84 CLINICAL BACTERIOLOGY. 



test-tubes. The disturbing water of condensation can be 

 removed from the surface of the plates by placing these in 

 an inverted position for forty-eight hours in the thermostat 

 at a temperature of 37 C. (98.6 R). 



Human blood-serum is obtained either by blood-letting 

 or from placentas. After the umbilical cord has been 

 ligated and divided, the maternal extremity is disinfected by 

 means of mercuric chlorid, which is rinsed off with distilled 

 water. An incision is made into the cord above the point 

 of ligation, and the blood is permitted to escape into steril- 

 ized flasks. The conversion into culture-media is effected 

 in the manner already described. A mixture of three parts 

 of sheep's blood-serum with one part of a one per cent, 

 grape-sugar bouillon (Loffler's blood-serum) may be em- 

 ployed with especial advantage for several purposes (diag- 

 nosis of diphtheria), being solidified and sterilized in tubes 

 or plates in the same way as ordinary blood-serum. 



Blood-serum agar (blood-serum glycerin- agar), which also 

 is employed with advantage for special purposes, is pre- 

 pared by adding to liquefied agar or glycerin-agar cooled 

 to 40 C. (104 F.) an equal amount or half the quantity 

 of sterile liquid blood-serum heated to 40 C. (104 F.). 

 The mixture is introduced into test-tubes or Petri dishes, 

 and made to solidify rapidly. 



(e) The Preparation of Potatoes. Potatoes constitute an 

 excellent nutritive medium for many purposes. They may 

 be prepared in various ways. Large potatoes are thor- 

 oughly cleansed with a brush and mercuric chlorid, are 

 carefully freed of their peels and their so-called decayed 

 spots and eyes, and are cut into slices about one cm. thick, 

 which are placed in glass double dishes. The peels, 

 together with the slices of potatoes, are exposed for an 

 hour to steam under pressure at a temperature of 110 C. 

 (230 F.), or for fifteen or twenty minutes at a temperature 

 of 120 C. (248 F.) ; or suitable oblique pieces are cut 

 out of the potatoes and are introduced into test-tubes with 

 some cotton at the bottom, and these are sterilized in the 

 manner already described. The cotton at the bottom of 

 the tubes is for the purpose of absorbing the fluid that 

 appears in the process of boiling, and with its aid the 

 potato is subsequently kept moist. Roux has devised 

 special tubes for potato-cultures which are narrowed near 

 the bottom. (Fig. 15.) Upon the shoulder or projection 



