The introduction of pus into the system has justly been regarded as the most important of this class of diseases. But the theory of the circulation of pus globules with the blood, supported as it has been by much ingenious reasoning, and most conveniently adapted to explain the formation of purulent deposits, has yet never obtained general belief. The stoppage of the pus globules in the capillary tubes, has appeared to many accustomed to the practical observation of diseases, too mechanical a solution of the origin of these abscesses; and it has become necessary to determine, with more precision than has hitherto been done, the actual conditions under which pus in substance can be received into the circulation. The simple experiment of mixing some pus with healthy, recently drawn blood, will at once shew that such a combination cannot circulate in the living body. It will be found that the blood coagulates round the globules of pus, and forms a solid mass which will adhere to the first surface with which it comes in contact; and it will be evident, that it is not till the coagulum thus formed is broken up or dissolved, that its elements can circulate with the blood. It appears not a little surprising that this, perhaps the simplest and the most instructive experiment that can be performed in reference to the subject of the formation of purulent deposits, should not have been resorted to in preference to others which have been difficult in their execution, and inconclusive in their results. It has been remarked by Sir Charles Bell, that we can seldom rely upon the answers that are extorted from living animals by experiments which go counter to the natural feeling of mankind; and that it is our duty, if experiments are performed, at all events to prepare for them by the closest previous application of our reason, and so to narrow the question as to be certain that advantage may be gained by our proceedings. Had the simple experiment mentioned above, illustrating the action of pus upon blood out of the body, been duly considered, it might have saved some of the vague and useless experiments which have been performed upon living animals in the investigation of the present subject. _Sept. 1850._ _13, Dover Street, Piccadilly._ PART I. ON INFLAMMATION OF THE VEINS: WITH EXPERIMENTS ILLUSTRATING THE EFFECTS OF A VITIATED CONDITION OF THE BLOOD.