Faraday to Charles Robert Leslie   25 May 1854¹

Royal Institution | 25 May 1854

My dear Sir

It gives me great pleasure to think that I may in any way be useful to you as it [is] very agreeable to my thoughts to suppose I have any knowledge which can bear upon your high & intellectual pursuit:- leaving you to judge of the applicability of what I may say, I shall plunge at once into the middle of your letter². We only know of two states of existence for the water in the atmosphere, one as clear transparent vapour and the other as the vesicles which form clouds, or what is commonly known as visible steam. We have no philosophical reason for supposing that in the first state it can produce the blue colour of the sky; for all experiment goes to shew, that, in that state, it is as transparent & colourless as the air itself. Neither, if we were to assume that the local colour of water is blue, and that in the state of transparent vapour this colour is retained, would that account for a blue sky; for supposing the whole of the atmosphere to its very summit, were retained at the high temperature of 80˚F, and that it were saturated with aqueous vapour, still the quantity of water present if condensed into the liquid state would not make a layer of more than 13 1/2 inches in depth. But considering the rapid diminution of temperature upwards, and other circumstances affecting the quantity of water present as transparent vapour at any one time in the atmosphere, we cannot suppose there is ever more than one fourth of this amount, and I leave you to judge how utterly insufficient this Would be to produce the blue skies seen in this country, much less those of Italy and other parts of the world. Three inches in depth of the Rhine water at Geneva, which is as blue as any water I know of, if held in a glass vessel between the eye & the sky would give scarcely an appreciable effect of colour.

But the other state of “vesicles” appears to be sufficient to account for the blue colour, not because of any colour they have in themselves, but because of their optical effect on the rays of light passing through the atmosphere[.] My own knowledge does not render me competent to give an opinion on this matter; but I place confidence in the investigations of a high mathematician, M. Clausius³, whose paper you will find (& I think read with interest) in Taylor’s memoires: and to facilitate your access to it I send you my copy, which if you can return it in a week or two I shall be obliged. His paper begins at p 326 and you will see that by the time he arrives at the end p 331, he considers that the blue colour of the sky, as well as the morning & evening red are fully accounted for⁴[.]

In reference to another part of your letter, which speaks of air without water being invisible, and therefore of the sun as intensely luminous and without halo or rays, and the space appearing black; there is reason to believe that such effects (except the rays) would occur. As to the rays, the irradiation of a very brilliant center of light does not depend upon the atmosphere, but upon effects produced in & by the parts of the eye; and would occur if the atmosphere could be entirely removed. On the other hand there is no reason to believe that the presence of water in the atmosphere, in its perfectly dissolved state, would produce any of the effects you refer to or change the appearances from those presented by perfectly dry air. It is more probable that the vesicles Clausius & others speak of, are the cause of the general diffusion of the light coming from the Sun to the earth, which takes place even in the clearest atmospheres[.]

Believe me to be | My dear Sir | Very faithfully Yours | M. Faraday

C.R. Leslie Esqre RA | &c &c &c