when reduced into thin Pellichs. 13 



there has been a progressive separation of molecules, in- 

 creasing, as in the case of my steel watch-spring, from the 

 spot scarcely reached by tbt heat to that which immedialdy 

 touched the flame. This movement, according to the prin- 

 ciples I have laid down, mirst have been sufficient for thie 

 production of the rings. 



We daily see many other examples of these kinds of co- 

 lours, the arrangement of the particles of which seems to 

 be the sole determining cause. Such are the stains formed 

 upon steel knives by the acid of fruits ; the black colour as- 

 sumed by silver plate from sulphurous emanations^ or from 

 the prolonged contact of certain substances ; and m che- 

 mical laboratories, the rainbow-coloured pellicles formed at 

 the surface of liquors containing some matter which has 

 been first dissolved, and afterwards separated by the progres- 

 sive evaporation of a volatile principle. The water of dung- 

 hills is also covered sometimes with similar coloured pelli- 

 cles. 



Now all these effects, whatever be the nature of the matter 

 primitively or secondarily formed, arise wholly from the ar- 

 rangement of the parts. Nothing is so easy to be ascer- 

 tained. Scrape the surfaces, rub the pellicles, and all these 

 colours are annihilated, and there remains nothing in the 

 ffagments of each substance, except particles uniformly pos- 

 sessing properties essential to their species. 



The following, among others, is a very convenient way 

 of observing these effects. I take a small quantity of the 

 green of Scheele and dissolve it in an acid ; after having di-. 

 luted this liquid in a considerable quantity of water, I pre- 

 cipitate by an alkali, and add ammonia, merely for the pur- 

 pose of redissolving the precipitate : leaving the whole after- 

 wards in an uncovered vessel, I find, after a few days, the 

 surface of the liquor covered with an iris-coloured pellicle, 

 very apparent, and in which we even distinguish a succes- 

 sion of rings, if the liquid has not been disturbed. I then 

 remove this pellicle, by slipping a sheet of paper or a piece 

 pf glass under it. The colours of the pellicle continue to 

 bevieiblc after its removal; and upon allowing the substance 

 to dry slowly, we may thus preserve them an' indefinite 



time 



