'Substantially as recommended by the Bacteriologic Committee of the American Public Health Association. Some minor changes, suggested by Dr. H. W. Hill, in the Report of the Health Department of Boston for 1898, have been incorporated without detracting from their value as standard media. 'Acid media are denoted by the plus sign, and alkaline media by the minus sign; the degree of acidity or alkalinity is denoted by parts per hundred. Thus, a medium marked + 1.5 indicates that the medium is acid, and that 1.5 per cent. of sodium hydroxid is required to make it neutral to phenc lphthalein. n ī Following is Fuller's method of obtaining the degree of reaction of culture-media : n (a) Prepare a solution of sodium hydroxid. 20 n (b) Prepare a solution of hydrochloric acid. 20 (c) Transfer 5 c.c. of the medium to be tested to a porcelain evaporating-dish. (d) Add 45 c.c. of distilled water. (e) Boil for three minutes. (f) Add 1 c.c. of a 0.5 per cent. solution of commercial phenolphthalein in 50 per cent. alcohol. (g) Titrate while still hot with the acid or alkali as required, and determine the reaction. To determine exactly when the neutral point is reached, notice that in bright daylight the first change that can be seen on the addition of alkali is a very faint darkening of the fluid, which on the addition of more alkali becomes a more evident color and develops into what may be described as an Italian pink. A still further addition of alkali suddenly develops a clear and bright pink color, and this is the reaction always to be obtained. (h) When the reaction has been obtained, calculate the amount necessary to neutralize the bulk of the medium or to produce the required reaction, and add the proper amount of a normal solution of the acid or alkali. 9. Heat over boiling water (or steam) bath thirty minutes. 10. Restore weight lost by evaporation to original weight of filtered meat-infusion, for bouillon and gelatin, and to twice that weight for agar— 1800 grams in each case. II. Titrate, reaction probably + 0.3 to + 0.5 per cent. 12. Adjust reaction to final point desired, +1.5 per cent. Graniteware funnel. Layer of absorbent cotton. Coil of wire to pre vent cotton from FIG. 27.-Filter for culture-media. 13. Boil five minutes over free flame, with stirring. 14. Add water if necessary to make up loss by evaporation to 1800 grams. 15. Filter through absorbent cotton, passing the filtrate through the filter repeatedly until clear. 16. Titrate to determine whether or not the desired reaction has been maintained. 17. Tube and sterilize for fifteen minutes in the steam sterilizer on three successive days.' Some of the gelatin- and agar-tubes after the FIG. 28.-Funnel for filling tubes with culture-media : a, funnel containing the culture-media in liquid condition; b, pinch-cock by which the flow of fluid into the test-tube is regulated; c, rubber tubing (Warren). 1 Allow the chamber to fill with steam before placing the media within. Do not leave the media in the sterilizer to cool. Plunge gelatin-tubes into ice-water immediately after each sterilization, in order to maintain a high melting-point. FIG. 29.-Providence Health Department tube of heavy glass, with etched surface for writing data. These tubes are etched by dipping for thirty seconds in "white acid." FIG. 30.-Potato-tube (Mallory and Wright). last sterilization should be allowed to solidify with slanting surfaces. IV. Media from Meat-extracts. (a) Mix thoroughly the white of 1 egg with 1000 c.c. of water for bouillon or gelatin; with 500 c. c. of water for agar. (b) Proceed then as directed in the above table, beginning with No. 4, substituting "white of egg solution" for "infusion." At No. 6 add 0.5 per cent. of Liebig's extract of beef for bouillon or gelatin, 1.0 per cent. for agar. V. Potato. (a) Select large sound potatoes and wash thoroughly. (b) Cut off the ends, and with a sterile cork-borer cut out cylinders of the potato a little smaller than the tubes in which they are to be placed. Handle the potatoes under water as much as possible, to prevent darkening of the surface. (c) Cut the cylinders into two equal parts by a diagonal cut. (d) Place in cold running water for twelve to eighteen hours. This will usually render the potato neutral. (If necessary to change the reaction of the potato, steam in a measured quantity of distilled water for one-half hour. Titrate and add the required n amount of sodium hydroxid, and repeat the boil I ing for thirty minutes.) (e) Distribute in tubes in the bottom of which a small amount of non-absorbent cotton or a short piece of glass rod has been placed, and sterilize in |