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 



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



a. Tripod, with adjustable feet. 



b. 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, 



