and Laboratory Methods. 1165 



" Straw Infusion. I procured about a handful of straw at a livery stable 

 and put it in a Mason jar three-quarters full of water, and put it in a warm place 

 where the temperature was on an average of 75°, on Thursday, March 22. Its 

 color was tan and the mixture smelt like musty straw. 



" The 23d, temperature 73°, mixture getting darker in color, and smell 

 becoming more noticeable. Saturday, temperature 74°. A thin scum is forming 

 and small things are coming up from the bottom and straw. The smell is get- 

 ting very strong. 



" Sunday, temperature 74°, scum becoming thicker and bubbles appearing in it." 



Discussion and microscopical examination in the class room brought out the 

 fact that the scum was composed of countless bacteria and other micro-organ- 

 isms which had grown from the germs on the dried hay. The inference was 

 drawn from the experiment that bacteria grow rapidly in a warm temperature, 

 when water and organic matter are present, and that decay is one of the results 

 of their activity. 



The cultivation of bacteria in the laboratory was the topic next considered. 

 Nutrient gelatin, the most useful medium in which to grow all kinds of bacteria, 

 may be readily prepared in the laboratory or in the home kitchen. The ingre- 

 dients necessary are the following : one pound of lean beef chopped fine (or 

 better run through a meat cutter) ; 60 grams (2 oz.) of the best French 

 gelatin ; 6 grams (1-5 oz.) of peptone, which can be bought for 10 cents at any 

 drug store ; a teaspoonful of salt, and a little baking soda. Put the beef in a 

 porcelain or agate dish, add a pint of cold water, and allow the mixture to boil 

 slowly for a half hour. Strain the broth through muslin and then allow the 

 liquid to run through filter paper. Pour in enough water to make the quantity 

 of broth equal to about a pint and a half.* The gelatin, cut into small pieces, is 

 then added to the broth, together with the peptone and salt. The mixture should 

 be heated sufficiently to cause the gelatin to dissolve, but should not be allowed 

 to boil. Just enough cooking soda is added to cause red litmus paper dipped 

 in the mixture to turn blue, that is, the liquid should be faintly alkaline. Filter- 

 ing the hot gelatin sometimes involves more or less difficulty. The process can 

 be easily carried on, however, within a steam cooker. A glass funnel should be 

 put in the mouth of a Florence flask (used commonly in a chemical laboratory) 

 and one or two layers of absorbent cotton placed within the funnel. If the gela- 

 tin, flask, and funnel are kept hot within the cooker the liquid will readily pass 

 through the cotton. After filtering, close the mouth of the flask with a plug 

 of absorbent cotton, and boil for a few moments. The flask may be set aside 

 as stock gelatin until needed for use. (If the gelatin mixture is not clear, it 

 should be filtered through the same cotton a second time.) 



Some of the liquid gelatin was poured into clean Petri dishes (Fig. 1), 

 or test tubes plugged with cotton may be used. After the gelatin had 

 solidified some of the dishes were opened to the air. Several days after 

 this exposure the cultures were placed upon the desks of the pupils, and they 

 were asked to make drawings of the bacteria colonies, and to answer certain 



*This broth may be prepared more easily from Liebig's beef extract. Four grams should 

 be dissolved in the pint-and-a-half (750 c. c. ) of water, and the solution should be filtered through 

 filter paper. 



