424 Professor Forbes o» the Colours of the Atmosphere. 



plains with remarkable elegance the actually observed phae- 

 nomena, and because it exposes the insufficiency of the theory 

 of iridescent colours to explain the hues of sunset. The theory 

 of vesicular vapour, or floating bubbles of water as constitu- 

 ting clouds, was j)revalent even at a far earlier period than this. 

 Leiljnitz had supported it in the seventeenth century*, and 

 had calculated the rarity of the asthereal fluid with which they 

 were supposed to be filled. Kratzenstein {1740) had, by ac- 

 tual experiment on the colours which they reflected, attempted 

 to estimate their thickness by direct measurement, to find their 

 diameterf. Saussure demonstrated the existence of bodies 

 apparently so constituted, in clouds themselves: but I nowhere 

 find that he has applied it to explain their coloration on the 

 principle which Melvill justly condemns in this passage. 

 Saussure's opinion of the blue colour of the sky was, so far 

 as I can judge, that of Mariotte and Bouguer:];, although he 

 alludes very particularly to bluish vapours as foreign matters 

 floating in the upper regions of the sky, which he says were de- 

 cidedly not aqueous, since they did not affect the hygrometer^;. 

 He thinks this may illustrate the obscure phjEnomena of dry 



The memoir ot Eberhard of Berlin on this subject^ con- 

 tains nothing to detain us. The author seems to coincide in 

 the theory of Mariotte, and spends much labour in refuting 

 that of Da Vinci. 



Delaval's elaborate Theory of the Colour of Bodies, we 

 may also rapidly dispose of. He adopts the idea of Fabri, that 

 the foreign matters suspended in the air become the means of 

 reflecting blue light, and transmitting red, on the same prin- 

 ciple as arsenic dispersed through glass. This comparison 

 to the acknowledged phsenomena of opalescence, is not unim- 

 portant**. 



The greater part of the optical writers of the present cen- 

 tury have closely followed one or other of those already 



* Opera Omnia, ii. p. ii. 82. Edit. 1768. " Cur vapores eleventur non 

 speriienda qusstio est, atque inter alia non mal^ concipiuntur in illis bullas 

 insensibiles ex pellicula aquae et acre incluso constantes, quales sensus in 

 liquoribus spuniescentibus ostcndit." 



+ Theorie de I'Elevation des Vapeurs et des Exhalaisons, &c. Bordeaux, 

 1740. Quoted in Saussure's Hygrometrie, § 202, and in Kamtz, Lehrbuch 

 der Meteorologie, iii. 48. The diameter he raade s-^, and the thickness 

 s incli. 



J Voyages dans les Alpcs, iv. § 2083. 



^ Hvgrometrie, § 355. 



II jkd. § 372. 



4 Rozier, Introduction, i. 618. 



** Manchester Memoirs, 1st Series, ii. 214, &c. 



