,120 CONSIDERATIONS ON COLOURS. 



Of flowers, &c. vegetation produces the fame effect on fome parts of plants. 



The arts and chemical procefles exhibit the fame in a multi- 



* • tude of circumfiances. 



Ufe to the ma- Hence the manufacturer may derive with advantage indica- 



nufaclurer. t j ons c ; t | lcr f tJie pr0 g re ( s G f combinations, or of the proper 



inftant for executing certain parts of his operations. 

 Coloured clouds. Our author next enters more particularly into the appear- 

 ance of* coloured clouds, particularly thofe we fee about the 

 riling and letting of the fun. This phenomenon fo generally 

 known, had hitherto remained without explanation, though 

 this had been attempted by natural philosophers of the flrft 

 rank. 

 Owing to ab- It is not owing to the refraftion of the folar rays, but to the 



nXTefraclion f ucceffive abforption of them, when they ftrike on the inferior 

 parts of the almofphere, which are more loaded with vapour. 



This abforption follows la ws analogous to thofe already men- 

 tioned. The quantity of vapours, and even their nature not 

 being the fame every day, produce correfponding differences 

 in their effects. 

 Order of ab- Commonly the firfr. rays attacked by thefe vapours are the 



n ' blue adjacent to the violet. Soon after they attack the con* 



tiguous rays, gaining with more rapidity the blue properly fo 

 called; then the green, the yellow, and thus proceeding to 

 the red. Hence the yellowifh, orange, and red colours 

 Sun-fet. exhibited by the clouds. This period of tints, the evening 



for example, difplays itfelf gradually as the fun approaches 

 the horizon. The fame hues tinge terreiirial objects, the part 

 of the air neareft. the fun, and this luminary itfelf. According- 

 ly when we can receive its rays on a prifm, we perceive that 

 the rays actually abforbed correfpond to the general tint of 

 the moment. 



From the fucceflive increafe of the vapours traverfed by 



the light in thicknefs and denfity, it follows liI<Lew»fe, that at 



the fame infUnt clouds differently placed mud be clothed in 



different hues. The highefi may be white, while others not 



fo high are yellow, and others ftill lower proportionately more 



red. At equal elevations thofe furtheii from the point where 



the fun fets will incline to red, and thofe neareft it to yellow. 



Blue and green, \\r e may then fee blue or green fliadows on bodies naturally 



owing to con- white, as Buffon and other philofophers have obferved. Thefe, 



*»**• as has been faid, are nothing more than the effect of contraft 



between 



