i88 



DYEING 



microtechnique. This is true although some of the most important 

 textile fibres are those of plant and animal origin. The chief 

 differences are tabulated here. 



Textile dyeing 



Cotton has been especially in- 

 vestigated, because it is so homo- 

 geneous chemically. It is extreme- 

 ly peculiar, because it is a nega- 

 tively charged object ordinarily 

 dyed by acid dyes. 



Whether, in particular circum- 

 stances, a dye is acting as a basic or 

 an acid dye may not be known 

 (p. 209). 



Dyes are generally used at or 

 near the boiling point of water. 



The dye-bath is usually ex- 

 hausted or nearly so. 



Almost perfect fastness to water 

 is generally required. 



The fibres — of cellulose or spe- 

 cial proteins, or synthetic — are 

 non-living and unfixed. 



No differential dyeing at the 

 microscopical level is required, and 

 there is no process of differential 

 extraction of the dye. 



Anionic chromium is used to 

 mordant for azo-dyes; other mor- 

 dants and mordant-dyes are seldom 

 used nowadavs. 



Dyeing in microtechnique 



The dyeing of cotton is of little 

 interest except to those studying 

 the cell-walls of plants. The bio- 

 logist does not ordinarily dye 

 negatively charged objects with 

 acid dyes. 



A glance down the microscope 

 at a dyed preparation usually 

 shows whether the dye used was 

 basic or acid. 



Dyes are usually used at room 

 temperature. 



The tissue takes up only a 

 minute part of the dye in the dye- 

 bath. 



Fastness to water is unneces- 

 sary, as one can quickly transfer 

 the tissue to some other medium 

 in which the dye is insoluble. 



The objects dyed are generally 

 either fixed or alive (p. 274). 



The whole purpose is differ- 

 ential dyeing at the microscopical 

 level; it is often achieved by 

 differential extraction. 



Iron, aluminium, and cationic 

 chromium are used to mordant 

 for haematein, carmine, and cer- 

 tain oxazine dyes (see p. 207). 



In this book every effort will be made to profit from the valuable 

 researches of the textile chemists, and it is necessary to make 

 special acknowledgement of the admirable presentations of this 

 subject by Vickerstaff,^^^ Bird,^^ and Venkataram; ^-^ but the 

 object throughout wall be to concentrate attention on the use of 

 dyes in microtechnique. 



In microtechnique we are primarily concerned wdth the electric 

 charges on the dye-ions and on the objects dyed. 



The electric charges on dye-ions are investigated by subjecting 

 dye-solutions to the action of an electric current. Cataphoretic 



