NUTRIENT JELLY— EXAMINATION OF WATER. 



93 



very incomplete information. Combined with this, the method of 

 staining a dried specimen with- one of the aniline dyes, and then 

 preparing by His's method (§ 32, p. 46) is extremely useful. In- 

 oculations with the water to be examined of one of the numerous 

 cultivation media, by means of a fine glass pipette or a platinum 

 wire, should always be made. In the case of spores of certain fungi, 

 bread paste " medicated " with nutrient fluids serves as a capital 

 medium ; whilst for bacteria, the transparent solid media answer best, 

 the growth of the micro-organisms being readily distinguished with 

 the naked eye, or by means of a very low magnifying power. The 

 methods now most commonly used are those based on Koch's gela- 

 tine method ; the first devised by Koch himself, the second devised 

 by the late Dr. Angus Smith. 



Koch mixes a definite quantity of water with a certain quantity of 

 warmed peptonised meat jelly. Whilst the mass is still fluid, it is 

 poured on to a level surface, on which the points of growth of the 

 micro-organisms are afterwards readily recognised. 

 The method of procedure is shortly as follows : — 

 On a tripod, which, by means of screws, may be raised or lowered 

 at any point as required, is placed a flat glass plate ruled into squares 

 by two series of lines at right angles to one another, and about half 



FlG. 48. — Apparatus for determining relative purity of water. 

 a. Tripod, with adjustable feet. 

 /'. Large glass plate, on which spirit level rests. 



c. Smaller plate, with inoculated gelatine in centre. 



d. Bell jar. 



an inch apart. The centre of this is covered with a bell jar ; care 

 having been taken to have both plate and bell jar thoroughly sterilised, 



