122 REPORT—1840. 
effect analogous to that of the powder of calcined bones in 
opaline glass*.” 
254. I have endeavoured to show} that the colorific property 
of watery vapour may not merely be gathered from induction, 
but demonstrated by direct experiment. MHaving first noticed 
that high-pressure steam, during a certain stage of condensa- 
tion, is coloured, and transmits orange-red light, I extended 
the observation to steam of low pressure. There seems no 
reason to doubt that the property of vapour, to be coloured in 
passing from its pure, elastic, colourless state to that com- 
monly called vesicular (such as it appears in clouds, or in 
issuing from the spout of a kettle), is a general one, and there- 
fore that great masses of vapour at any temperature under- 
going condensation must pass through the colorific stage. Now 
the development of the brightest atmospheric colours is in- 
variably attended with change of temperature. And Forster, 
without the remotest reference to theory, has recorded that the 
sunset glow is contemporaneous with the dew-point tempera- 
ture; hence he argues that “ some sudden change produced by 
the first falling dew is the cause of the simultaneous change of 
colour in all the clouds then visible{”’.. The application of this 
doctrine to atmospheric colours, as a prognostic of weather, is 
likewise evident and satisfactory §. 
255. Dry Fogs.—Of atmospheric colours, which may be con- 
sidered unusual, the blood-red colour of dry fogs, which have 
occurred at various times over a vast extent of country, is 
amongst the most remarkable. It is hardly possible to believe 
that they are not due to the accidental intermixture of foreign 
matter with the atmosphere. Remarkable fogs of this kind 
* Edinburgh New Philosophical Journal, vol. xv. Count Maistre explains the 
colour of water by similar reasoning. He considers it blue for reflected, and 
yellowish orange for transmitted, light; and the green colour of the sea and 
some lakes he attributes to diffused particles which reflect a portion of the 
transmitted tint, and mingle with the blue. This is well confirmed by Davy’s 
observations (Salmonia, third edition, p. 317). Arago has very ingeniously 
applied the same reasoning to the ocean, showing that when calm it must be 
blue, but when ruffled, the waves, acting the part of prisms, refract to the eye 
some of the transmitted light from the interior, and it then appears green 
(Comptes Rendus, 23rd July, 1838). Most authors have admitted the intrinsic 
blue or green colour of pure water, as Newton (Optics, b. i. part ii. prop. 10), 
Mariotte, and Ever. Humboldt seems doubtful (Voyage, 8vo, ii. 133.). 
¢ “On the Colour of Steam under certain circumstances.” ‘ On the 
Colours of the Atmosphere.”—Edinburgh Transactions, xiv. 371; Philoso- 
phical Magazine, Third Series, xiv. 121,419; xv. 25; and Poggendorff’s 
Annalen. 
+ Researches about Atmospheric Phenomena, third edition, p. 87. 
§ Those who wish for fuller details on the history of this part of the sub- 
jeet, will find them in my papers above referred to. 
