TEXT-BOOK OF BACTERIOLOGY. 57 



either to discard alcohol altogether and replace it by some other 

 substance, or to find some method of diminishing 1 its excessive 

 bleaching power. The first of these two plans was adopted by 

 Unna, who treats the sections, after they have been properly decol- 

 ored, with acetic acid and distilled water over a flame, thus remov- 

 ing all moisture l>y evaporation, a process which must of course be 

 performed on the slide. Opinions are much divided as to the value 

 of this dry method. If the wet section be held on the glass above a 

 flame till the water is dispelled, then given transparency in xylol 

 as Unna recommends, mounted in Canada balsam, and examined 

 under the microscope, it will be found that, under some circum- 

 stances, bacteria can be seen which could not have been made visi- 

 ble by the ordinary process. 



But the tissue is seriously damaged. It is full of gaps and cracks ; 

 it looks lumpy and curdled. Therefore when we wish to display 

 the typical structure of the tissue, the dry method is scarcely ap- 

 plicable. If, on the other hand, it is only desired to display the 

 micro-organisms that may be in the tissues, it may in some cases 

 prove of advantage. 



There is another similar process by which the preparation suf- 

 fers less and in which the water is not removed by heat, but by a 

 current of air. The section, lying on the slide, is dried by means of 

 a small balloon-bellows, rendered transparent in xylol, and im- 

 bedded in balsam. 



H. Klihne uses alcohol, but previous^ adds to it a considerable 

 quantity of the same stain with which the preparation was treated. 

 In the process for removing water almost every atom of coloring 

 matter which is removed by diffusion is replaced by another, and 

 thus the decoloration is reduced to a minimum. 



Far better results, however, are gained by another process 

 that of Weigert, which is the best of all wherever we have to deal 

 with bacteria in tissue that are difficult to stain. 



Weigert employs, instead of alcohol, anilin oil, or a mixture of 

 two parts anilin oil and one part xylol. The anilin oil acts similar 

 to the alcohol, but far more mildly; it removes water quickly and 

 decolors comparatively little. If necessary to avoid even this little, 

 like H. Kuhne, first color the anilin oil with a little of the stain. 

 To do this, rub as much of the powdered stain as can be raised on 

 the point of a knife into about 10 c.cm. of oil in a little dish. If the 

 mixture be allowed to stand for a time it becomes clear. Then add 

 as much of it to the pure anilin oil as is required to produce the de- 

 sired tint.. The anilin process has this advantage, that the sections 

 on leaving the water are apt to roll up and contract, so much so 



