject of the 

 masterly investigations of Sir Gabriel Stokes. The phenomenon of 

 fluorescence is very frequently met with in dye-stuff's, and in the raw 

 materials used for their manufacture. It can be exhibited in a very 

 striking way with the help of electricity, either by allowing an arrow 

 of electric light to penetrate into the solution of a fluorescent sub- 

 stance or by working a Geissler tube of suitable shape submerged in 

 such a solution (Exp. VI.). The fact that the fluorescence of many 

 substances is chiefly caused by the ultra-violet light, I shall try to 

 demonstrate by the following, somewhat delicate, experiment : I have 

 here, submerged in a solution of cosine, a Geissler tube, the lower 

 part of which is ground out of a piece of rock-crystal, whilst the 

 upper half is made of glass. When the electric current passes this 

 tube the fluorescence round the quartz part of it is stronger than 

 that in the neighbourhood of the glass, because the latter absorbs a 

 good deal of ultra-violet light, whilst the quartz is almost free from 

 such absorption. (Exp. VII.) 



An immense amount of patient work has been accomplished by 

 many chemists in the hope of establishing definite views on the con- 

 stitution of the azo-colours, that group of dye-stuff's the introduction 

 of which into the colour-industry was the direct consequence of our 

 early eff'orts to cast off" empiricism, and to conduct our search for new 

 colouring-matters according to definite scientific principles. Simple 

 and transparent as the constitution of azo-colours appears to be if 

 viewed superficially, yet it off'ers some problems of extraordinary 



• The isomerism of the two forms of the ammonium salt of phenolphtaleine 

 is best explained by their constitutional formulee : 



/0NH4 

 C=o c=0 



»i,.A^^.™. „.A '■'\ 



Cycloid Form. Quinonoid Form. 



