The biologist has rendered sanitary science great service not only in discovering the causes of certain diseases, but also by aiding to determine the nature of the disease in any outbreak. It makes a vast difference if a given case is one of true diphtheria or not, or of Asiatic cholera or not, and often the symptoms alone are not conclusive. Here the biologist comes to our aid, as is seen so often in cases of supposed diphtheria. A portion of the throat secretion is sent him under such precautions that no bacteria from the outside can possibly contaminate. With this secretion he stabs or inoculates a jelly composition which he has placed in a test-tube, stuffs a wad of absorbent cotton in the mouth of his tube and puts it in a warm chamber or incubator. If there are any microbes present, they will begin to grow, and the expert biologist can tell the bacteria from its manner of growth as readily as the gardener can distinguish between his radishes and lettuce when they sprout in the spring, and in this way is able to report the nature of the germs. If he is in doubt, he carries his cultivation further and employs other tests to prove his observation. [Illustration: LABORATORY OF THE UNIVERSITY OF PENNSYLVANIA.] The biologist has also rendered great aid to sanitary science in discovering many other species of bacteria that are helpful to man. Our polluted waters could not be purified, our air could not be cleared from foul odors, nor the proper decomposition of organic matters go on, without the aid of bacteria. These little vegetable growths, while working much harm upon humanity, contribute far more to their comfort, well-being, and happiness than they do to their ill. Possibly no better illustrations can be given of the value of bacteriology to sanitary science, and the great progress it has brought about, than to contrast a cholera outbreak of a few years ago with one occurring more recently; or to point to the efficacy of purifying water by the assistance of bacteria. Another disease, pulmonary consumption, may also be noticed, but the triumph here is not so marked as yet.