The employment of barytes in the colour industry is often regarded as adulteration, which, however, it is not when the case is considered from the right point of view. For instance, the only preparation which can properly be termed white lead consists of basic lead carbonate. This, when pure, is a rather expensive pigment, whereas, for certain purposes, the consumer requires a product that can be obtained at a low price. In order to satisfy this demand, the only course open to the colour-maker is to mix the white lead with a cheap white substance, which enables him to turn out different grades of white lead, which, although low in price, are far inferior to the pure article in covering power. Pure white lead being itself a very heavy substance, the only bodies suitable as adjuncts are such as are also of high specific gravity; and of all the cheap pigments known, heavy spar is the only one endowed with this property. Consequently, this substance is extensively used in making the cheaper grades of white lead and the pale kinds of chrome yellow. The only cases in which the addition of heavy spar to a colour can be regarded as an intentional fraud on the consumer is when he is sold, as pure white lead, chrome yellow, etc., a product really composed of a mixture of such colour and barytes. Moreover, the presence of barytes in white lead can be easily detected by a simple examination, pure white lead readily dissolving, with considerable effervescence, in strong nitric or acetic acid, whereas barytes is insoluble in all acids, and therefore remains, as a heavy white powder, at the bottom of the vessel. In this way both the presence and amount of barytes contained in a sample of white lead or chrome yellow can easily be ascertained. The preparation of barytes for the purposes of the colour-maker is entirely a mechanical operation. The barytes, which though fairly hard is easily reduced, is crushed with stamps, ground in a mill and finally levigated, it being impossible to obtain a sufficiently fine powder even by repeated grinding. Native barytes must not be confounded with the artificial barium sulphate sold as permanent white or blanc fixe, which is an extremely finely divided barium sulphate obtained by precipitating a solution of a barium salt with sulphuric acid or a soluble sulphate, and is a painters’ colour that is highly prized for certain purposes. Both the native sulphate and the artificial variety have the property of remaining completely unaltered by exposure to air, and they can therefore be mixed with any kind of pigment without fear of the colour deteriorating. As a rule, barytes is first roughly crushed in edge-runner mills or stamps, and then ground to the extreme degree of fineness obtainable in ordinary mills. Even with the greatest care, however, it is impossible by this means to obtain sufficient fineness of division for mixing with fine colours, the only way in which this can be accomplished being by levigation.