378 CIIEMISTKY. 



in such cases, some substance must be employed which has 

 a strong attraction for both the coloring matter and the 

 substance of the cloth, and can therefore unite the two firm- 

 ly together. Such a substance is called a mordant, for rea- 

 sons already given. The cloth to be dyed is first charged 

 with the mordant, and then is immersed in the dye. 



534. Colors Modified by Mordants. The mordant not 

 only fixes the color, but modifies it, so that with different 

 mordants different colors can be produced with the same 

 dye. Thus, by using with madder the acetate of aluminium, 

 produced by mixing common alum with acetate of lead, a 

 red color is obtained ; but if ferrous sulphate be mixed with 

 the acetate of lead instead of the alum, a deep black color 

 is the result. Then, again, if some arsenious acid be added, 

 together with the ferrous sulphate, the madder gives a rich 

 purple color. Now all of these madder colors can be pro- 

 duced upon the same piece of calico by printing the different 

 figures with different mordants before introducing it into the 

 dye. The printing is done by rollers, between which the 

 calico is passed. These rollers are engraved with the fig- 

 ures, and the pastes containing the mordants are each put 

 upon its appropriate set of rollers. The calico is passed 

 through the several sets successively. After all the mor- 

 dants are thus printed upon the cloth it is immersed in the 

 dye. The process is not finished yet, for the common color 

 of the madder is in all those parts of the cloth not touched 

 by the mordants. But the color is not fast, and is easily 

 washed out, leaving a white ground, the washing out not 

 affecting at all the colors fastened by the mordants. 



QUESTIONS. 



508. Why must the common idea that acids are sour be abandoned ? 

 Name some of the sources of organic acids. What is said of the proper- 

 ties of the members of the fatty acid series ? In what conditions do the 



