After a sufficient time the tube is inspected.  To the untrained eye nothing appears.  The bouillon still remains in the little bulb apparently unchanged.  Its color and clearness have not been affected.  But the chemist notices that it does not stand so high in the closed end of the tube as it did when placed in the incubator.  The observation seems trivial, but to the man of science it is significant.

What has happened?  The water contained some minute organisms which when acted upon by the chemical in the tube have set up a fermentation.  Gradually, one by one in the little bulb, bubbles of gas have formed and risen to the surface of the liquid in the closed upper end of the tube.  As this gas was liberated, it took the place of the liquid in the tube, and the liquid was forced downward until there was quite a large space, apparently vacant but really filled with gas.

It was this phenomenon that had attracted the attention of the chemist.  What did it mean?  It was the evidence that the water which was being furnished to the city for half a million people to drink contained some living organism.

Now that, in itself, was enough to make an official of the health department begin to take an interest.  It was not, however, in itself a danger signal.

Not all bacterial life is a menace to health, the chemist will tell you.  Indeed, humanity has come to live on very peaceable terms with several thousand varieties of bacteria and to be really at enmity with but a score or more.  Without the beneficent work of a certain class of bacteria the world would not be habitable.  This comes about through a very interesting, though rather repulsive condition—­the necessity of getting rid of the dead to make room for the living.

What would be the result if no provision had been made for the disintegration of the bodies of all the men and animals that have inhabited the earth since the beginning?  Such a situation is inconceivable.  But very wisely providence has provided that myriads and myriads of tiny creatures are ever at work breaking up worn-out and dead animal matter and reducing it to its original elements.  These elements are taken up by plant life, elaborated into living vegetable growth and made fit again for the nourishment of animal life, thus completing the marvelous cycle.  And so we must not get the notion that all bacteria are our mortal foes.  We could not live without them, and our earth, without their humble services, would no longer be habitable.

Neither need we fear the presence of bacterial life in our drinking water.  Drinking water always contains bacteria.  We, ourselves, even when in the best of health, are the hosts of millions upon millions of them, and it is fair to suppose that they serve some useful purpose.  At any rate, it has never been demonstrated that they do us any harm under normal conditions.