As repeatedly stated, no indigo vat can be worked continuously with good results; the continual agitation induced by the passage of the yarns or cloths into the liquor brings the liquor into contact with the air, and oxidation sets in, resulting in the indigo being thrown out of the liquor in its original form. When this happens the vat loses its original clear yellow or yellowish-brown colour and becomes greenish, a sure sign that the vat is getting out of condition to give good results. The remedy has been pointed out in dealing with each kind of vat, and consists essentially in adding to the vat more of the active reducing agent and allowing the vat to rest a while. The dye-vats may be either round tubs or square wooden tanks; for yarn in hanks, when cloths or warps are being dyed, these may be fitted (p. 149) with winces and guide rollers so as to draw materials through the liquor. The hawking machine shown in figure 22 is also very good for indigo cloth dyeing, and is largely used for this purpose. [Illustration: Fig. 23.--Indigo Dye-vat.] Figure 23 also shows an excellent machine for indigo dyeing on cloth. In this the vat has a frame carrying guide rollers, round which the cloth passes, so that it travels several times through the vat liquor in its passage from one end of the vat to the other, the amount of liquor in the vat being so arranged that the cloth is entirely immersed the whole time. After going through the liquor the cloth passes between a pair of squeezing rollers, in order to have any surplus liquor taken out, then it traverses the space between sets of guide rollers arranged over the vat, during which time the indigo becomes oxidised and the blue develops, while finally it is (p. 150) plaited down on a table. The illustration clearly shows the working of the machine. #Dyeing Wool with Indigo Extract.#--Sulphonated indigo, prepared by dissolving indigo in sulphuric acid, is sold under the name of "indigo extract," or "indigo carmine," in two forms--paste (containing, perhaps, 25 to 30 per cent. actual colour) and powder. Both forms are freely soluble in water, although some makes are more so than others. This quality of solubility is dependent upon the proportion of sulphuric acid which may have been used in the preparation of the extract. When this is small, what is termed indigo monosulphonic acid only is formed, which is but slightly soluble in water, and gives red shades. If a larger proportion of acid be used, then the indigo disulphonic acid is formed, which is fairly easily soluble in water, and gives bluer shades than the former.