220 The Microscope. 



as the standard quantity in which the bacterial content is expressed. 

 It should be remembered, in studying the results of the analysis 

 which are to follow, that one cubic centimetre (i. c. c.) of water is a 

 little less than one-third of a teaspoonful. 



The portion of gelatin mixed with the measured quantity of 

 water is now poured out into a sterilized glass plate, and is spread 

 upon it in a thin, even layer. The plate, while this is being done, 

 rests upon a level surface kept cool by ice, so that the gelatin soon 

 solidifies. The plate with its gelatin layer is now put into a clean 

 glass saucer, covered so as to be free from dust, and set away in a 

 moderately warm place. Now, the single bacteria which are evenly 

 distributed through the gelatin, commence to grow, but as they are 

 each inclosed by a solid wall of the nutrient gelatin they grow in a 

 little heap or mass, which is called a "colony." 



In the course of from two or four days these colonies, each now 

 containing thousands of individual bacteria, have grown large enough 

 to be readily visible to the naked eye, or with the aid of a low power 

 magnifying glass. We have now only to count these colonies and 

 we know exactly how many living bacteria were present in the one 

 cubic centimetre of water used for analysis, because each visible col- 

 ony is the result of the growth on the spot of a single invisible bac- 

 teriiam which was caught here by the solidifying gelatin. 



Of course, if two or more bacteria chance to have clung together 

 when the water was mixed with the nutrient gelatin, our estimate of 

 the number of original germs will be too low, but if the mixture 

 was carefully made this so seldom occurs that the experimental error 

 is slight. To facilitate the counting of the colonies, a glass plate 

 ruled off into equal squares is usually placed over the gelatin film so 

 as not to touch it, and thus if we do not count all the colonies, we 

 can count those found in several of the squares, and then by multi- 

 plying the average number under the squares which are counted by 

 the whole number of squares, we find the number of colonies, which 

 is the same as the number of living bacteria in the volume of water 

 analyzed. 



If we wish to study these bacteria further, which is sometimes 

 of the utmost importance, so as to identify their species, study their 

 effects on animals and find out whether or not they are disease pro- 

 ducing, or pathogenic, we put the glass plate under the microscope, 

 and with a tine, sterilized platinum wire set in a glass handle, we 

 take out a minute portion from one of the colonies and put it into a 

 fresh tube nutrient gelatin, where it will grow until a sufficient 

 ■quantity of the material is produced for experimentation. 



