THE USE OF STRONGER STAINING SOLUTIONS. 121 



care, as it has a very powerful effect, even occasionally 

 decolourising the tubercle bacilli themselves. 



Grain's method of staining is exceedingly useful for 

 differentiating, similar organisms. The preparations are 

 first treated for a few minutes with aniline water solu- 

 tion of gentian violet. They are next introduced into 

 a liquid prepared in the following manner : to 300 

 ccm. of water one grm. of iodide of potassium and one 

 grm. of iodine are added. (As a rule, the iodine does 

 not become completely dissolved.) The cover-glasses 

 remain in this liquid for exactly two minutes ; after 

 which they are rinsed with absolute alcohol, until no 

 more stain is given oft'. By means of this method some 

 species remain stained, whilst others are decolourised. 

 It must, however, be borne in mind that many bac- 

 teria are differently affected by these reagents accord- 

 ing to the age of the cultivation ; young cultures, 

 perhaps, yielding preparations which remain intensely 

 stained, whilst those from very old ones may be quite 

 decolourised. This is, for example, the case with one 

 of the most common pus bacilli, the Staphylococcus 

 pyogenes aureus. 



Preparations made according to Gram's method 

 sometimes contain minute quantities of precipitates 

 which here and there make them indistinct. These 

 can generally be got rid of by placing the preparation 

 for five to ten seconds in 5 per cent, acetic acid or in 

 hydrochloric acid solution in alcohol. 



