Common usage differs from the scientific in the description of colours. In the physical sense, yellow, red and blue are the so-called “simple colours,” between which lie orange, green and violet as “mixed colours”. Physics knows no white colour and no black colour, but describes white as a mixture of all the simple colours and black as the absence of colour. A grey or brown shade, produced by different mixtures of simple colours, is just as little known in the scale of colours as white or black. The colour maker follows, as we have said above, the common manner of speaking; to him white and black are equally as much colours as red and green. Besides the pure principal colours (yellow, red and blue), and the mixed colours obtained from them (orange, green and violet), colour makers distinguish many shades of each colour—lemon yellow, sulphur yellow, cherry red, blood red, violet blue, etc. For the present purpose it is of great importance to accurately distinguish the several shades, for the value of many colours is in proportion to their beauty of shade. The colour maker is often required to produce a colour of some particular shade, which he accomplishes in many cases by a suitable alteration in the process by which the colour is made, in other cases by mixing different colours, in which event chemistry is of no help to him; he must depend on the sensitiveness of his eyes to colour. =White Mineral Pigments.=—We are acquainted with a great number of white or, more properly, colourless mineral compounds; they possess the property of reflecting, undecomposed, all the rays of light which fall upon them, in consequence of which they produce that impression upon the eye which we call white. According as a white substance reflects every ray of light or absorbs a portion, we see it as a brilliant pure white or, in the latter case, as a white with a grey tinge. If a white body reflects the majority of the rays of light falling on it, but decomposes a small number, we perceive a white which has a yellow, blue or red tinge.