Up to the 7th day 50 per cent. were dark red; after the 7th day only 13.3 per cent.; and after the 15th day none. Up to the 7th day 83.3 per cent. were either dark red or sero-sanguineous and not one had attained to translucency. After the 7th day only 8 per cent. were of port wine hue, and by the 15th day 24 per cent. of all cases of over seven days standing were already transparent. Of all cases of over 15 days standing, 80 per cent. were perfectly translucent and none showed the dark red hue. Finally after the 30th day all remaining cases were limpid. This of course must not be applied with the same confidence in both directions. While translucency of the effusion bespeaks seven days standing and probably fifteen or twenty, the dark red hue must not be held to imply a recent date for the attack. A relapse in the course of convalescence may easily and quickly stain anew a liquid that was already limpid, or had advanced far toward this condition. The appearance of the lung tissue in a case of confirmed pleurisy is characteristic. The lung is of a dull red color, shrunken, slightly collapsed, flabby, scarcely crepitant under pressure and heavier than water or floating in water. It is tough, not friable like hepatized lung, and its cut surface is dry, smooth, and presents the interlobular septa very well marked. This is due to the compression by effused liquid, and by the organizing and contracting false membranes covering the lung and implies nothing more than simple condensation. The air cell may be collapsed, but contains no new product and has not parted with its epithelium and the lung can be inflated through the bronchia. _Differentiation according to the nature of the effusion._ Pathologists have divided acute pleurisy into the _dry_, _sero-fibrinous_, and _sero-fibro-purulent_. =1. Dry or fibrinous pleurisy= has usually a more acute type and the exudate containing an excess of the fibrinogenous elements forms a coagulum or false membrane on the affected surface tending to bind that to the part adjacent—the lungs to the costal pleura. The serum, small in quantity, is in the main retained in the exudate or if set free is actively reabsorbed by the healthy pleura. =2. Sero-fibrinous pleurisy.= This form is usually less acute and more extended involving perhaps an entire pleural sac, or even both sides of the chest. This is the common form of pleurisy and is that referred to in the experiments of St. Cyr and others above. The earliest lesions in experimental cases (with chloride of zinc solution) in dogs are an uniform bright red congestion, with a bright, shining surface as yet perfectly dry. There is already shedding of patches of the endothelial cells, swelling and proliferation of the superficial connective tissue cells and the formation of a few pus globules. This is seen in from half an hour to six hours after the application of the irritant.