a Chemically Clean Surface. 213 



lization. After some hours the tubes were taken out, the cot- 

 ton-wool removed, when the solution in one tube immediately 

 became solid, and that in the other tube did the same the mo- 

 ment it was touched with a bit of wire. 



I sent in a note of these experiments to the Royal Society by 

 way of addendum to a paper " On Supersaturated Saline Solu- 

 lutions" which I had the honour of submitting to that body on 

 the 28th of May last*. I also brought the substance of that 

 note before the Chemical Section of the British Association at 

 Norwich on the 20th ult. The subject excited some interest, 

 and several gentlemen who spoke on the occasion called for a 

 definition of a chemically clean, in contradistinction to a dirty 

 surface, one gentleman quoting the adage that dirt is something 

 in the wrong place. 



As a suggestion from any one of the distinguished obser- 

 vers who spoke on this occasion must be highly prized by me, 

 and as all of them seemed, to think I had not sufficiently defined 

 the basis of my theory, I adopt the course recommended by no 

 less a man than Descartes — that is, not to answer an objection at 

 the time it is made, but to carry it home, consider it well, and 

 then reply to it in writing. I therefore suspend for a short time 

 the further prosecution of this subject in order to define more 

 precisely what I mean by chemical purity, and to illustrate that 

 definition by briefly recapitulating the work that has been done 

 by means of chemically clean surfaces. The work that remains 

 to be done seems to me to be as important as any that has 

 yet been accomplished. There are several sets of phenomena at 

 present wrapped up in vague molecular theories, which I am 

 presumptious enough to think admit of simple explanation under 

 the theory I am advocating. 



In the first place, what is a chemically clean surface ? 



A chemically clean surface is one that has on it no film or 

 coating of any substance whatsoever foreign to its own compo- 

 sition. As oxidation by the air, organic matter, and floating- 

 motes are the most usual forms of films, we might say loosely, 

 that any substance which has been exposed for some time to the 

 air is chemically unclean; but speaking strictly, a film of any 

 foreign matter will render a surface unclean for some conditions 

 or other in the experiments in hand. 



A chemically unclean surface, then, may be generally defined 

 as anything that is exposed to the products of respiration or of 

 combustion, or to the touch, or to the motes and dust of the 

 air, and so becomes covered with a film more or less organic. So 



* Proceedings of the Royal Society, vol. xvi. p. 40,'3. See also ' Che- 

 mical News ' for July 3rd, 1868, and the Philosophical Magazine for Sep- 

 tember 1868. 



R2 



