The next group may be called hydroxy-azo dyes, and are quite of modern introduction. They are azo dyes, one of whose constituents is a body like salicylic acid, amido-benzoic acid, dihydroxy-naphthalene-sulpho acid, which contain the group OH, hydroxyl with carboxyl COOH. The first group imparts phenolic characters, while the second gives true acid properties, and both of these acting together cause the dyes to be able to form colour lakes with metallic oxides. There is one point of difference between the two groups of dyes, the phenolic dyes are as a rule not dyes of themselves, some of them are practically free from colour, and it is only when brought into combination with the metallic oxide or mordant that they form a colour and dye a fibre. On the (p. 115) other hand the azo mordants are in general colouring matters, and can be used to dye wool without the aid of a mordant, the latter only serving to make the colour faster to light, acids, milling, etc., and it often has no material effect on the shade or tone of colour being dyed. Alizarine Yellow G G, Gambine Yellow, Anthracene Yellow, Chrome Violet, are examples of such dyes. There are, however, some dyes (such as the Chromotrops, Azofuchsine, Anthracene Acid Browns, etc.) on which the mordant has a marked effect. The methods adopted in practice for the application of this class of dyes are many and varied. The mordants used are alum, alumina sulphate, acetate of chrome, chrome alum, fluoride of chrome, ferrous sulphate and tin chloride, while, in addition, along with these true mordanting materials, assistant mordants are used, such as argol, tartar, tartaric acid, lactic acid, lignorosine, oxalic acid and sulphuric acid. The mordanting may be done either before or after the dyeing, the first plan being that commonly adopted with the phenolic colours, while the second method may be used and is the best to use with azo-mordant dyes. Sometimes the mordanting and dyeing may be done in one bath, but this method is one which leads to a loss of colouring matter and often to the production of colours which are loose to rubbing, and cannot, therefore, be recommended. #Mordanting.#--This operation is carried out in the same way in all cases. The goods are entered into the bath at a temperature of about 150 deg. F. The heat is raised to the boil, and is then maintained for one and a half hours, after which the mordanted wool is lifted and well rinsed, when it is ready for the dye-bath. As mordanting materials bichromate of potash and fluoride of chrome are chiefly used when chrome mordants are required, sometimes chrome alum. With these (p. 116) are used sulphuric acid, oxalic acid, cream of tartar or argol, lactic acid, etc.