622 



LABORATORY EXERCISES 



LESSON 21 



Cullivatton of Yeash on Gypsum Blocks.— Spore CuUivation. — Blocks of gypsum 

 are used generally for the cultivation of the spores of the yeasts. The block is in 

 the form of a truncated cone, and the cover of the vessel fits quite loosely. The 

 dishes used in the Carlsberg laboratory are the so-called bird troughs (Vogelnapfe). 



Fig. 219. — Method of pouring gelatin into Petri dishes. {After iMhnis.) 



A suitable size for these, taking outside measurements, is as follows: height 4.5 to 

 S cm.; diameter of the bottom about 7 cm. The gypsum block is 3 cm. high; the 

 diameter of the lower surface is 5.3 cm., that of the upper surface 3.8 cm. To make 

 a gypsum block, 2 parts of powdered gypsum are mixed with % part of water and the 

 mixture poured into a tin mould. The block should be hard, and the mould must 

 not be rubbed with fat, oil or such material A culture on a gypsum block in such 



Fig. 220. — Petri dish. (After Williams in Schneider, Pharmaceutical Bacteriology, 



P- 59.) 



a vessel cannot, as a rule, be kept free from bacterial infection, for the cover must 

 not be closed down tightiy, but should allow free access of the air. The dishes with 

 gypsum blocks are sterilized for one to one and a half hours at 110° to ti5°C., the 

 dishes first being wrapped in a crepe napkin or in iilter paper. The gypsum blocks 

 are sterilized in a moist condition before planting the yeast on their upper surface. 

 The gypsum blocks can be used several times. 



Method of Pouring Plates (Fig. 219). — Place three sterile Petri dishes (Fig. 220) 



