of Edinburgh, Session 1881 - 82 . 
481 
bility is that the colour is due to the water, as the impurities will 
be different in the two samples, and they would probably give 
different colours. The result was, the colour of the samples of water 
prepared in both sets of apparatus was the same — namely, blue. This 
conclusion was further confirmed by preparing another sample of 
water, and condensing it this time in a platinum tube. The water so 
prepared was also found to be of a fine blue. All three samples 
were almost exactly the same colour as Prussian blue. Standards 
of colour were kept with which the different samples of water 
were compared, both for the colour and for the amount of 
the colour. As all the different samples of distilled water — • 
after the apparatus was thoroughly purified — had the same colour 
and amount of colour, it seems almost certain that water is a 
blue transparent substance, and that the colour in these experi- 
ments could not be due to impurities, which must have varied 
both in kind, and in amount in the different samples of water. 
Further, as the amount of colour in the Mediterranean water, and in 
the bluish well waters, was as near as could be judged the same as 
in pure water, it does not seem necessary to call in the aid of im- 
purities to account for the blue colour seen in lakes and seas, the 
colour being principally due to the water itself, and the different 
substances in solution, instead of making the water blue, tend to 
change its proper colour and make it green, or yellow. 
The addition of impurities to water seems generally to change its 
colour from blue to green, or to yellow, though there seems to be no 
reason why some impurity may not change it to a deeper blue. The 
selective-absorption of the water remains the same, while the 
impurities add their selective-absorptions to that of the water, and 
while they change the colour they also decrease its transparency. 
This explains why it is that the yellow well waters are so much less 
transparent than the blue. This must necessarily be so, as a very 
small depth of water destroys the rays which give yellow, and the 
transparency of yellow waters can only be the transparency of 
water for yellow light, which is very much less than for blue light. 
FTo attempt was made to find out what the different discolouring 
substances in water are. The task would evidently be an endless 
one, and of little value. 
The effect of the light reflected from the surface of the water is 
