CHEMICAL REAGENTS. 25 



have to be examined as they grow on or in some of the solid 

 cultivating media — gelatine, blood serum, bread paste, potatoes, &c. ; 

 fourthly, as they occur in the tissues of the body ; and lastly, in 

 various organic materials — foods, dust, mould, vegetables, and so on. 

 In whatever position they are found, these micro-organisms react 

 as though they were perfectly free, and the differences of treatment 

 are necessitated, not from any change in the organism itself, but 

 from the different surroundings it has assumed. Thus, it would not 

 be necessary to add any reagent to a cultivation of micro-organisms 

 in a clear meat extract ; all the micro-organisms are perfectly dis- 

 tinguishable, and in such a fluid there is nothing else for which they 

 can be mistaken. Alcohol, chloroform, ether, or even strong alkalies 

 or acids, have not the slightest effect on them ■ there is no alteration 

 in appearance, and the micro-organisms appear as sharply defined, 

 strongly refractile, rounded, ovoid, or rod-shaped bodies, the former 

 arranged in pairs, chains, or masses, the latter most frequently in 

 pairs or chains. On the addition of some of the aniline colouring 

 reagents the micro-organisms in the above fluid stain just as if they 

 were embedded in the tissues or in a mass of sputum. 



It is only when they are mixed up with other granular inorganic or 

 organic matter, or when they are embedded in tissue cells, or in inter- 

 cellular or other spaces in the body, that there is any great difficulty in 

 identifying these micro-organisms, and in assigning to them their 

 proper position in the scale of nature. 



These organisms, with a single exception (the Spirochete Ober- 

 meyeri), exhibit an extraordinary resistance (optically) to all chemical 

 reagents, for, as noted, they are not changed in the slightest degree 

 by the above-mentioned reagents ; and it is to this fact that we owe 

 the possibility of distinguishing them from the tissues or granular 

 material in which they lie, all of which may be more or less altered 

 by one or other of these chemical reagents. The addition of acids 

 will, in most cases, dispose of inorganic granular matter, granular 

 looking fibrin, and even of tissue structure. Ether or chloroform, 

 and alcohol, dissolve out fatty granules or fat crystals. Caustic 

 potash or soda may be used instead of or along with the acid to 

 remove granular material, and to clear up the tissues. The following 

 may be taken as an example of the routine which should be observed 



