It has already been stated, that the blood is stagnated in the capillaries occupying the centre of the inflamed part, as well as extravasated in the contiguous cellular tissue. Pus has long been considered as a secretion; but, from the time of Mr. Hunter downwards, a great similarity between the globules of pus and those of blood has been recognised: the former, however, are spherical, larger, and rougher on the surface, and are not so regular in size as those of blood; their diameter is between 1/2000 and 1/3000 of an inch, although some of the globules may be occasionally seen a little larger or smaller than these measurements. Smaller particles are also detected, the molecules of the pus globules, each of which contains two or three. These molecules are insoluble in acids, soluble in caustic alkalis, and can be freed from the fibrous capsule in which they are contained, and of which the pus globule is composed, when treated with acetic acid. The appearance of the pus globules and molecules is here shown upon a scale, the squares of which are 1/4000 of an inch. These small particles were pointed out to me by my friend, Mr. Gulliver, nearly two years ago, as constituting an important element in suppuration. It results from his observations that the entire pus globule is composed in its central part of these minute molecules, the diameter of which is from 1/10666 to 1/8000 of an inch, cemented together, as it were, by a superficial deposit of matter possessing all the properties of coagulated lymph. And these minute nuclei constitute a proximate animal principle, possessing such well marked characters as to justify us in regarding them as peculiar in their nature, and essential to the composition of the pus globule. Thus they resist putrefaction with remarkable pertinacity, are very dense, and spherical in form, and are insoluble in some acids which act with facility on albumen, fibrin, or the blood-corpuscle. They are easily seen by treating a little recent pus with sulphurous acid, which so acts on the pus-globule as to render the nuclei distinctly visible through its fibrinous capsule; by sulphuric acid the external part of the pus-globule is quickly dissolved, and the nuclei, somewhat shrunk, are seen in considerable numbers floating separately about the field of vision. The instrument used by Mr. Gulliver in these observations was a compound achromatic microscope, with a deep object glass, having one-eighth of an inch focal length. [Illustration]