STAINS 31 



delusive resemblance to long slender bacilli are frequently de- 

 posited in it. A similar formation of crystals also occurs if 

 the stain be allowed to dry on the slide. 



In cold weather the thionin may crystallize nearly com- 

 pletely out, and the fluid stain very badly. If kept in a warm 

 place for a few hours it will recover its properties, the sedi- 

 ment being redissolved. 



8. Eosin is used in a 4 or 5 per cent, watery solution. This . 

 keeps well. Red ink (slightly diluted) will answer most 

 purposes. 



9. Acid methylene blue is prepared by mixing 2 parts of 

 borax methylene blue (vide supra), i part of -glacial acetic acid, 

 and 7 parts of water. It is only used for staining cells, casts, 

 etc., in w r et preparations, and has the advantage of dissolving 

 the red blood-corpuscles, which when numerous often obstruct 

 the view of the more important elements. It keeps well. 



10. Watery neutral red (J per cent.) is perhaps the most 

 generally useful counterstain for Gram's stain. 



Stains should be filtered before use. Where much work is 

 to be done, it is convenient to keep them in bottles which are 

 closed with a perforated cork throug'h which a small glass 

 funnel is placed. A filter-paper is kept permanently in this 

 funnel, and the stain is filtered directly on to the slide or cover- 

 glass. 



Gram's iodine solution may be mentioned here, though it is 

 not a stain. It consists of a solution of iodine, i part; iodide 

 of potassium, 2 parts ; water, 300 parts. It keeps indefinitely. 

 In preparing it, dissolve the iodine and iodide in a small 

 volume (a few c.c.) of water, and then add the remainder, as, 

 if they are dissolved in the full amount, solution is very slow. 



CLEANING SLIDES AND COVER-GLASSES 



Slides and cover-glasses must be absolutely clean when used 

 in the bacteriological laboratory; it is especially necessary that 

 they should be free from the slightest trace of grease, for this 

 will prevent fluid from spreading out into a thin and uniform 

 film. As a rule, new slides and cover-glasses are not greasy, 

 and require no elaborate cleaning. It is usually sufficient to 

 rub the former with an old handkerchief or linen rag, whilst 

 cover-glasses should be dropped into absolute alcohol first. 



