We have seen also that _local foci of rarefaction_ may be met with in _infective_ arthritis of _non-gouty_ type. Moreover, the _proliferative_ and marked _atrophic_ bony changes found in some instances of gouty arthritis are quite compatible with their _infective_ origin. Witness how impossible it is to differentiate skiagraphically between _atrophic_ arthritis and _gouty_ arthritis, and at the same time let it not be forgotten, on the other hand, that the dividing line between the _infective_ arthritides and the more acute forms of _atrophic_ or rheumatoid arthritis is by no means sharply defined. Review this also in light of the fact of the close resemblance that obtains between _acute articular gout_, especially the polyarthritic variety, and types of arthritis of avowedly _infective_ origin, and we see at once how close is the clinical similitude. We turn to _radiography_, and here again we are met with the same family resemblance, suggestive of a probable community of origin. How clear then the inference that it is on _tophi_, and tophi alone, that we must base an absolute diagnosis of _gouty_ arthritis. Moreover, since tophi are not detectable by _skiagraphy_, our mainstay must be physical examination directed to their detection. As for those cases of so-called “clinical gout,” viz., unattested by tophi, it is probable that their absolute identification, as such, will never be attainable on purely clinical, but on _bacteriological_, data, which, it is to be hoped, will before long be forthcoming. DIFFERENTIAL DIAGNOSIS _Infective Arthritis._—According to Goldthwait, of Boston, no changes can be detected by skiagraphy in bone or cartilage in infective arthritis. The density of the former is in no way diminished, while the cartilage retains its normal thickness. If, however, the infective arthritis is of destructive character, new bone may be thrown out in the process of repair. If then in such cases sole reliance be placed on the X-ray appearances without any reference to the clinical history and course of the disorder, then, as Goldthwait says, this irregular formation of bone is likely to be confused with the X-ray findings in osteoarthritis. It is, however, possible, according to him, to distinguish the nature of the case by careful scrutiny of the skiagraphs.