CONSIDERATIONS OK COIOVKS., j ] 9 



ceffion, never by fits, till it reaches the laft. In confeqoence 

 the body grows darker and darker, and always finiibes with 

 becoming black. Sometimes it extends only on one iide from 

 the rays firft abforbed ; at other times on both fides at once, 

 and either with equal pace, or more rapidly on one fide than 

 on the other. 



If we vary each particular that affeels the experiment fepa- Change of dr- 

 rately, we fhall have a diitina progreffion of remits. That ^ t t^efJt% 

 depending on the density of the futbflanceis not always fimilar 

 to that arifing from change of thicknefs. In receiving light 

 of different kinds too on the fame fubflance, the progrefs of 

 abforption is differently modified, and consequently the colours 

 changed. 



Our author adduces in fiances of all thefe cafes. He takes 

 them from the numerous experiments he has made with 

 coloured glafs, acid, or alcaline folutions of metals, and fluids 

 tinged by the infufion or decoction of vegetable fubftances. 

 Thefe exhibit curious particularities, but we mall not here 

 relate them, both for the fake of brevity, and becaufe it is 

 eafy for any perfon to obferve them, when once the track is 

 pointed out. 



From all thefe obfervations taken together, many very im-May lead totb* 

 portant conferences refpe&ing the reciprocal action of bodies ^ u n ° t ^'" 

 and light on each other are drawn; and perhaps at fome 

 future period they will tend to elucidate the grand queftion 

 concerning the caufe to which their permanent colours are to 

 be afcribed. 



After thefe hints, the author dedicates a concluding para- Colonr of bodies 

 graph to the examination of feveral phenomena of different *' djffere ^ 



n m r r h degrees of m- 



kinds. He points out the modifications that coals heated to candefcence. 



different degrees of incandefcence undergo in their colours. 



His remarks apply to other fubftances likewife, as iron in the Lamps feen 



irate of ignition, a long row of lamps with reverberators feen ^"Lou"*/ 



through a fog, or a white light feen through a glafs blackened fmoaked glafs, 



by progreffive applications of fmoke. In all thefe cafes the 



colours neceflfarily pafs through a feries of tints from white to 



yellow, orange, and red of a deeper and deeper (hade, the 



reafon of which he gives. 



Metallic oxides too have a gradation of tints, according to Colours of me- 

 their proportion of oxigen. A certain continued change in P f * ° t *^" e ^ 



vegetation the oxigen. 



