292 STAIXS AND STAINING. 



syrupy fluid two of water be added, no crystallisation takes 

 place with the potassium iodide, and the solution answers every 

 requirement. At this point it agrees in consistence and sp. gr. 

 with strong sulphuric acid, about 1*84. If much weaker than 

 this it will dissolve potassium iodide to almost any extent, but the 

 solution, when finished, stains the cellulose such an ugly brick-red 

 colour, that as a re-agent it is next to useless. 



It might readily be supposed that the iodide of potassium 

 only acted as an aid to getting sufficient iodine into the solution ; 

 but there is no doubt that it modifies largely the colour produced 

 on the cellulose. When only traces of iodide are present the 

 colour produced is a pure blue ; with increasing quantities of 

 iodide the colour becomes more and more violet. The amount 

 of iodine which the solution is capable of dissolving depends 

 upon the strength of zinc chloride and the proportion of 

 potassium iodide, but there is no advantage in adding more than 

 oT per cent. The final result therefore is : — 



Chlor-zinc Iodt?ie {Imp?-oved formula). 



Zinc chloride solution (sp. gr. i'85)* 70 c.c. : potassium 

 iodide, 10 grammes : iodine, o*i gramme. The solution can 

 only be used as a reagent and not as a dye. Structures stained 

 with it cannot be mounted in any of the ordinary mounting 

 media. I have kept them for a short time by mounting them in 

 some of the fluid, and ringing the preparation with caoutchouc 

 cement. 



Cellulose can be stained permanently by carmine, haematoxy- 

 lin, nigrosine, methylene blue, safranine. and fuchsine. 



I use Grenacher's alcoholic borax carmine undiluted, and my 

 ammoniated haematoxylin, or Delafield's solution, diluted i to 9 

 of distilled water. The aniline dyes are used in dilute aqueous 

 solutions, containing one-eighth or one-fourth per cent, of dye. 



When the cellulose undergoes the change known as lignifica- 

 tion, its reactions are altered. It is coloured yellow by chlor-zinc 

 iodine, red with phloroglucin, yellow by aniline chloride. The 



* A solution of this sp. gr. may be obtained by evaporating 100 c.c. of 

 liquor zinci chloridi B.P. , remembering that the sp. gr. of this liquor should 

 be about i"53, and not 1*46 as officially stated. 



