(2) Concentrated hydrochloric acid 25 c.c. Distilled water 100 c.c. When the phosphotungstic acid is completely dissolved, mix the two solutions. This reagent is quite stable if kept in a dark glass bottle. Before examination, the milk should be diluted according to the probable amount of proteid, and allowance made in the subsequent reading. For human milk the optimum dilution is 1:10; for cow's milk, 1:20. Dilution must be accurate. Lactose.--The proteid should first be removed by acidifying with acetic acid, boiling, and filtering. Purdy's method may then be used as for glucose in the urine (p. 79); but it must be borne in mind that lactose reduces copper more slowly than glucose, and longer heating is, therefore, required; and that 35 c.c. of Purdy's solution is equivalent to 0.0268 gm. lactose (as compared with 0.02 gm. glucose). * * * * * It is frequently desirable to detect formalin, which is the most common preservative added to cow's milk. Add a few drops of dilute phenol solution to a few cubic centimeters of the milk, and run the mixture gently upon the surface of some strong sulphuric acid in a test-tube. If formaldehyd be present, a bright-red ring will appear {281} at the line of contact of the fluids. This is not a specific test for formaldehyd, but nothing else likely to be added to the milk will give it. SYPHILITIC MATERIAL [Illustration: FIG. 129.--Spirochæte pallida: _a_ and _b_, Typical pallida; _c_ and _d_, atypical curves in pallida; _e_, thick pallida, apparently splitting into three; _f_, two pallida partly so closely coiled as to resemble thick portion of _e_ (X1800) (Goldhorn).] In 1905 Schaudinn and Hoffmann described the occurrence of a very slender, spiral micro-organism in the lesions of syphilis. This they named _Spirochæte pallida_, because of its low refractive power and the difficulty with which it takes up staining reagents. Its etiologic {282} relation to syphilis is now almost universally admitted. It is not found in tertiary lesions. _Spirochæte pallida_ is an extremely slender, spiral, motile thread, with pointed ends. It varies considerably in length, the average being about 7 µ, or the diameter of a red blood-corpuscle; and it exhibits three to twelve, sometimes more, spiral curves, which are sharp and regular and resemble the curves of a corkscrew (Fig. 129). It is so delicate that it is difficult to see even in well-stained preparations; a high magnification and careful focusing are, therefore, required. Upon ulcerated surfaces it is often mingled with other spiral micro-organisms, which adds to the difficulty of its detection. The most notable of these is _Spirochæte refringens_, which is distinguished by being coarser and having fewer curves of wider and less sharp contour (Fig. 130). [Illustration: FIG. 130.--Spirochæte refringens (X1800) (Goldhorn).]