MANY attempts have been made to utilise electrolysis in aneurysms, especially in those thoracic ones that are not amenable to ordinary surgical treatment; it has been hoped that the clotting which occurs around the poles might serve as a nucleus for further coagulation and deposits of fibrin, and that the aneurysm cavity might in this way become partially filled up. Such attempts have not met with much success hitherto, and the purpose of this research has been to endeavour to determine by experiments on blood serum outside the body what the actual effect of electrolysis is so far as regards clotting. Various methods of electrolysing the blood in an aneurysm have been used by surgeons. Ciniselli introduced needles connected with both poles, and reversed the direction of the current every five minutes; of 38 cases so treated, 27 were ameliorated, but none were cured. In the unipolar method one pole only was introduced, and the other was connected with an indifferent pad placed in the vicinity. The difficult question was which pole to introduce. The positive pole gave the firmest clot, but it was thought that it might be difficult to withdraw, and that hæmorrhage or even rupture of the vessel might follow.