294 CLEAN MILK 



be given save that hot water and soap are necessary. New glass- 

 ware should be treated first with 1% HC1. Used glassware that 

 contains the remains of gelatin or other media must first be boiled 

 in water, preferably containing a little sal soda or powdered soap. 

 After boiling, wash well in hot water and then rinse thoroughly 

 in clear cold water; drain and allow to dry. 



2. Comparison of Yeast and Bacteria. — Rub up in a watch- 

 glass a bit of a cake of yeast with a little water. Place a drop 

 of the liquid on a slide and examine directly with the microscope, 

 using a %-inch objective. Dry a little of the material on a slide 

 and stain exactly as in staining bacteria (No. 3). Study with the- 

 immersion lens, comparing the yeast cells as to size and shape with 

 bacteria already studied. Hunt for cells showing budding. If these 

 are not found they may be obtained in quantity by planting a little 

 yeast in a weak solution of molasses in water and allowing to grow 

 for a few hours in a warm place. Make a sketch of the yeast 

 cells with buds, showing their relative size to that of bacteria. 



3. Direct Microscopic Study of Milk. — For this a centri- 

 fuge is needed, having tubes with straight sides, and closed at the- 

 bottom with a rubber cork. Place 2 c.c. of the samples of milk- 

 to be tested, after thorough shaking, in the tubes. It is necessary- 

 to fill enough tubes with milk from different samples, to balance 

 the centrifugal machine on both sides. Rotate in the machine 

 at a rate of 2,000 to 3,000 per minute, for 5 minutes. Remove 

 the tubes from the machine and they will be found to have a layer 

 of cream at one end, and a small slimy deposit at the other end, next 

 the cork. Holding the tube with the cream end down, remove the 

 cream with a platinum loop, and pour out the milk gently. A slimy 

 sediment will be left attached to the cork. Carefully remove the 

 cork with its adhering sediment, and smear over the surface of a 

 glass slide, with a drop of sterile water, to cover an area of exactly 

 four square centimeters. Special slides marked with wax pencils 

 into such areas are needed. This distribution of the sediment must 



