When the condition is dependent on disease of the heart, kidney or other organ, these must be attended to according to their special requirements. Disconnected from such complications hydrothorax will often give way to an active treatment similar to that indicated above. In some cases however our only hope of even temporarily prolonging life lies in the operation for drawing off the fluid. =Tapping the chest= or as it is technically called =thoracentesis= or =paracentesis thoracis= has proved sufficiently successful in the lower animals to warrant its continuance in cases that resist other modes of treatment. It is highly probable that the larger proportion of unsuccessful cases is due in great part to the hopelessly advanced stage at which it is often had resort to, to the insufficient precautions adopted in its performance, and to the want of appropriate dietetic and medicinal treatment. Dr. Bowditch’s treatment by _paracentesis_ saved in the human subject at the rate of about two patients in five and we ought by availing of similar precautions to reach the same standard. The cannula employed in veterinary practice is a silver tube two inches in length, a quarter of an inch in diameter and furnished with a shield of the same metal at one end. The trochar by which it is introduced is of steel or brass. To carry out Dr. Bowditch’s system the operator must supply himself with a syringe of a somewhat smaller bore and an intermediate brass piece of a size adapted to fit accurately into the cannula and supplied with a stopcock. By an instrument of this kind the fluid can be drawn off by means of the syringe without any risk of the introduction of ærial germs which always tend to induce suppuration and even a putrefactive decomposition in the contained fluid.