Wallace Letters Online

Share this:

Record number: WCP476

Sent by:
John Aitken
Sent to:
Alfred Russel Wallace
23 November 1900

Sent by John Aitken, Ardenlea, Falkirk, Scotland to Alfred Russel Wallace [address not recorded] on 23 November 1900.

Record created:
01 June 2002 by Lucas, Paula J.


Re. Prof. Dewar's experiments, light, atmospheric particles and the causes of the blue colour of the sky.

Record contains:

  • letter (1)

View item:

LETTER (WCP476.476)

A typical letter  .

Held by:
Natural History Museum
Finding number:
NHM WP1/8/5
Copyright owner:
Copyright of the John Aitken Literary Estate.

Transcription information






23 Nov[ember] [19]00

Alfred R. Wallace Esq.

Dear Sir,

Owing to the kindness of Prof. Dewan who 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 Dewan 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 selection absorption of a body in its liquid & in its gaseous state[,] It appears to 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 multiplies into the square of the density is the same in each case, & this law looks true from the gaseous to the liquid state. from this it follows that the small amount of blue in liquid oxygen when explained to the density of our atmosphere is practically nil for the total depth of our atmosphere, he leaves not only to consider the expansion of the oxygen to the low pressures & gaseous condition, but its mixture with nitrogen –- which is colourless. This mixing [[4]] has the effect of further reducing the density of the oxygen & reducing its atmosphere of the red end of the spectrum to so small an amount that it cannot be observed by experiment 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 Aitken [signature]


1. Dewar, James (1842-1923). Chemist and physicist

2. Aitken, John (1839-1919). Meteorologist

Please note that work on this transcript is not yet complete. Users are advised to study electronic image(s) of this document, if available.