Bacteria are everywhere. In the water we drink, in the milk, butter, cheese and in dust. We cannot avoid them, try as we will; it is fortunate, therefore, that the majority are harmless. You may be surprised that, with this ubiquity, you have never seen one. When, however, you learn that most of them are only about twenty-five thousandths of an inch long and that a thousand million of them could be packed comfortably into a little box, whose sides measured but a twenty-fifth of an inch in length, it is not really so surprising after all. Being so small, the activities of a single bacterium are insignificant; that “union is strength” was never better exemplified than amongst these lowly plants. There are no male and female bacteria, in the majority of cases they increase by splitting, in fact they are often called splitting plants. The change may be watched under the microscope. The plant elongates somewhat, it becomes narrower and narrower in the middle, it develops a waist in fact; finally the two halves part company and each one leads a separate existence as a bacterium. This splitting progresses at an extraordinary rate. A celebrated scientist once wrote: “Let us assume that a microbe divides into two within an hour, these two into four in the next hour, these again into eight in the third hour and so on. The number of microbes thus produced in 24 hours would exceed 16-1/2 millions; in two days they would increase to 47 trillions, and in a week the number expressing them would be made up of 51 figures. At the end of a day (24 hours) the microbes descended from a single individual would occupy one fortieth of a hollow cube with edges one twenty-fifth of an inch long, but at the end of the following day would fill a space of twenty-seven cubic inches, and in less than five days their volume would equal that of the ocean.” It is hardly necessary to add that these alarming figures represent what would happen if no accident befell the bacteria, they show the enormous vitality possessed by the smallest of all plants. Even allowing for misadventure their increase is alarming; actual tests, with a sample of milk containing originally 153,000 bacteria per cubic inch, show that the cubic inch contained after one hour, 539,750; after two hours, 616,250; after seven hours, 1,020,000; after nine hours 2,040,000 and after 25 hours 85,000,000 individuals. The writer whom we have just quoted calculated that a single bacterium weighs about 0.000,000,000,024,243,672 of a grain, that forty thousand millions weigh one grain and that two hundred and eighty-nine billions weigh a pound. The descendants of one bacterium weigh 1/2666 of a grain, after twenty-four hours; more than a pound after two days, and sixteen and a half million pounds after three days. The assumption in this case, also, is that no harm comes to any of them; the mortality amongst bacteria is, clearly, very great.