3IO BACTERIA, YEASTS, AND MOLDS 



about half an hour. Set aside for twenty-four hours, and steam 

 again ; and after another twenty-four hours steam a third time. If 

 properly made, the material will still be clear, and, being now sterile, 

 will remain clear indefinitely. It differs from the medium prepared 

 for molds chiefly in being alkaUne instead of acid. 



Agar Culture Medium 



For most purposes a modification of the above is desirable. It is 

 made in the same way, except that, instead of using loo grams of 

 gelatin, there are placed in the mixture 1 .5 grams of agar-agar (a prep- 

 aration from a sea moss which may be purchased from dealers). This 

 is known as agar culture medium. In other respects it is made pre- 

 cisely as above, except that more heat is required to melt agar than 

 to melt gelatin. 



51. Bacteria in Tap Water. Melt six of the agar tubes by mod- 

 erate heat. By means of a sterilized pipette, preferably one that holds 

 exactly one cubic centimeter, place in each of the six tubes a cubic 

 centimeter of water drawn directly from the tap. Mix the water thor- 

 oughly with the agar and pour the contents of each tube into a 

 petri dish, covering it at once and allowing it to cool. Set aside 

 at a temperature not above 70°. In about two days the dishes will 

 be found to be covered with little dots known as colonies. These 

 will be somewhat variable in appearance, but since each colony repre- 

 sents what was a single bacterium in the original drop of water, the 

 counting of these colonies in the plate will give the number of bac- 

 teria in the tap water. 



52. Bacteria in Well Water. Proceed in the same way with the 

 water drawn from a well if it is obtainable. 



53. Bacteria in Miscellaneous Waters. Obtain samples of water 

 in sterilized bottles from several sources — horse troughs, gutters, run- 

 ning water of the streets, snow, etc. — and treat them in the same way 

 as described above. Comparison of the plates will give an idea of 

 the relative number of bacteria in water from different sources. 



54. Bacteria in Ice. Obtain a piece of ice and melt it in a steril- 

 ized beaker. Place a cubic centimeter of the water in gelatin and 

 proceed as above described. 



