"One-cow" or "whole" milk, if taken from the apparently healthy animal (that is, an animal without any obvious lesion of the udder or teats) with ordinary precautions as to cleanliness, avoidance of dust, etc., contains but few organisms. In dealing with one-cow milk, from a suspected, or an obviously diseased animal, a complete analysis should include the examination (both qualitative and quantitative) of samples of (a) fore-milk, (b) mid-milk, (c) strippings, and, if possible, from each quarter of the udder. "Mixed" milk, on the other hand, by the time it leaves the retailer's hands, usually contains as many micro-organisms as an equal volume of sewage and indeed during the examination it is treated as such. It is possible however to collect and store mixed milk in so cleanly a manner that its germ content does not exceed 5000 micro-organisms per cubic centimetre. Such comparative freedom from extraneous bacteria is usually secured by the purveyor only when he resorts to the process of pasteurisation (heating the milk to 65° C. for twenty minutes or to 77° C. for one minute) or the simpler plan of adding preservatives to the milk. Information regarding the employment of these methods for the destruction of bacteria should always be sought in the case of mixed milk samples, and in this connection the following tests will be found useful: 1. _Raw Milk_ (Saul). To 10 c.c. milk in a test tube, add 1 c.c. of a 1 per cent. aqueous solution of ortol (ortho-methyl-amino-phenol sulphate), recently prepared and mix. Next add 0.2 c.c. of a 3 per cent. peroxide of hydrogen solution. The appearance of a brick red color within 30 seconds indicates raw milk. Milk heated to 74° C. for thirty minutes undergoes no alteration in color; if heated to 75° C. for ten minutes only, the brick red color appears after standing for about two minutes. 2. _Boric Acid._ Evaporate to dryness, 50 c.c. of the milk which has been rendered slightly alkaline to litmus, then incinerate. Dissolve in distilled water, add slight excess of dilute hydrochloric acid and again evaporate to dryness. Dissolve the residue in a small quantity of hot water and moisten a piece of turmeric paper with the solution. Dry the turmeric paper. _Rose_ or _cherry-red_ color = borax or boric acid. 3. _Formaldehyde_ (Hehner). To 10 c.c. milk in a test tube add 5 c.c. concentrated _commercial_ sulphuric acid slowly, so that the two fluids do not mix. Hold the tube vertically and agitate very gently. _Violet zone_ at the junction of the two liquids = formaldehyde. 4. _Hydrogen Peroxide._ To 10 c.c. milk (diluted with equal quantities of water) in a test tube add 0.4 c.c. of a 4 per cent. alcoholic solution of benzidine and 0.2 c.c. acetic acid. _Blue coloration_ of the mixture = hydrogen peroxide. 5. _Salicylic Acid._