True pus has certain properties which, when taken singly, may belong to other secretions, but which conjointly form the peculiar character of this fluid, namely, globules swimming in a fluid which is coagulable by a solution of the muriate (hydrochlorate) of ammonia, which no other animal secretion is, and, at the same time, a consequence of inflammation. This fluid-like serum is coagulable by heat. Pus also contains abundance of fibrine; if water be poured upon pus until the solid part which remain? at the bottom of the vessel be entirely deprived of its serum and globules, numerous portions of fibrine are found remaining, and although not exactly of the same size, yet they have a great uniformity of appearance. Thus pus is composed of serum, fibrine, and globules, say some authorities; but if I were to hazard a theory upon this subject, I should say that pus was composed of the. constituent parts of the blood, slightly changed in their character by inflammation. The colour and the consistence of pus are the two qualities which first attract the notice of the most superficial observer. The colour arises from the largest portion of this fluid being composed of very small round bodies, much resembling the globules of cream. The fluid in which the globules of pus swim might at first be supposed to be the serum of the blood, for it coagulates with heat, like the latter fluid. Pus is also probably mixed with a small quantity of coagulating lymph, as it partly coagulates after it is secreted. However, the fluid part of pus is found to have properties which serum has not. There being a similarity between pus and milk, experiments have been made to ascertain whether the fluid of pus could be coagulated with the gastric juice of animals, but no coagulation could be effected in this manner; a solution of muriate of ammonia made the fluid parts of pus coagulate, but not any other secretion or natural fluid, and hence it was concluded that whenever globules were found swimming in a fluid coagulable by muriate of ammonia, the matter was to be considered as pus.

Now, besides the above properties, pus has a sweetish mawkish taste, very different from that of most other secretions, and this whether it be pus from a sore or from an inflamed surface. It has likewise a smell, in some degree peculiar to itself, but different in different cases.

Pus is specifically heavier than water, consequently sinks in that fluid. It is probably about as heavy as blood.

Pus also communicates to water an uniformly white troubled colour; mucus gives the appearance of stringy portions floating in it. Again, mucus is said to be more readily dissolved by sulphuric acid than pus is. It has also been asserted, that if water be added to such solutions, the pus is precipitated to the bottom of the vessel; while the mucus, instead of being completely precipitated, forms swimming flakes. These and other distinctions between pus and mucus are not, however, deemed of much importance, at the present day, when pus is no longer regarded as a sure proof of the existence of ulceration.

Pus has been suspected to have a great tendency to putrefaction; but this is not the case with pure pus, which, when first discharged from an abscess, is perfectly sweet. But if the abscess has any communication with the air while the matter is confined in it, or if the collection has been so near the rectum as to have been affected by the faeces, then the matter will quickly become putrid.

When blood is blended with pus, when sloughs are mixed with it, when the parts forming the seat of the abscess are in a gangrenous state from an erysipelatous affection, the matter has a greater tendency to putrefy than the pure pus discharged from sound abscesses or healing sores.

Pure matter, though easily rendered susceptible of change by extraneous additions, is in its own nature tolerably uniform and immutable. It appears so unchangeable that we find it retained in an abscess for weeks without having undergone any alteration. These qualities, however, only belong to perfect pus. If a healthy sore inflames, the matter now produced from it, though unmixed with extravasated blood or dead solids, becomes much sooner putrid, and more irritating than the discharge formed before this alteration of the ulcer. When dead bone or other extraneous bodies are present, and keep up irritation, or when blood becomes mixed with the purulent matter, the discharge is always fetid and offensive. This state of it is one mark of the presence of carious or dead bone. The discharge of an unhealthy sore blackens a silver probe.