In order to use this apparatus to compare the intensity of colour of two liquids, the following process is performed: A liquid is made, the colour intensity of which is taken as 100. The colour intensity of the liquid under examination will then be represented by a number indicating the relation between its intensity and that of the standard liquid. A solution of caramel in water is generally used as the standard, since this substance has very great intensity of colour. The preparation of absolutely pure caramel is difficult, and it is therefore advisable, in order always to have the standard solution of the same intensity, to prepare a large quantity of caramel solution at once, to add carbolic acid to prevent it from decomposing, and to keep it in well-closed bottles. When the standard solution is almost used up the colorimeter is employed to prepare a fresh quantity of equal intensity. To render possible an exact comparison of two substances they must be tested under the same conditions, that is, the solutions of the colouring matters must be made in exactly the same manner. Finely-powdered materials dissolve more readily than coarse powders. In making the solutions which are to be examined for intensity of colour, the raw materials must be brought into a condition of fine division by the same instrument, for example, a rasp. The colour solution is made by boiling 100 grammes of the dye-wood with exactly a litre of distilled water for precisely thirty minutes. The liquid is then filtered into a 1-litre flask. The dye-wood absorbs water, and some is lost by evaporation, so that considerably less than 1 litre of liquid is collected in the flask. Distilled water is added to make up the volume to 1 litre.