392 BACTERIA. 



ture (at least 90 C.) to melt it, although, as should be borne in mind, once 

 melted it will remain fluid at 40 C., at which temperature any ordinary 

 inoculation may be made A mixture of agar and gelatine prepared by 

 adding 5 per cent, of gelatine and .75 per cent, of agar to the peptone beef 

 broth, or other nutrient fluid combines the advantage of both gelatine and agar- 

 agar media. It is almost as clear as gelatine, is readily manipulated, and 

 melts at a considerably lower temperature, although it will remain solid 

 between at 30 and 40 C., and it is not nearly so difficult to prepare as the 

 pure agar. The number of organisms in water is always calculated per 

 cubic centimetre, but having obtained the number of organisms in the fraction 

 of a cc. , it is easy enough to convert the figures to the desired standard. 

 In every case a sample of water should be allowed to stand in conical 

 glasses, so that any sediment may be obtained for microscopic and biological 

 examination. This is especially important where the presence of pathogenic 

 micro-organisms such as those of typhoid is suspected. These organisms 

 are usually brought in along with solid particles of sewage matter, and to 

 these they usually adhere so that they may all be deposited along with 

 such particles. 



The conditions under which micro-organisms are most 

 numerous in water have been already referred to in general 

 terms. It is of course found that where great rain storms, 

 or thaws after the action of frost, break up the earth's 

 surface, many organisms are set free and are carried away 

 into the water supplies, the number usually varying to a 

 certain extent with the amount of solid matter that is 

 suspended in the water ; as a general but by no means in- 

 variable rule it may be stated that most bacteria are found 

 where there is most ammonia. It is sometimes said that if 

 water does not contain more than one thousand organisms 

 per cc. that it may be used with safety for drinking purposes, 

 but it must be borne in mind that this thousand organisms 

 may contain a larger number of pathogenic organisms, 

 whilst on the other hand five thousand organisms in the 

 same quantity might not include a single pathogenic germ. 

 It has been found indeed that no general biological examina- 

 tion will give us absolutely accurate indications as to the 

 nature of bacteria in water ; to obtain such information a 

 rigid examination of every species present must be carried 

 out. The number of liquefying organisms has indeed 

 been taken as giving an indication of the quality of water, 

 and this is undoubtedly a safer plan than to take into 

 consideration the mere number. A safer rule still, however, 

 is to take the number of different species of organisms in a 

 drinking water as indicating its purity or impurity for 

 drinking purposes, for it follows that if any water contains a 



