An organism which has exceptional opportunities for finding its way into cows' milk is the _Bacillus coli communis_, normally present in the fæces of all animals. This microbe is a very undesirable adjunct to milk, and may greatly interfere with the souring process, by multiplying extensively, and so producing a change in the milk which renders it impossible for the particular souring bacteria to carry on their work, resulting in their collapse and ultimate extinction. But this is not the only injurious effect which these Coli bacilli can produce in milk, for there is a growing conviction that their presence is responsible for many intestinal disturbances with which young children are specially troubled. Quite recently determinations of the bacterial contents of cow-dung have been made, and it has been ascertained that _a single gramme_,[6] freshly collected, of this material may contain as many as 375,000,000 bacteria, of which the majority were found to be the above undesirable organism, the _B. coli communis_. [6] One gramme = 15 grains. Milk may also contain bacteria characterised by their remarkable resistance to heat, which is due to their possessing what is known as the hardy spore in addition to the ordinary rod form. The numbers in which they are present in milk varies with different samples; but they may be taken as a sort of index as to the care observed in milking, for they are always present in great quantity in uncleanly-collected milk. Careful studies have been made of this class of milk bacteria by Professor Flügge and others, and it has been found that when added to milk upon which puppies were subsequently fed the latter succumbed under symptoms of violent diarrhoea. The danger of even a few bacteria gaining access to milk is serious, on account of the fabulous rapidity with which they multiply when they find themselves in such congenial surroundings. Professor Freudenreich has made very exhaustive investigations to show how milk microbes may multiply in the time which elapses between milking and the receipt of the milk by the consumer. The following example will convey some notion of what bacterial propagation under these circumstances is capable of. The sample of milk in question was found to possess on reaching the laboratory, two and a half hours after milking, a little over 9,000 bacteria in a cubic centimetre. The sample was divided into three portions, which were kept at different temperatures, and after definite intervals of time they were examined. The following table shows at a glance the results obtained:-- NUMBER OF BACTERIA IN ABOUT TWENTY DROPS OF MILK.