DYEING AND OTHER PROCESSES OF COLOURING 297 



We shall then be in a position to formulate more exactly what we 

 mean by dyes and dyeing. 



It must be noted at the outset that noun and verb are not in- 

 separable, for we may use dyes without dyeing. The most obvious 

 example is the use of a dye such as carmine as a component of an 

 injection medium. Since carmine can be dispersed in water, though 

 insoluble, we may suspend it in a suitable vehicle (such as a gelatine 

 sol) and force it into the vessels or other cavities of an organism, 

 through the walls of which it cannot escape. When microscopical 

 preparations are subsequently made, the colour of the dye defines 

 the form of the cavities. There is here no question of a process of 

 dyeing, or of any reaction with tissue-constituents. Any strongly 

 coloured substance whatever, provided it were insoluble but 

 capable of dispersion in water, could be used instead. 



The uptake of carmine and certain other dyes by phagocytic 

 cells (p. 276) is another example of a process that has nothing to 

 do with dyeing. The particles may equally well be of carbon in 

 the form of lamp-black : the cells make no distinction, but pile up 

 the substance in their cytoplasm. Neither the cytoplasm nor any 

 pre-existent object in it is coloured. The process shows some re- 

 semblance to the sweeping up into a dustpan of a powdered dye 

 that has fallen on the floor, but none at all to the dyeing of one's 

 clothes. 



The simplest way in which a colouring agent can act is by 

 solution in a fluid contained within the tissues. Water itself is not a 

 suitable solvent for this purpose, since it is distributed almost 

 throughout the bodies of organisms, and very diffuse results 

 would therefore be obtained; beyond this, many coloured sub- 

 stances that are soluble in water would be taken up also by pro- 

 teins, so that the distribution of the water itself in the tissues 

 would not be displayed. Separate liquid droplets lend themselves 

 particularly to colouring by solution. The chief liquids that occur 

 in the tissues of organisms in this form are lipids and the essential 

 oils of plants. These can be coloured by solution. Solid (crystalline) 

 lipids will not dissolve colouring agents. ^^' ^^^ (The essential oils 

 will not be mentioned again. It is to be remembered that most 

 colouring agents that are soluble in lipids dissolve also in essential 

 oils and their thickened products, the resins.) 



A substance that is used in microtechnique to colour tissue- 



