BLUE WATER 77 



tubes, which reflected the most vivid blue tints when 

 illuminated by the electric arc-lamp. Later Aitken, of 

 Edinburgh, showed that the " fog " which forms in smoke- 

 ridden towns is due to the condensation of previously 

 invisible watery vapour as "cloud" around the solid 

 floating particles. of carbon of the smoke. Aitken further 

 used this property of solid floating particles, namely, 

 that they cause the formation of fog and cloud in vapours 

 — to test the question as to whether the excessively minute 

 odoriferous particles which affect our noses as " smell " are 

 distinct solid floating particles as often supposed, or are of 

 the nature of gas and vapour. He admitted strong per- 

 fumes such as musk into tubes containing watery vapour, at 

 such a temperature that the vapour was in a " critical " state 

 — ^just ready to condense and precipitate as "cloud." If 

 he had admitted fine solid particles such as a minute 

 whiff" of smoke, or some "dusty" air — the cloud would 

 have formed. But the admission of the perfume had no 

 such effect. Therefore, he concluded that the odoriferous 

 emanations used by him are not distinct particles like 

 those of smoke or dust, but are gaseous. 



The beautiful blue tint of the semi-transparent " white " 

 of a boiled plover's egg is due to a fine-particled cloud 

 dispersed in the clear albumen. London milk used to 

 be " sky-blue " for a similar reason, before the recent 

 legislation against the adulteration of food. The blue eyes 

 of our fair-haired race and of young foxes are not due to 

 any " pigment " — that is to say, a separable self-coloured 

 substance — but to a fine cloud floating in a transparent 

 medium which reflects blue rays of light as blue smoke does. 

 The iris of the eye can and often does develop a pigment, 

 but it is a brown one. When present in small quantity it 

 produces a green-coloured iris, the pale yellow-brown 

 being added to the blue cloud-caused colouring. When 

 present in larger quantity the same pigment gives us brown 



