244 Mr. C. Tomlinson on some Effects of 



also any vessel or surface wiped with a cloth that has been ex- 

 posed to the air is chemically unclean. In some cases bodies that 

 have been exposed to the oxidizing influences of the air become 

 chemically unclean. 



A good test of the chemical purity of the surface of water is 

 afforded by camphor. If a few fragments be scraped from a fresh 

 cut surface with the point of a knife, and be allowed to fall upon 

 water, they rotate with amazing velocity, and sweep over the 

 surface, if the water be chemically clean ; but if not, the frag- 

 ments lie on the surface perfectly motionless. 



The morning of September 1st was sunny and bright and 

 the air dry, conditions highly favourable to the camphor- mo- 

 tions, which depend as much on evaporation as on solution. 

 On this occasion I washed four shallow glasses, A, B, C, D, 

 each 3J inches in diameter, with strong sulphuric acid, rinsed 

 them out with water, and then filled them with water from the 

 cistern-tap. Camphor was very active on all four surfaces. I 

 now put my finger into A, my tongue into B. Fresh fragments 

 were inert upon A, but as active as before upon B — showing that 

 the finger was chemically unclean, and that the tongue was 

 clean, or rather that, instead of depositing a film on the water, 

 it absorbed water and any possible film with it. The water was 

 emptied from C, and the glass filled up from a so-called clean 

 jug from the kitchen filled from the same tap that had been used 

 for the glasses ; but the camphor fragments on C were motionless, 

 showing that the jug had imparted a film to the water in C 

 which prevented adhesion. The water from D was also thrown 

 out and the glass rubbed and polished with a dry cloth. On 

 again filling up the glass from the tap, the camphor fragments 

 were motionless on the surface of the water, showing that the 

 glass-cloth, commonly called clean, had imparted a film to the 

 water. 



It will be seen from these experiments how easily a chemi- 

 cally clean glass or a water-surface may become contaminated 

 and rendered chemically unclean. In some cases mere expo- 

 sure to the air during a few minutes is sufficient to convert 

 a clean into an unclean surface, and to arrest processes which 

 appear to be going on with great vigour. In other cases these 

 very processes provide for and maintain the conditions of che- 

 mical purity, as, for example, in the camphor-wheels and cur- 

 rents which I have elsewhere described * A stick of camphor, 

 held by forceps, is lowered so as just to dip below the surface of 

 water which has been very lightly dusted with lycopodium 

 powder. As soon as the camphor touches the surface, a clear 



* 'Experimental Essa3's,' published in Weale's Series, p. 38, figs. 9, 10, 

 and 11. 



