3. These microbes are among the minutest objects which can be studied under the microscope. Bacteria average about 1/9000 of an inch in their longest diameter; micrococci and spores (Dauersporen, Billroth) are yet smaller. Much care, therefore, as well as skill, must be exercised in making observations upon them.[32] Huxley asserted a few {144} years ago that a distinguished English pathologist had mistaken for movements of minute living organisms the "Brownian movements" seen in the particles of many not living substances under a high magnifying power. One observer, at least,[33] considers that the forms designated as bacteria and micrococci, etc. are either forms of coagulated fibrin or granules from morbidly-altered blood-corpuscles (zoogloea of Billroth, Wood, Formad, and others). Koch denies the validity of the observation of organisms in tubercle by Klebs and Schüller, while insisting upon his own demonstration of a bacillus tuberculosis. Authorities must, by mutual confirmation or correction, remove these obscurities. [Footnote 32: A very interesting discovery was made by Tyndall, to the effect that while one boiling of a liquid would sterilize it for the time by destroying all the bacteria present, their spores might still retain vitality and be afterward developed. By repeated exposure to a boiling temperature, taking these spores in their developing stage, they were destroyed, and complete sterilization was effected.] [Footnote 33: R. Gregg, _N.Y. Med. Record_, Feb. 11, 1882. Sternberg, however, has replied to him (_N.Y. Med. Record_, April 8, 1882, p. 368). The latter admits a doubt as to whether the granules seen within the leucocytes by Wood and Formad in diphtheritic material, and believed by them to be micrococci, are such, or are merely granules formed or set free by disorganization of protoplasm within the leucocytes. This uncertainty well illustrates the difficulty of these investigations. A chemical test much relied upon is, that bacteria resist the action of acids and alkalies, which destroy granular material of animal origin; also, that all these organisms are deeply stained by aniline dyes and by hæmatoxylin. The most decisive test, however, is cultivation in a liquid sterilized by heat. Koch prefers a process of dry culture for the bacillus of tubercle. Gradle (_Lectures on the Germ Theory of Disease_, Chicago, 1883, p. 28) says that the absolute criterion of the life of bacteria is their power of multiplication.] 4. Bacteria and micrococci have been abundantly discovered (Kolaczck; J. G. Richardson) in healthy bodies upon the various mucous membranes and in the blood. The correctness of such observations has been denied, but, so far at least as the mucous membranes are concerned, it has been well established by Nothnagel, Sternberg, and others. Bacteria have sometimes been found in countless numbers in fecal discharges.