400 APPENDIX. 



of common salt, recommended by Miqiiel, or of 5 per cent, glycerine, 

 which is added before the nutrient fluid is finally sterilized (first used by 

 Roux and Nocard). This glycerine meat extract is, as we have already 

 seen, an excellent medium for the growth of the bacillus tuberculosis. 

 Albumen peptone, cane or grape sugar, acetic acid, mannite, &c., have all 

 been added in various proportions and for various purposes. Liebig's extract 

 of beef, in the proportion of five parts to one thousand, or Cibil's extract, 

 twenty grammes to a litre, may also be used. These latter require to be very' 

 carefully sterilized by Tyndall's method of discontinuous heating, in which 

 the fully developed organisms are killed off in a very short time on exposure 

 to a comparatively low temperature. Some of the spores that remain in 

 these develop into the vegetative form during the next twenty-four hours. 

 This crop is again killed by a second heating. The remaining spores, if 

 any, are again encouraged to develop, and then this crop is also killed off, 

 usually leaving the fluid sterile, though in some cases the process may have 

 to be repeated three, four, or even five' times. Various infusions and 

 decoctions of wheat or hay, of different fruits or vegetables, yeast water, 

 beer wort (the latter especially for the culture of the mucors and yeasts), 

 a mixture of beer wort and prune juice (especially useful for the growth of 

 the various aspergilli) may be used. These should all be sterilized by 

 discontinuous heating at 100° C. in the steam sterilizing apparatus for 

 twenty minutes on three or four successive days. Urine, aqueous humour, 

 or other fluids of the body drawn with antiseptic precautions, may all be 

 used as cultivation media for certain organisms. 



Milk. 



Milk may also be used as a culture medium, but although it is a substance 

 easily obtained, it is a somewhat difficult matter to render it absolutely 

 sterile. If heated under pressure to 120° C, milk may be sterilized in 

 from ten to fifteen minutes ; but in the steam sterilizing chamber, at 100° 

 C. , it is necessary to heat it for an hour on the first day, and from twenty 

 to thirty minutes on each of the two following days. 



Solid Culture Media. 



Bread Crumb. 



One of the simplest of the solid culture media, bread crumb, is prepared 

 by taking the crumb of a loaf, drying it in small pieces, spreading it out on 

 a sheet of clean paper in an oven, or on the top of a stove, or even in front 

 of a warm fire ; then rubbing it through a fine sieve, or passing it through 

 a coffee-mill. A small quantity of this dried crumb, sufficient to cover the 

 bottom of a flask to about the depth of a quarter of an inch, is put into a 

 wadding-stoppered small sterilized flask ; distilled water is added until the 

 bread crumb is thoroughly moistened,-no superfluous water, however, being 

 left unabsorbed. If after allowing the bread crumb to stand for about a 

 quarter of an hour, it is found to be properly moistened, the flasks con- 

 taining it are heated in the steam chamber on each of three successive 

 days for half an hour. In place of using water, the crumb may be moistened 

 with beef extract, sugar solution, dilute glycerine, or any other of the fluid 

 media already referred to. This medium is used chiefly for mucors and 

 should be rendered slightly acid by the addition of a small quantity of 

 Tartaric or other organic acid. 



