7G8 The Right Hon. Lord RmjMgh [Feb. 25, 



very various. Sea-water from outside the grotto of Capri, from 

 Suez, and from near the Seven Stones Lightship off the Cornish 

 coast, I owe to tlie kindness of friends. Of these the two former 

 showed a greenish bhie, the latter a full, or, perhaps, rather yellow- 

 green, and these colours were not appreciably modified after the 

 water had stood in the tubes for weeks. It is important to 

 remember that the hue may, to some extent, depend upon thickness. 

 It is quite probable that in a greatly increased thickness the Capri 

 and Suez waters would assume a more decided blue colour. But I do 

 not think the Seven Stones water could so behave, the colour, with 

 12 feet, seeming to involve the absorption of blue light. 



Further observations on greater depths of sea-water would be de- 

 sirable. A naval son informs me that off the coast of G-reece a plate 

 lying in 6 fathoms of water looked decidedly blue, although the sky 

 was a dirty grey. I have doubts Avliether this would be generally the 

 case in the Mediterranean ; the green due to moderate thicknesses 

 seems too decided. 



Of natural fresh waters that I have tried, none were better than 

 that frojn a spring in my own garden. This water is hard, but bright 

 and clear, and it shows a greenish-blue, barely distinguishable from 

 that of the Capri and Suez water. Distillation does not improve the 

 blue. Neither did other treatments do any good, such, for example, 

 as partial precipitation of the lime with alkali, or passage of ozone 

 with the idea of oxidising humus. Wisliing to try water of high 

 chemical purity, I obtained — through tlie kind offices of Sir J. Dewar 

 — water twice distilled from alkaline pei'manganate, and condensed in 

 contact with silver, but the colour was no bluer. In the light of this 

 evidence, I can hardly avoid the conclusion that the blueness of water 

 in lengtlis of 4 metres has been exaggerated, especially by Spring, 

 although I have no reason to doubt that a fully developed blue may 

 be obUiined at much greater thicknesses. I should suppose that suf- 

 ficient care has not been taken to start with white light. It may be 

 recalled that overcast days are not so common in some parts of the 

 world as in England. 



A third possible cause of apparent blueness of the sea must also 

 be mentioned. If a liquid is not absolutely clear, but contains in 

 suspension very minute particles, it Avill disperse light of a blue 

 character. Although, undoubtedly, this cause must operate to some 

 extent, I have seen no reason to think that it is important. But the 

 existence of three possible causes of blueness complicates the inter- 

 pretation of the phenomena. Hitherto observers have not been 

 sufficiently upon their guard to distinguish blueness having its origin 

 in the sky from blueness fairly attributable to the water itself. 



As regards the light from the sky, the theory which attributes it 

 to dispersal from small particles, many of which are smaller than the 

 vrave-length of light, is now pretty generally accepted. To a first 



