obstruction of the common duct, the biliary acids slowly and gradually diminished in the urine, until they at length almost entirely {61} disappeared as the case approached a fatal termination. Here the disappearance of the biliary acids went on step for step with the impairment of the secreting powers of the liver, in consequence of the pressure exercised on its parenchyma by the retained bile. [Footnote 18: Professor Hoppe tests for bile-acids in the following manner:--The urine is boiled with an excess of milk of lime for about half an hour, and filtered to free it from the precipitate thus formed. The filtrate is evaporated to dryness, decomposed with hydrochloric acid, washed with water, and then extracted with alcohol. The alcoholic extract contains the bile-acids, which are recognised by Pettenkofer's test.] The readiest mode by which the biliary acids may be detected is the following: To a couple of drachms of the suspected urine add a small fragment of loaf-sugar, and afterwards pour slowly into the test-tube about a drachm of strong sulphuric acid. This should be done so as not to mix the two liquids. If biliary acids be present, there will be observed at the line of contact of the acid, and urine--after standing for a few minutes--a deep purple hue.[19] This result may be taken as a sure indication that the jaundice is due to obstructed bile-ducts. On the other hand, the absence of this phenomenon, and the occurrence of merely a _brown_ instead of a _purple_ tint, although, in the earlier stages of jaundice, equally indicative of suppression, is of course, for the reasons already given, no indication of the cause of the suppression. That must be gleaned from other circumstances. [Footnote 19: The immediate formation of a reddish coloured line is due to the acid setting free urohæmatine, the normal colouring matter of the urine.]