WCP591

[1]

45. Frederick Road

Edgbaston

March 21. 1899

Alfred R. Wallace Esq

Dear Sir,

I was greatly honored by receiving your letter of the 19th.¹

(1) Fine Particles. This is quite a mistake (I mean the idea that the blue colour of sea water is one to scatter light). So far as I know it originated with Tyndall² who in a Lecture at the Royal Institution hinted rather than stated that the colour of some water off the N. W. coast of Africa might be so explained.

(2) Of course I have tried the colour of distilled water often — & it is nearly if not quite the same as pure sea water. This however is well known [2] of most work by aitken³ & also I think Forel or Morges.

The experiment is not very easy as you require a black tube which does not soil the water. I know one Professor of chemistry who for at least 20 years has annually shown to his class that distilled water is green — The mistake arising from the contamination of the water by colouring matter dissolved from the paint used for the inside of the tube.

(3) Fine particles again. I have made many examinations of sea water for scattering of light & never found a trace of such an effect: as the scattered light is polarized the experiment is easy.

(4). I quite agree that the differences in composition by sea water are insufficient to account for the [3] differences in colour obscured.

(5) with regard to the atlantic [sic]; my own experience is quite in accordance with your statement. In this case I fancy that fine particles of mud of a yellowish colour are at the bottom of the colour of the N. atlantic [sic].

(6) The only problem to be solved is the cause of the colour of exceptionally clear green water such as one sees near coral Islands—e g at Honolulu— or on the coasts of W. australia [sic]. I think the spectroscope would settle this but I don’t know when I shall have another opportunity, for I have rejected a Professorship which I held at Sydney.

Yours very truly | R. Threlfall⁴ [signature]