I. John Hunter expressed his belief that the blood has "the power of action within itself",[1] and that when it coagulates, it does so in consequence of an "impression" which it receives. Such an impression may be communicated by separation from the living vessels, or from "cessation of natural action"[2] in them. In certain circumstances also the living vessels themselves may be the means of exciting coagulation.[3] In others, the admixture of extraneous substances may either retard or hasten this operation.[4] The experiments made to determine the last point, Mr. Hunter informs us, "were rather imagined than fully carried out; and the subject rather broached and touched upon, than prosecuted".[5] In these experiments, different articles used in medicine were mixed with portions of blood taken from the body; and it was found that, in some cases, they altered both "the time," and "the firmness of the coagulation".[6] The circumstance of medicines being used in such experiments, conveys the idea, that, in instituting these researches, Hunter conceived that substances which would tend to produce such actions out of the body, might likewise produce some effect upon the blood in living animals. In endeavouring to prosecute the idea thus thrown out, I have been led to try the effect of different substances upon the blood, and to consider the changes which may be produced in that fluid, by the admixture of animal secretions. The experiments which will be hereafter detailed, not only confirm Mr. Hunter's notion, that foreign substances may induce actions in the blood when withdrawn from the body, but also show that some of these effects may be produced still more rapidly in the living vessels. [1] HUNTER on the Blood. Ed. 1794, p. 21. [2] P. 25. [3] P. 24. [4] P. 97. [5] P. 94. [6] P. 98. In these experiments, pus was used in preference to any other fluid; first, because the power of coagulating the blood which it was found to possess, enables its influence to be traced within the body; and secondly, because, being an animal secretion, the results obtained are likely to be analogous to those produced by the admixture of other secretions with the blood.