78 BACTERIOLOGY 



bacilli the dye is allowed to act for twelve hours, and in 

 that of lepra bacilli for half a day. Superfluous stain is 

 removed with blotting-paper, and the preparation is brought 

 for two minutes into solution of iodine and potassium 

 iodide, then for half a minute into pure alcohol, for 

 exactly ten seconds into 3 per cent, hydrochloric acid in 

 alcohol, and immediately afresh into plain alcohol for 

 several minutes, changing the spirit as long as any colour 

 is extracted from the preparation. The cover-glass is now 

 dried and mounted in balsam, and sections of tissue are 

 laid in xylol (which renders them transparent in half a 

 minute), and then mounted on slides with Canada balsam 

 dissolved in the same liquid. 



Weigert's modification of Grain's process. The sections 

 stained with gentian or methyl violet are not transferred 

 to alcohol from the iodine solution, but laid upon slides and 

 covered with aniline oil, which dehydrates and differentiates 

 them. The aniline oil is then removed with blotting-paper, 

 xylol is poured upon the preparation, and it is put up in 

 Canada balsam in xylol. 



Impression preparations. These are made for the purpose 

 of rapidly gaining an idea, when examining plates, regarding 

 the arrangement of the colonies and the microscopic 

 peculiarities of the organism under investigation. A cover- 

 glass is laid on the plate, pressed gently down, lifted care- 

 fully with a forceps, and laid aside to dry. It can then be 

 stained like an ordinary cover- glass preparation. 



Examination of micro-organisms in sections of tissues. 

 The examination of micro-organisms in the tissues, whether 

 in the interior of the individual cells, or in the structures 

 which are formed by them, is of pre-eminent importance in 

 research directed to medical ends. Not only have the nature 

 of the micro-organisms and their mode of entrance into 

 the body to be discovered, but attempts must be made to 



