CHAPTER I I 



The Indirect Attachment of Dyes 



to Tissues 



It is characteristic of dyes that when they are dissoh^ed in water 

 or a mixture of w^ater and alcohol, they attach themselves directly 

 to certain tissue-constituents in one or more of the ways described 

 in the last chapter. Some of them, however, have an alternative 

 method of attachment, involving the presence of another substance 

 besides dye and solvent; and when this substance is present, their 

 behaviour is quite different. Great advantage can sometimes be 

 taken of this difference in behaviour, both in the textile industry 

 and in microtechnique. A dye that is almost useless in simple 

 solution may become of major importance. 



The salts of certain metals are the chief substances that radically 

 alter the behaviour of particular dyes. These salts were called 

 mordants because they were thought to bite into certain textile- 

 fibres and thus give attachment to dyes that would not work 

 satisfactorily by themselves. The word was taken into our language 

 from French. 



A mordant is capable of entering into chemical combination 

 with a dye. The resulting substance is called a lake. This word 

 had a curious origin. It derives from lac, a Hindustani word mean- 

 ing the waxy material produced by the scale-insect, Tachardia 

 lacca. The females of this species attach themselves in great 

 numbers to the twigs of certain Indian trees and produce an 

 abundance of waxy material, which fills up all the spaces between 

 them. The waxy material is sold as lac, or, if in flat plates, as 

 shellac. The insects themselves contain a large quantity of lac-dye, 

 which is chemically related to carminic acid; ^^^ this is said to be 

 stored chiefly in the ovaries. ^^ The dye is dissolved out and then 

 precipitated by potassium alum. The name lac became attached 

 not only to the wax but also to the precipitated dye, and afterwards 

 to the product of the cochineal insect precipitated in the same way 

 ('crimson lake'). The alum plays a double role. It precipitates the 



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