60 Br. Edward FranMand [Feb. 21, 



extreme mimiteness as, practically, to defy detection by ordinary 

 microscopical examination. But, although the microscope cannot 

 detect with certainty single bacteria or their spores, even the naked 

 eye can easily discern towns or colonies consisting of thousands or 

 even millions of such inhabitants. 



Dr. Koch's method accomplishes at once two things : it isolates, 

 in the first place, each individual microbe or germ ; and secondly, 

 l^laces it in conditions favourable for its multiplication, which takes 

 place with such amazing rapidity that, even in a few hours, or at 

 most in two or three days, each organism will have created around 

 itself a visible colony of innumerable members ; a town, in fact, com- 

 parable to London itself for population. 



By operating upon a known volume of the water under in- 

 vestigation, such as a cubic centimetre for instance, the number of 

 separate organit>ms or their spores, in a given volume of the water, 

 can thus be determined. 



The following is the method now adopted in carrying out Koch's 

 process for the investigation of drinking water : — 



1. Preparation of the nutritive medium. 



2. Sterilisation of the medium. 



3. Collection of the sample of water in a vacuous tube to be 

 hermetically sealed immediately afterwards. 



4. Transport of the sample to the bacteriological laboratory. 



5. Mixture of a known volume of the water sample with the 

 nutrient medium. 



6. Casting of the mixture into a solid plate. 



7. Incubation of the solid plate. 



8. Counting of the colonies (suitable time for the colonies to 

 develop being given as shown in diagrams, Figs. 6, 7, 8 and 9). 



Fig. 6 shows a gelatine culture of unfiltered Thames water placed 

 on a ruled surface to assist counting ; whilst Figs. 7, 8 and 9 illus- 

 trate the gradual development of the colonies in a gelatine culture of 

 y|-Q of a cubic centimetre of unfiltered Lea water collected at the 

 East London Company's intake on January 13, 1896. Fig. 7 shows 

 the condition of the colonies on the third day ; Fig. 8 the further 

 development on the fourth day ; and Fig. 9 the condition of the 

 colonies on the fifth day, when many colonies are mingled together 

 and counting is no longer possible. 



9. Examination of separate colonies, or rather of the individual 

 members, under the microscope. 



Sometimes the cultivations are made upon a plate of the substance 

 called agar, which resembles isinglass, and bears a temperature of 

 blood heat without melting (Fig. 10). There is a very remarkable 

 colony on this plate, showing an apjDarently organised city, with 

 suburbs stretching far into the country, and containing many millions 

 of inhabitants. 



In order to ascertain the eflect of filtration upon the bacterial 

 quality of the water, it is absolutely necessary that the sample should 



