1910] on Golours of Sea and Shj. 7G7 



discusses the difficult questions involved. He tried columns of great 

 length — up to 26 metres ; but even when the distance traversed was 

 only 4 or 5 metres, he finds the colour a fine blue only to be com- 

 pared with the purest sky-blue as seen from a great elevation. But 

 when the tubes contain ordinary water, even ordinary distilled water, 

 the colour is green, or yellow-green, and not blue. 



The conversion of the original blue into green is, of course, 

 explicable if there be the slightest contamination with colouring 

 matter of a yellow character — i.e. strongly absorbent of blue light. 

 Spring shows that this is the effect of minute traces — down to one 

 ten-millionth part — of iron in the ferric state, or of humus. The 

 greenness of many natural waters is thus easily understood. Another 

 question examined by Spring is not without bearing upon our present 

 subject — viz. the presence of suspended matter. I am the better 

 able to appreciate the work of Spring, that many years ago I tried a 

 variety of methods, including distillation in vacuo, in order to obtain 

 water in the condition which Tyndall described as " optically empty," 

 but I met with no success. Spring has shown that the desired 

 result may be obtained by the formation within the body of the 

 liquid of a gelatinous precipitate of alumina or oxide of iron, by 

 which the fine particles of suspended matter are ultimately carried 

 down. 



Perhaps the most telKng observations upon the colour of water 

 are" those of Count Aufsess, who measured the actual transmission 

 of light belonging to various parts of the spectrum. The principal 

 absorption is in the red and yellow. In the case of the purest water, 

 there was practically no absorption above the line F, and a high 

 degree of transparency in this region was attained even by some 

 natural waters. That these waters should show blue, when in 

 sufficient thickness, is a necessary consequence. 



In my own experiments, made before I was acquainted with the 

 work of Aufsess, the light traversed two glass tubes of an aggregate 

 length of about 4 metres (12 feet). On occasion the light was 

 reflected back so as to traverse this length twice over. I must con- 

 fess that I have never seen a blue answering to Spring's description, 

 when the original light was white. For final tests I was always 

 careful to employ the light of a completely overcast day, which was 

 reflected into the tubes by a small mirror. The colour, after trans- 

 mission, showed itself very sensitive to the character of the original 

 source. The palest clear sky of an English winter's day gave a 

 greatly enhanced blue, while, on the other hand, isolated clouds are 

 usually yellowish, and influence the result in the opposite direction. 

 I should myself describe the best colour of the transmitted light on 

 standard days as a greenish blue, but there is some variation in the 

 use of words, and, perhaps, in vision. Some of my friends, but not 

 the majority, spoke of blue simply, but all were agreed that the blue- 

 ness of a good sky was not approached. The waters tried have been 

 Vol. XIX. (No. 104) 3 ji 



