Further, the condition of the bath has some influence on this point; it is found that an alkaline bath tends to considerably increase the felting properties of the wool, and on this account dyers invariably avoid the use of both the caustic and carbonated alkalies. Strong soap liquors have also some influence in the direction of increasing the felting, therefore soap should not be used if it can possibly be done without. Ammonia has not so strong a felting action as the other alkalies. Acids, on the other hand, exert a retarding action on the felting of the wool, and this is a matter of some interest and importance in the dyeing of wool, as an acid condition of the bath is necessary for dyeing by far the great majority of colouring matters on this fibre. Alkaline salts, such as Glauber's salt and common salt, exert little or no influence on this felting property, and can be added to dye-baths with impunity, and in many cases with good effect, so far as the quality of dyeing is concerned. So far as the properties of the wool are concerned, it is seen (p. 061) that an acid condition of the dye-bath will work better than an alkaline condition, and wherever it is possible to use acids such should be added. What has been said in regard to wool is equally true of all fibres derived from animals in the same way as wool is, such as horse-hair, fur of rabbits, hares and other animals, although, of course, there are some minor differences between different furs in their resistance to the action of acids and alkalies. The next feature that influences the methods of dyeing wool is the varying properties of the dye-stuffs, or colouring matters. It is obvious that those which, like Magenta or Saffranine, have a strong affinity for the wool fibre must be dyed differently from those which, like Alizarine and Gambine, have no direct affinity for the wool fibre, and, further, which require the aid of mordants before they can be dyed, and on the character of which mordants the colour that is fixed on the fibre depends. The dye-stuffs, independently of the question whether they be derived from natural sources or be of artificial origin, may be roughly divided into five groups, some of which may also be subdivided again as will be shown later on. These groups may be named the (1) Neutral, (2) Basic, (3) Acid, (4) Mordant, and (5) Indigo dye-stuffs. The first two classes are practically dyed in the same way; but as there is a great difference in the chemical composition of the colouring matters comprised in them, it will be best to consider them separately.