By gently palpating with the finger-tips over the softened area, a fluid wave may be detected--_fluctuation_--and when present this is a certain indication of the existence of fluid in the swelling. Its recognition, however, is by no means easy, and various fallacies are to be guarded against in applying this test clinically. When, for example, the walls of the abscess are thick and rigid, or when its contents are under excessive tension, the fluid wave cannot be elicited. On the other hand, a sensation closely resembling fluctuation may often be recognised in œdematous tissues, in certain soft, solid tumours such as fatty tumours or vascular sarcomata, in aneurysm, and in a muscle when it is palpated in its transverse axis. When pus has formed in deeper parts, and before it has reached the surface, œdema of the overlying skin is frequently present, and the skin pits on pressure. With the formation of pus the continuous burning or boring pain of inflammation assumes a throbbing character, with occasional sharp, lancinating twinges. Should doubt remain as to the presence of pus, recourse may be had to the use of an exploring needle. _Differential Diagnosis of Acute Abscess._--A practical difficulty which frequently arises is to decide whether or not pus has actually formed. It may be accepted as a working rule in practice that when an acute inflammation has lasted for four or five days without showing signs of abatement, suppuration has almost certainly occurred. In deep-seated suppuration, marked œdema of the skin and the occurrence of rigors and sweating may be taken to indicate the formation of pus. There are cases on record where rapidly growing sarcomatous and angiomatous tumours, aneurysms, and the bruises that occur in hæmophylics, have been mistaken for acute abscesses and incised, with disastrous results. #Treatment of Acute Abscesses.#--The dictum of John Bell, "Where there is pus, let it out," summarises the treatment of abscess. The extent and situation of the incision and the means taken to drain the cavity, however, vary with the nature, site, and relations of the abscess. In a superficial abscess, for example a bubo, or an abscess in the breast or face where a disfiguring scar is undesirable, a small puncture should be made where the pus threatens to point, and a Klapp's suction bell be applied as already described (p. 39). A drain is not necessary, and in the intervals between the applications of the bell the part is covered with a moist antiseptic dressing. In abscesses deeply placed, as for example under the gluteal or pectoral muscles, one or more incisions should be made, and the cavity drained by glass or rubber tubes or by strips of rubber tissue.