96 Professor Bunsen on the Colour of Water. 



tensity of which will diminish in proportion as the column of 

 water is shortened, so that the shade of colour becomes at 

 length too faint to be perceived. This blue coloration may 

 also be recognised when a white object is illuminated through 

 the column of water by sunlight, and seen at the bottom of 

 tlie tube through a small lateral opening in the black coating. 

 The blue tint so frequently observed in water cannot, there- 

 fore, be regarded as in any way strange. The question there- 

 fore arises, why this blue colour is not seen evei*yvvhere, and 

 why it should not occur in many seas \ Why, for instance, 

 the lakes of Switzerland, the waters of the Gej'sers in Iceland, 

 and in the South Sea Islands, should exhibit every shade of 

 green, whilst the waters of the Mediterranean and Adriatic 

 are occasionally of so deep a blue as to vie with indigo \ 

 These questions are easily answered, since clearness and 

 depth are the primary, if not the sole requirements for im- 

 parting to water its natural colour. Where these fail, the 

 blue tint will likewise be wanting. The smallest quantity of 

 coloured elements which the water may take from the sand or 

 mud of its bottom, the smallest quantity of humus held in so- 

 lution, the reflection of a dark and strongly coloured bottom, 

 are all sufficient to disguise or alter the colour of water. It 

 is well known, that the yellowish-red colour of the waters 

 which traverse the lower group of the ti'ias formations de- 

 pends upon hydrated oxide of iron, contained in the mud of 

 the variegated sandstone. From a similar cause, tlie vast 

 glacier streams of Iceland, which, in these desolate regions 

 where there are neither roads nor bridges, the traveller finds, 

 to his discomfort, that he must ford, are rendered opaque and 

 milk-white from the detritus of dark volcanic rocks, which, 

 crushed into a white powder by the overwhelming mass of 

 the descending glaciers, are carried to the sea, in the form 

 of white mv;d and sand, and again deposited there in vast 

 deltas. 



In like manner, the natural colour of the small lakes in the 

 marshy districts of northern Germany is concealed by the 

 black tint imparted by the dissolved humus derived from the 

 turf. These waters often appear brownish or black, like the 

 water in most of the craters of the Eifel and Auvergne, where 



