572 SUMMAEY OF CUEEENT EESEAEOHES EELATING TO 



for permanent preservation over a shallow cell containing a solution 

 of potassium acetate (Koch's method), carbolic acid water (2-5 per 

 cent.), camphor water, or simply distilled water. 



To make satisfactory photographs of the smallest bacteria, it is 

 necessary to use a staining fluid which will give a stronger photo- 

 graphic contrast, as the violet is transparent for the actinic rays. 

 For this purpose aniline brown (recommended by Koch) may be em- 

 ployed, or iodine solution (iodine 2-5 grains, potassium iodide q. s. 

 to dissolve, distilled water 100 grains). 



A recent writer (Soubbotine *) advises the use of osmic acid as a 

 fixing solution to be used in advance of staining. This is doubtless 

 desirable when specimens of blood or thin sections of tissue containing 

 bacteria are to be examined, as the normal histological elements are 

 better shown, but the method possesses no special advantages so far 

 as the demonstration of vegetable organisms is concerned. It must be 

 remembered that aniline solutions often contain a granular precipitate 

 which might be mistaken by a novice for deeply stained micrococci. 



Ehrlich's Method of Exhibiting the Bacteria of Tuberculosis.! — 

 Dr. Ehrlich, Prof. Koch's assistant, has lately explained J a new 

 method of preparing tuberculous bacteria, which is a great improve- 

 ment on the original process of Koch. It renders the demonstration 

 much easier, and its results are so certain, that it may be applied to 

 establish the diagnosis of the disease in doubtful cases. The pre- 

 parations made according to the method recommended by Koch in his 

 first paper (i. e. double staining by alkaline methylene-blue and 

 vesuvin §) have left doubts in the minds of some observers, which 

 they could not have had if they had seen the preparations obtained 

 by Ehrlich's process which Koch himself has now adopted in prefer- 

 ence to any other. 



Tuberculous bacteria as well as all the micro-organisms, bacteria, 

 or micrococci, have a great affinity for aniline colours, and are 

 strongly stained by them. Koch's researches have shown that the 

 bacteria of tuberculosis have special and characteristic properties, and 

 that their cellular membrane is very easily penetrated by alkalies. 

 It is upon this experimental fact that Koch based his ingenious 

 method, which consists essentially in impregnation by an aniline 

 colour, rendered alkaline by the addition of a small quantity of 

 caustic potash. 



But this alkali exercises a modifying action on the different 

 histological elements and on the bacteria themselves. Under its 

 influence the albuminoid corpuscles swell to excess, and the coagulated 

 layers of morbid matter are easily detached from the cover-glasses. 

 Ehrlich, therefore, sought for another base, acting in a less powerful 

 manner, and found it in phenylamine or aniline. Other alkaloids, 

 perhaps even vegetable alkaloids, which can be transformed into 

 colouring matters by means of various reagents, might be equally 

 useful. 



* Arch, de Phys., viii. p. 479. 



f Bull. Soc. JBelg. Micr., vii. (1882) pp. cxvii.-cxxii. 

 % See Berl. Klin. Wochenschrift, 6th May, 1882. 

 § Cf. this Journal, ante, pp. 385-8. 



