APPARATUS AND METHODS. 19 



place in it the specimen, and cover but this may damage the speci- 

 men, and it is better to apply as little heat as possible. 



17. Staining with Dyes and Mounting in Balsam. In 



addition to the various " microchemical " reagents which give 

 characteristic reactions with certain cell-contents and cell-walls 

 e.g. iodine, chlor-zinc-iodine, aniline sulphate, Millon's reagent, 

 alkannin it is often useful to stain specimens with dyes in order 

 to see clearly certain structures which are otherwise not readily 

 distinguished on account of transparency or lack of colour, or to 

 bring out differences between bodies of nearly the same general 

 appearance. 



It is not proposed to give here a full account of the various stains 

 used, the majority of which are aniline dyes, the chief non-aniline 

 stains being the haematoxylins and carmines. Delafield's haema- 

 toxylin is perhaps the best general dye to use when single staining 

 is required ; other useful stains for this purpose are safranin, eosin, 

 and aniline blue, all of which may be used for specimens which are 

 to be mounted in glycerine as temporary preparations, or made per- 

 manent by sealing or by transference to glycerine jelly. 



For various purposes specimens may be stained with two or even 

 more dyes in succession. A simple form of double staining is that 

 which has for its object the production of one colour in cellulose 

 walls and a second colour in lignified walls. Beginning with sections 

 of Marrow stem, for instance, we may either (1) transfer the sec- 

 tions from one liquid to the next in a series of watch-glasses or 

 pots, or (2) perform all the processes on the slide, applying drops of 

 the various liquids in turn by means of the glass rod belonging to 

 each bottle. 



First, treat the section with strong alcohol for a minute or two ; 

 then drain this off and add some safranin ; after ten or fifteen 

 minutes treat with 50 per cent, alcohol, and examine the specimen 

 until you find that the red colour has nearly disappeared from the 

 cellulose walls, though still present in the lignified walls. Treat 

 the section for two or three minutes with Delafield haematoxylin 

 this will stain the cellulose walls, but should not displace the 

 safranin from the lignified walls. Treat with water ; if the purple 

 colour is very deep, add a trace of hydrochloric acid (a drop to 

 50 c.c. of water), and as soon as the sections begin to turn reddish 

 rinse them in plain water. Treat with ordinary alcohol for two or 

 three minutes, then with absolute alcohol for five or ten minutes 

 to dehydrate the section, which is very important ; then drain off 

 the alcohol, and cover with a drop of clove oil, to clear the section ; 

 then drain off the oil, put on a drop of balsam, and cover. In this 

 way we get a permanent double-stained balsam preparation ; the 

 lignified and suberised walls are stained red, the cellulose walls 

 violet. 



Various other combinations of stains are used for double staining, 

 on the same general principles, sections with lignified and cellulose 



