BLUE WATER 75 



some doubt as to whether water was self-coloured blue, 

 or whether its blue colour was due to the action on light 

 of excessively minute solid white particles of chalk sus- 

 pended in the water. Such fine suspended particles in 

 some cases act on the light which falls on to them so 

 as to reflect the blue rays. This occurs in certain 

 natural objects which have a blue colour. But these 

 can be distinguished from transparent self-coloured blue 

 substances by the fact that whilst the light reflected from 

 their surface is blue, the light which is made to traverse 

 them (when they are held up to the light so that they 

 come between one's eye and the sun's rays) is brown. 

 This is the case with very hot smoke, and can be well 

 seen when a cigar is smoked in the sunlight. The smoke 

 which comes off from the lighted end of the cigar is very 

 hot, and its particles are more minute than those of 

 cooler smoke. The hot smoke shows a bright blue 

 colour when the sunlight falls on it and is reflected, but 

 when you look through the smoke-cloud at a surface 

 reflecting the sunlight, the cloud has a reddish-brown 

 tint. As the smoke cools its particles adhere to one 

 another and form larger particles, and the light reflected 

 from the cloud is no longer blue but grey, and even 

 white. Thus the smoke which the smoker keeps for 

 half a minute in his mouth is cooled and condensed, and 

 reflects white light — is, in fact, a white cloud — when he 

 puffs it out, and contrasts strongly with the blue cloud 

 coming off from the burning tobacco at the lighted end of 

 the cigar. The blue colour of the sky is held by many 

 physicists (though other views have been of late advanced) 

 to be due to the same action on the part of the very 

 finest particles of watery vapour, which are diffused 

 through vast regions of our atmosphere above the con- 

 densed white-looking clouds consisting of larger floating 

 particles of water. 



