480 
Proceedings of the Royal Society 
be tested was poured in till the tube was half full, so that by looking 
through the upper half of the tube the coloured surface could be 
seen, and through the lower half the effect of the absorption of the 
water on the colour, and on the brilliancy, of the transmitted light. 
The transmitted light was also examined by means of a spectroscope. 
The colours of the different waters were found to vary greatly. 
One sample was of a fine blue, others were green-blue, some green, 
whilst others were of colours between green and yellow, but all were 
of colours between blue and yellow. It was observed that the more 
transparent a water was, the nearer its colour was to blue. Scarcely 
any light could be seen through 7 m. of any of the yellowish waters, 
whilst through this length all the bluish waters were quite trans- 
parent, and the spectroscope showed that some of the waters trans- 
mitted almost the entire light of the blue end of the spectrum, and 
only stopped the rays of the red end. When one of the bluish 
waters was examined in a tube 15 m., or nearly 50 feet long, it 
appeared of a fine blue green as transparent as a piece of glass. 
Only very little relation could be traced between the colour of a 
water — when tested in long tubes — and its suitability for dietetic 
purposes. 
The cause of the colour of water has been a frequent subject of 
speculation. Every substance which has been discovered in water 
has in turn been suggested as the cause of the colour. When no 
useful purpose could be given for its presence, it was told off to do 
the ornamental, and make the water beautiful to the eye. All these 
speculations assume that the colour is due to some impurity in the 
water. This, however, is obviously begging the whole question. 
It is first necessary to find out whether water has any colour in 
itself, and what that colour is, before we can say anything about the 
effect of impurities. 
As it would be impossible to prepare absolutely pure water, and 
as we might still be in doubt as to whether any colour seen in 
purified water was due to the water or to the impurities, the follow- 
ing method of experimenting was adopted : — Distilled water was 
prepared in two sets of apparatus ; in one set the condensing tube, 
the collecting bottle, and the testing tube were all of glass ; in the 
other set they were all made of brass. If the waters prepared in 
these two sets of apparatus have the same colour, then the proba- 
