At the same time, the dust-particles suspended in the water have a great effect in making the water appear more beautiful, brilliant, and varied in its colouring; because little or no light is reflected by the interior of a mass of water itself. If a dark metal vessel be filled with a weak solution of Prussian blue, the liquid will appear quite dark and void of colour. But throw in some fine white powder, and the liquid will at once become of a brilliant blue colour. This accounts for the change of depth and brilliancy of colour in the several shores of the Mediterranean. When, then, you look at the face of a deep-blue lake on a summer evening--the heavens all aglow with the unrivalled display of colour from the zenith, stretching in lighter hues of glory to the horizon--though to you the calm water appears like a lake of molten metal glowing with sky-reflected light, so powerful and brilliant as entirely to overpower the light which is internally reflected, yet blue is the normal colour of the water: _blueness is its inherent hue_. Looking upwards, we observe three distinct kinds of blue in the sky from the horizon to the zenith. All painters in water-colours know that. Newton thought that the colour of the sky was produced in the same way as the colours in thin plates, the order of succession of the colours gradually increasing in intensity. CHAPTER XXIII A SANITARY DETECTIVE The impure state of the air in the rooms of a house can now be determined by means of colour alone. Dr. Aitken has invented a very simple instrument for that purpose; and this ought to be of great service to sanitary officers. It is called the koniscope--or dust-detective. The instrument consists of an air-pump and a metal tube with glass ends. Near one end of the test-tube is a passage by which it communicates with the air-pump, and near the other end is attached a stop-cock for admitting the air to be tested. It is not nearly so accurate as the dust-counter; but it is cheaper, more easily wrought, and more handy for quick work. All the grades of blue, from what is scarcely visible to deep, dark blue, may be attached alongside the tube on pieces of coloured glass; and opposite these colours are the numbers of dust-particles in the cubic inch of the similar air, as determined by the dust-counter.