~Qualitative.~--The qualitative bacteriological examination of milk is chiefly directed to the detection of the presence of one or more of the following pathogenic bacteria and when present to the estimation of their numerical frequency. Members of the Coli-typhoid group. Vibrio choleræ. Streptococcus pyogenes longus. Micrococcus melitensis. Staphylococcus pyogenes aureus. Bacillus enteritidis sporogenes. Bacillus diphtheriæ. Bacillus tuberculosis. Some of these occur as accidental contaminations, either from the water supply to the cow farm, or from the farm employees, whilst others are derived directly from the cow. In milk, as in water examinations, two methods are available, viz.: Enrichment and Concentration--the former is used for the demonstration of bacteria of intestinal origin, the latter for the isolation of the micro-organisms of diphtheria and tubercle. The first essential in the latter process is the concentration of the bacterial contents of a large volume of the sample into a small compass; but in the case of milk, thorough centrifugalisation is substituted for filtration. _Apparatus Required_: A large centrifugal machine. This machine, to be of real service in the bacteriological examination of milk, must conform to the following requirements: 1. The centrifugal machine must be of such size, and should carry tubes or bottles of such capacity, as to enable from 200 to 500 c.c. of milk to be manipulated at one time. 2. The rate of centrifugalisation should be from 2500 to 3000 revolutions per minute. 3. The portion of the machine destined to carry the tubes should be a metal disc, of sufficient weight to ensure good "flank" movement, continuing over a considerable period of time. In other words, the machine should run down very gradually and slowly after the motive power is removed, thus obviating any disturbance of the relative positions of particulate matter in the solution that is being centrifugalised. 4. The machine should preferably be driven by electricity, or by power, but in the case of hand-driven machines-- (a) The gearing should be so arranged that the requisite speed is obtained by not more than forty or fifty revolutions of the crank handle per minute, so that it may be maintained for periods of twenty or thirty minutes without undue exertion. (b) The handle employed should be provided with a special fastening (e. g., a clutch similar to that employed for the free wheel of a bicycle), or should be readily detachable so that, on ceasing to turn, the handle should not, by its weight and air resistance, act as a brake and stop the machine too suddenly. One of the few satisfactory machines of this class is shown in figure 212.