iigC Consider at 10713 on Colcnrs, 



decoctions. They exhibited curious peculiarities; hut we 

 shall not detail them, both for the sake of brevity, and be- 

 cause they may be easily tried. 



In short, very important consequences in regard to the 

 reciprocal action of bodies and light arise from the whole 

 of these observations, and perhaps they will throw some 

 light on the grand question of the cause to which their per- 

 manent colours ought to be ascribed. 



After these researches the author concludes witli an exa- 

 mination of different phaenomena of various kinds. Me 

 indicates the modifications experienced in their coloration 

 by burning coals at different degrees of incandescence. His 

 remarks are applicable also to other bodies, such as iron in 

 a state of ignition, or a long series of reverberated lanips 

 seen during foggy weather, or a white light seen through a 

 piece of glass blackened by progressive strata of smoke. 

 In all these cases, the colours necessarily pass through a 

 series of shades, which proceed fron? white to yellow, to 

 orange and to red, more and more dark ; the reason of which 

 he explains. 



Metallic oxides have also a gradation of shades according 

 to the proportion of oxygen. A certain continued altera- 

 tion in vegetation produces one in some of the parts of 

 plants. The arts or chemical processes present one also 

 under a multitude of circumstances. 



The manufacturer may, with advantage, derive from them 

 indications either in reaard to the progress of combinations, 

 or to enable him to judge of the moment proper for per- 

 forming certain operations of his labours. 



The author then dwells more particularly on the appear- 

 ance of the coloured clouds, and especially those seen near 

 the time of the rising and setting of the sun. This phae- 

 nomenon, so generally known, had hitherto never been ex- 

 plained, though the ablest philosophers had made it an ob- 

 ject of their research. 



It does not arise from the refraction of the ravs of the 

 sun, but the successive absorption of these rays when they 

 strike the lower parts of the atmosphere and those most 

 charged with vapours. 



This absorption follows laws analogous to those already 

 mentioned. As the quantity of the vapours, and even their 

 nature, are not similar for two days insnccession, tliis irre- 

 gularity produces correfepondmg diflerences in their efiects. 



In general, the first rays attacked by these vapours arc 

 blue approaching to violet. Soon after, they absorb the 

 contiguous rays, gaining with more rapidity the blues pro- 

 perly 



