WCP476

[1]

ARDENLEA

FALKIRK

23 Novr 00

Alfred R Wallace Esq^r.

Dear Sir

Owing to the kindness of Prof. Dewar¹who 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 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 hold 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 nil² for the total depth of our atmosphere,³we have not only to consider the expansion of the oxygen to the low pressure & gaseous condition, but its mixture with nitrogen — which is colourless.⁴ 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 experiment⁵ 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 Aitken⁶ [signature]