{65} Tyrosine may be obtained in a pure state by adding to four ounces of urine a solution of acetate of lead, till a precipitate ceases to form, filtering, and freeing the liquid from the excess of lead by a current of sulphuretted hydrogen, again filtering, and evaporating the clear solution. The tyrosine is now colourless, and crystallizes with the microscopic characters above alluded to, but still better marked. Tyrosine may be further recognised by putting a few crystals on a platinum spatula, adding a drop or two of nitric acid, and evaporating to dryness. If present, the yellow residue thus obtained assumes a pumpkin hue on the addition of potash, and leaves on incineration a dark greasy stain. Frerichs recommends another test, namely, the following:--Put the suspected substance into a watch-glass, along with some sulphuric acid, and after they have been in contact about half an hour, dilute the mixture with water. Next boil, and then neutralize with carbonate of lime. Filter, and to the clean filtrate add a few drops of perchloride of iron, devoid of free acid. The formation of a dark violet blue colour indicates the presence of tyrosine. Leucine is known by its flat, circular, oily-looking discs, without any crystalline structure. At the first glance, a globule of leucine might be mistaken for oil, not only on account of its {66} microscopical characters, but also on account of its being lighter than water. The globules of leucine are distinguished from those of oil by their being soluble in water, and boiling alcohol, and insoluble in ether. Besides this, the discs are occasionally opaque and laminated like the granules of potato starch. They are then not at all unlike microscopic crystals of the carbonate of lime; but the carbonate of lime crystals sink in water. [Illustration: FIG. 6. Dark globules of leucine of various sizes, resembling in appearance globules of carbonate of lime.] Both tyrosine, and leucine are usually deeply impregnated with the colouring matter of the urine. Since Frerichs' views were first published I have found tyrosine, and leucine in the urine of cases of chronic, as well as of acute atrophy of the liver. Their presence may therefore aid in diagnosing the latter as well as the former condition of the hepatic organ. {67} I have little doubt that future research will discover other conditions of the liver, besides those just mentioned, in which tyrosine, and leucine, may appear in the urine; for, as will be subsequently pointed out at page 96, I have been successful in producing them artificially in the urine of animals in which there was no evidence either of acute or chronic atrophy of the liver having taken place. In the cases cited, indeed, it will be seen that the jaundice was the result of suppression, consequent upon congestion of the liver, produced by blood poisoning.