WCP476

[1]¹

Ardenlea

Falkirk

23 Nov[ember] [19]00

Alfred R Wallace Esq^r.

Dear Sir,

Owing to the kindness of Prof. Dewar²who gave me some references[,] I can now answer some of the questions omitted in my last.

In the ordinary production of liquid air[,] filters are not used, but Prof Dewar [h]as[?] frequently used filtered air, but has never been [2] able to detect any scattered blue light[.]

Then with regard to the relation between the selective absorption of a body in its liquid & in its gaseous state — It appears that their selective absorption is of the same nature but is very much less in the gaseous than in the fluid condition for equal quantities of the substance. The law is [3] that the absorption in different columns of a gas under different pressures is identical when the length multiplied into the square of the density is the same in each case, & this law held[?] true from the gaseous to the liquid state. From this it follows that the small amount of blue in liquid oxygen when expanded to the density of our atmosphere is practically nil³ for the total depth of our atmosphere,⁴ we have not only to consider the expansion of the oxygen to the low pressure & gaseous condition, but its mixture with nitrogen — which is colourless.⁵ This mixing [4] has the effect of further reducing the oxy density of the oxygen & reducing its absorption of the red end of the spectrum to so small an amount that it cannot be observed by experiment⁶ in the air of our atmosphere. From all of which it would appear that neither oxygen nor nitrogen have anything to do ^directly with the blue of the sky[.]

With best wishes | Yours truly | John Aitken⁷ [signature]