106 CLINICAL BACTERIOLOGY. 



method, as by this means the blood coloring-matter and a 

 portion of the plasma are removed. 



The specimens prepared in the manner described show 

 the size and the shape of the bacteria, but fail to disclose 

 their mutual positions in the colonies. It this information 

 be desired, so-called contact-preparations or impress-prepa- 

 rations must be made. A carefully cleaned cover-slip is 

 lightly applied or impressed upon an isolated bacterial 

 colony of a gelatin-plate or an agar-plate, and immediately 

 removed. After it has been dried, this preparation is further 

 treated exactly in the manner described. 



Staining of Sections of Tissue. Bits of tissue hard- 

 ened according to the usual methods and embedded in cel- 

 loidin are cut with the aid of the microtome. The sections 

 are next placed in distilled water, and from this are trans- 

 ferred to one of the staining solutions mentioned. In this 

 they are permitted to remain in general from two to fifteen 

 minutes. The excess of coloring-matter is removed by 

 means of dilute acetic acid (i : 1000), and the sections are 

 rinsed in water for the removal of the acid, dehydrated with 

 absolute alcohol, cleared with xylol, and mounted upon a 

 slide in xylol-Canada balsam. It is advisable first to spread 

 the sections upon slides, and to undertake the staining, 

 the decolorization, the dehydration, and the clearing upon 

 these. On the whole, this mode of procedure agrees with 

 that originally described by Weigert. If the tissue has 

 been embedded in paraffin, this must be extracted with 

 xylol before staining, and the xylol be then removed with 

 alcohol. 



The methods of staining thus far described have the 

 property of staining all bacteria in the same way. There 

 is, however, a method that stains some of the bacteria 

 specifically that is, Gram's method. 



Gram's Method. The prepared cover-slips from a fresh 

 culture from twenty-four to forty-eight hours' old are 

 placed in aniline-water gentian-violet solution (cover-glasses 

 for one or two minutes, sections directly out of alcohol for 

 ten or fifteen minutes), and then for half a minute, or for two 

 and a half minutes or three minutes, in an iodin potassium- 

 iodid solution, consisting of iodin i.o, potassium iodid 2.0, 

 water 300.0. Decolorization is next effected in alcohol, and 

 is continued as long as any color is given off*. The prepara- 

 tion, which finally appears colorless (light gray), is then 



