List of firms where microscopes, chemicals, and the various apparatus mentioned can be obtained:

Microscope and Accessories.

Bausch & Lomb Optical Company, Rochester, New York. Messrs. R. & J. Beck. Agents, Williams, Brown & Earle, Philadelphia.

Messrs. Powell & Lealand, 170 Euston Road, London, England. C. Zeiss, Jena, Germany.

C. Reichert, VIII., Bennagasse 26, Vienna.

Chemicals, Bacteriological Apparatus, Stains, etc.

Eberbach & Son, Main St., Ann Arbor, Michigan.

Staining solutions ready for use can be obtained from this firm which are thoroughly reliable.

PART II.

PRACTICAL BACTERIOLOGY.

CHAPTER VIII.

MICROORGANISMS have now such an intimate relation to disease that the study of their growth, life history, and the chemical products they give rise to, forms an important part of pathological investigation. This subject is one that has received more attention of late years than any other branch of pathology, and an enormous amount of work has been done in it. Much of this work is, however, valueless, as has been shown by the many organisms accepted as being the virus of particular diseases, which have, however, upon extended observation been given up, while in some cases a single organism has been replaced by a number of forms. This all tends to show that the work is only in its infancy.

At the present time great attention is being paid to chemical products which have poisonous properties and which are supposed to be formed by microorganisms. This portion of the investigation—that is, the isolation of these chemical products—requires the services of a skilled chemist; but even then, if the chemical product should produce naked-eye changes and death in an animal, it must not be assumed that they have caused any given disease, unless pathological changes can be demonstrated in the tissues of the animal similar to those found in the human subject dying of the same disease. (See Koch's laws.)

In bacteriological investigation it must be always borne in mind that the air is teeming with different forms of microorganisms, and that everything used in the work is covered with them. And it must also be remembered that these minute forms are very much more numerous in some localities than in others; in crowded cities the certainty of contamination is so great that it is often impossible to carry on special investigations, while in the country or in small towns,

where the laboratories are some distance from the more crowded parts, the difficulty of keeping cultivations sterile is not very great.

The one absolute necessity in this work is cleanliness; not the ordinary form, but an attention to minute detail which results in rendering all the apparatus and instruments used absolutely sterile—that is, free from any form of microorganism.

The aim of the bacteriologist is to free any one form of organism he may be working with from accidental contamination acquired in obtaining it, and to separate it from other organisms which may have been present in the diseased parts—that is, to obtain what is called a pure cultivation of the organism in question.

When this is obtained his object is to free it from any disease products which have been removed with it from the affected part. To do this successive inoculations are made from one tube of sterilized nutrient medium to another until it is presumed that all trace of any disease product is removed and nothing but the particular form of organism is left. That this has ever been done it is impossible to say positively. The next step is, with the pure cultivation of the given organism, now presumably free from any contamination. whatever, to inoculate an animal susceptible to the disease the organism is supposed to be the virus of, and reproduce in that animal all the pathological changes present in the human subject from which the organism was first obtained. That this is difficult needs no demonstration-as in how few cases are all these changes known-and yet we have seen in the past few years announcements of the discovery of the bacillus of this and that disease, of which in many cases there is not any susceptible animal known. In making bacteriological examination of tissue removed from an animal inoculated, it must always be remembered that death causes changes in the blood eminently favorable to the growth of organisms in it, and that, if after death the vessels are full of bacilli and there is no inflammatory change in the surrounding tissue, these bacilli have increased after death. It must also be remembered that if one or two organisms exist in the intestine of man or a lower animal in hot weather or in a hot country, and the post-mortem examination is not made for several hours after death, there will be ample time for the production of an almost pure cultivation of the organisms in question.

In the following account the simplest manner of making bacteriological investigation will be given, and it will be shown that expensive apparatus is not required. As a matter of fact, the cultivation and

isolation of bacteria is one of the simplest proceedings in the whole range of pathology, and any ingenious student can construct an apparatus enabling him to do the work thoroughly at a small expense -two or three dollars. This has been done by one or more students of the University of Michigan with very creditable results.

APPARATUS, GLASSWARE, ETC, REQUIRED.

Steam Sterilizer.

Hot-air sterilizer.

Incubator.

Hot-water filter.

Gas blowpipe.
Test-tubes.

Glass flasks.

Shallow cultivation dishes (Esmarch's dishes).

Glass funnels.

Glass tubing.

Large cultivation dishes for potatoes.

Platinum needles.

Sputum-spreader.

STEAM STERILIZER.

This is a cylindrical vessel made of leaded iron with a copper bottom. It is covered with felt and has a conical lid in which is a hole to receive the thermometer; on one side is a gauge to show the height of the water and a tap for running it off. It is fixed into a frame of sheet-iron which has an opening in front through which a Fletcher burner or two large Bunsen's are introduced. In the inside is a grating which separates the water from the steam chamber, and on which rests the cages in which test-tubes are placed to be sterilized. These cages are four in number and are made in segments of a circle, so that no room is lost; and they can be used singly when a small number of tubes require sterilization, which is a great convenience, as if circular cages are used the whole of their interior must be filled up each time. The diameter of this steam sterilizer is sufficiently large to admit of a number of flasks being placed on the grating without the cages, or two cages can be used and the remaining half filled with flasks. This is the most convenient and cheapest steam sterilizer that can be procured, and it is made by Eberbach & Son, Ann Arbor.

Such a sterilizer as the above is required for laboratory work, but the student can do all the sterilizing he requires in an ordinary

potato steamer, where the arrangement is practically the same. The water cannot go higher than the boiling-point in either, and can be maintained at that point as long as is requisite equally well in the