1894] NEW-YORK MICROSCOPICAL SOCIETY. 71 



mercially dyed fabrics this will often be found to have been done 

 very imperfectly. Thus a piece of Turkey red appears in the 

 piece to be well dyed, and practically it is ; but when examined 

 microscopically it will be found that the individual fibres have 

 taken the dye very unevenly. This is often the case with fabrics, 

 especially cotton and linen, which have been dyed in the piece ; 

 whereas if the fibre is dyed before weaving, as in the ginghams, 

 the result on the individual fibres is more uniform. 



A mordant is a substance which has an affinity for both organic 

 tissues and coloring matters, and which by virtue of this property 

 is employed in dyeing to fix the color upon the tissue. 



The most common mordant in staining microscopic prepara- 

 tions is alum, a solution of which is usually mixed with the stain 

 previous to its application. Other substances, as dilute solutions 

 of organic and mineral acids, are sometimes used under the name 

 of "fixers." Their action is seldom that of a true mordant, their 

 efficiency, when they possess any, being confined to some decom- 

 position of the coloring matter or to some action upon the tissue 

 itself. 



In staining animal and vegetable sections some stains require a 

 mordant, while others do not. Thus hasmatoxylin, or the stain- 

 ing principle of logwood, is often used without a mordant, while 

 most of the carmine stains require one. 



The staining of tissues may be effected in four ways. First, 

 when the stain has sufficient affinity for the tissue to be retained 

 by it without the intervention of any outside agent. Second,, 

 when the stain and mordant are mixed and applied to the tis- 

 sue in one solution. These two are the simplest and easiest 

 methods of staining. Third, when the tissue is first immersed in 

 the staining liquid and then transferred to some other liquid which 

 shall fix the color upon the tissue. Fourth, when the tissue is 

 first impregnated with the mordant, or fixing agent, and then im- 

 mersed in the stain. The last method is the one usually followed 

 in commercial dyeing establishments, and is to be recommended 

 in the staining of microscopical preparations which do not readily 

 take the stain. 



It is easy to see that, in substances which are difficult to stain, 

 there is less chance of effecting the object when the mordant and 

 stain are both presented to the object at the same time, than when 



