degradation of the sharpness of the border-line. On the other hand, with a sufficient quantity of solution, the border-line is surprisingly sharp. The refractometer is now suspended on the frame, and the measurement proceeded with as before described. After measurement, the cover is first removed, and the prism allowed to fall into the hollow of the hand, then the beaker is removed to enable the refractometer to be conveniently cleaned. Strengths of Various Solutions.-The most extensive work on the quantitative determination of the strength of a large number of common aqueous solutions with the immersion refractometer has been done by Wagner, who has published a large number of tables. These tables show the percentage strength (grams per 100 cc. at 17.5° C.) of a large number of salt solutions and of acids, corresponding to the range of scale readings of the instrument, as well as of cane sugar, dextrose, formalde SCALE READINGS ON IMMERSION REFRACTOMETER OF VARIOUS STANDARD REAGENTS USED IN VOLUMETRIC ANALYSIS.* Hydrochloric acid: Sulphuric acid: Normal.. Fifth-normal. Tenth-normal. Oxalic acid: Half-normal. Potassium bitartrate: Tenth-normal. Potassium hydroxide: Potassium bichromate: Tenth-normal.... Silver nitrate: Tenth-normal. Sodium chloride: Tenth-normal. Ammonium sulphocyanate: Tenth-normal..... Temperature C. 15°. 16°. 17°. 17.5°. 18°. 19o. 20°. 21°. 22°. 37.45 37.20 36.95 36.85 36.70 36.45 36.20 35.95 35-70 17.80 17.60 17.40 17.30 17.20 17.00 16.80 16.55 16.30 30.60 30.40 30.20 30. 10 29.95 29.75 29.50 29.25 29.00 18.75 18.60 18.40 18.30 18.20 18.00 17.80 17.55 17.30 17.15 16.95 16.75 16.65 16.55 16.35 16.15 15.90 15.65 22.45 22.30 22.10 22.00 21.90 21.70 21.50 21.25 21.00 17.15 16.95 16.75 16.65 16.55 16.35 16.15 15.90 15.65 17.75 17-55 17-35 17.25 17.15 16.95 16.75 16.50 16.25 43.90 43.65 43.40 43.25 43.10 42.80 42.50 42.20 41.95 18.45 18.30 18.10 18.00 17.90 17.70 17.50 17.25, 17.00 18.50 18.35 18.15 18.05 17.95 17.75 17-55 17.30 17.05 24.20 24.05 23.85 23.75 23.65 23.45 23.20 22.95 22.70 17.75 17-55 17-35 17.25 17.15 16.95 16.75 16.50 16.25 20.20 20.05 19.85 19.75 19.65 19.45 19.25 19.00 18.75 18.20 18.00 17.80 17.70 17.60 17.40 17.20 16.95 16.7c 20.60 20.45 20.25 20.15 20.05 19.85 19.65 19.40 19.15 According to Wagner, all these solutions were made up at 17.5° C. Readings at different tem peratures are given for convenience. hyde, alcohol, etc. All these observations have been based on the Mohr liter, at a temperature of 17.5°. More convenient for the American analyst would be tables based on the use of a higher temperature, say 20°, and the analyst is recommended to work out his own standards for comparison, at the temperature best suited to his special locality and convenience. The instrument is especially useful in preparing normal and tenthnormal solutions. The table on page 106, from Wagner, shows the strength of various common laboratory reagents. SCALE READINGS AT TEMPERATURES FROM 10-30° C. 30 3.20 3.15 3.25 3.40 3.55 3.65 3.90 4.05 4.20 4.60 4.80 2.90 2.85 2.95 3.10 3.25 3.35 3 55 3.75 3.90 4.25 4.45 4.85 4.50 3.65 25 1.75 1.75 1.80 1.90 2.05 2.15 2.25 2.45 2.60 2.70 2.95 -- 2222 23 21 24 50 1.45 1.55 1.60 1.75 1.85 1.95 2.10 2.25 2.35 2.55 20 0.50 0.50 0.55 0.60 0.65 0.65 0.75 0.75 0.85 0.90 0.95 19 18 0.30 0.30 0.30 0.35 0.40 0.40 0.45 0.45 0.45 0.55 0.55 17-5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 17.5 17 16 -0.10 0.10 0.10 0.10 0.10 0.10 0.15 0.15 0.15 0.15 0.20 0.20 17 0.30 0.30 0.30 0.30 0.35 0.35 0.40 0.45 0.45 0.50 0.55 15 0.50 0.45 0.45 0.50 0.60 0.60 0.65 0.75 0.75 0.80 0.85 14 13 0.70 0.60 0.60 0.70 0.80 0.85 0.90 0.95 1.05 1.10 1.25 CHAPTER VII. MILK AND ITS PRODUCTS. MILK. CHARACTERS AND CONSTITUENTS.-Milk is the secretion of the mammary glands of female mammals for the nourishment of their young. Containing as it does all the requisites for a complete food, i.e., sugar, fat, proteins, and mineral ingredients, combined in appropriate proportion, there is ample reason why it occupies so high a place in the scale of human foods. It is a yellowish-white opaque fluid, denser than water, containing in complete solution the sugar, soluble albumin, and mineral content, and, ir. less complete solution, the casein, while the fat-globules are held in suspension in the serum, forming an emulsion. The specific gravity of pure milk ranges from 1.027 to 1.035. Milk from various animals has the same general physical properties and the same ingredients, differing, however, in percentage composition. Of all the varieties, the milk of the cow is by far the most important from its universal use, and, unless otherwise qualified, the term milk wherever it occurs in this volume will be understood to mean cow's milk. Acidity. When perfectly fresh, milk of carnivorous mammals is, as a rule, acid to litmus, while human milk is alkaline. Cow's milk, when freshly drawn, reacts acid to phenolphthalein, and alkaline to methyl orange and lacmoid, but amphoteric to litmus, i.e., reacts acid to blue and alkaline to red litmus paper. Bordas and Touplain * believe the original acidity to phenolphthalein is due to casein; as a result of the decomposition of lactose, before lactic acid appears, casein is separated from calcium caseinate and monocalcium phosphate is formed from the dicalcium salt. It soon becomes distinctly acid, and the acidity increases as the lactose gradually becomes converted into lactic acid. Microscopical Appearance. Under the microscope pure milk shows a conglomeration of various-sized fat globules having a pearly lustre. * Compt. rend., 152, 1911, p. 1274. |