96 Geheimrath Professor Otto N. Witt [March 21, 



When I received from Sir William Crookes the flattering invita- 

 tion to speak before you this evening, my thoughts naturally wandered 

 back to some recollections in connection with this Institution. I re- 

 membered vividly several brilliant lectures to which I had the privilege 

 of listcDing in this room, where the spirits of Davy and Faraday, of 

 Graham and Huxley, of Wiirtz and of my immortal friend A. W. von 

 Hofmaun seem still to be hovering. I felt loth to raise my own voice 

 in such hallowed precincts. But then I also remembered an almost 

 forgotten episode in my own life, which I ask your permission 

 to tell. 



I remembered, that almost exactly five-aud-twenty years before 

 receiving this invitation, I, then a very young chemist, had read 

 before the Chemical Society of London, a paper containing a then 

 somewhat daring speculation on the connection of the constitution 

 of colouring-matters with their properties, a paper which the Publi- 

 cation Committee refused to print. A lively discussion followed, 

 which was wound up by some encouraging remarks from the presi- 

 dent, the late Mr. De la Rue. He said, that he hoped, that this 

 speculative paper would prove useful in clearing up the complicated 

 domain of colouring-matters, and that perhaps on some future occa- 

 sion I should be in a position to place before the world, in a Boyal 

 Institution lecture, the results which had been obtained by its help. 



This strange reminiscence, coupled with the curious fact that 

 Mr. De la Eue's prophetic words were fulfilled just when the period 

 commonly assigned to a Jubilee had elapsed, gave me the courage 

 to accept Sir William's kind invitation. For though I have done 

 comparatively little towards the increase of our knowledge of colouring- 

 matters, the five- and-t wen ty years past have sufficed to shed a 

 brilliant light on what Mr. De la Eue could then justly call a very 

 imperfectly known domain of chemistry, and innumerable facts 

 brought to light during this period by a whole army of assiduous 

 workers are now by common assent being classified under a theory 

 which is neither more nor less than the suggestions contained in that 

 rejected paper of mine, which I had fortunately published in another 

 journal. 



I may add, that I have been guilty in later times of another 

 theory, which refers to the domain of dyeing, and which has still 

 many opponents. This theory is the direct outcome of the theory 

 of colouring-matters, and may be illustrated by some simple, yet 

 striking experiments, some of which I intend to show you. 



A fundamental question in the chemistry of dye-stuflfs, and one 

 not at all easy to answer, is this : " What is a dye-stuff"? " Clearly 

 it is something totally different from a substance only endowed with 

 the power of selective absorption of light, a power which causes it 

 to appear coloured. We know now that there are more substances 

 in creation which possess this power than bodies which lack it. In 

 this very room we have learned, that the air itself, through which 

 the solar rays penetrate on to the surface of the earth, is blue and 



