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.
While the number of particles was counted by means of the dust-counter, the depth of blue given by the koniscope was noted; and the piece of glass of that exact depth of blue attached. A metal tube was fitted up vertically in the room, in such a way that it could be raised to any desired height into the impure air near the ceiling, so that supplies of air of different degrees of impurity might be obtained. To produce the impurity, the gas was lit and kept burning during the experiments. The air was drawn down through the pipe by means of the air-pump of the koniscope, and it passed through the measuring apparatus of the dust-counter on its way to the koniscope. It may be remarked that, by a stroke of the air-pump, the air within the test-tube is rarefied and the dust-particles seize the moisture in the super-saturated air to form fog-particles; through this fog the colour is observed, and the shade of colour determines the number of dust-particles in the air. These colours are named “just visible,” “very pale blue,” “pale blue,” “fine blue,” “deep blue,” and “very deep blue.”
When making a sanitary inspection, the pure air should be examined first, and the colour corresponding to that should be considered as the normal health colour. Any increase from the depth would indicate that the air was being gradually contaminated; and the amount of increase in the depth of colour would indicate the amount of increase of pollution.
As an illustration of what this instrument can detect, a room of 24 by 17 by 13 feet was selected. The air was examined before the gas was lighted, and the colour in the test-tube was very faint, indicating a clear atmosphere. In all parts of the room this was found the same. A small tube was attached to the test-tube, open at the other end, for taking air from different parts of the room. Three jets of gas were then lit in the centre of the room, and observations at once begun with the koniscope.