calorific capacity of the gas between the initial temperature and 100° C. The following empirical equation expresses the line p = 0.124 calculated into a line of variation of specific-heat with temperature: Cya+2b(100-t)+3c(100-t), where t is the initial temperature of the experiment in centigrade degrees; a = 019020000. C 0'00000182. Theoretical considerations led Rankine and others to predict a decrement of the specific, heat of a gas at constant volume with rise of temperature. The characteristic-equation of carbon dioxide does not seem to have been given as yet in a satisfactory form for application to the higher pressures. Probably on this account only an approximate agreement is found to exist between the numerical amount of the decrement predicted and that observed experimentally at the higher densities. At the lower densities the sensible absence of variation with temperature over the range of the observations is in perfect agreement with the theoretical numbers. III. "A Contribution to the Study of the Yellow Colouring Matter of the Urine." By ARCHIBALD E. GARROD, M.A., The uncertainty which still surrounds the origin of a phenomenon so familiar as the yellow coloration of the urine bears eloquent testimony to the difficulties which beset the investigation, by ordinary chemical methods, of such substances as the urinary pigments, and to the importance of the part which the spectroscope has played in the acquisition of such knowledge of them as we possess. Indeed, our acquaintance with the individual pigments is proportional to the distinctive character of their absorption spectra, rather than to the time which has elapsed since they first attracted attention; and in not a few modern works doubt is thrown upon the very existence of a distinct yellow pigment, having negative spectroscopic properties, but to which normal urine owes its characteristic tint, the chief part in the coloration of the excretion being assigned to urobilin. In this connexion the spectro-photometric researches of Vierordt* are of much importance, for they appear to show conclusively that more than one pigment is present in normal urine. Vierordt found that with different specimens of the urine of a single healthy individual, examined at considerable intervals, the extinction coefficients for different parts of the spectrum exhibited relative as well as positive differences. The variations of positive value are of course dependent upon the depth of colour of the specimen, but the relative variations can only be explained by the presence, in varying proportions, of two or more distinct pigments. It must not, however, be forgotten that, as Vierordt himself points out, pigments which yield definite absorption bands may influence the extinction coefficients, even when present in such small quantities that their bands are not visible as such; and it can be shown that at least three colouring matters, apart from a yellow pigment, may be present in any given specimen of the urine of a healthy individual, which may, nevertheless, exhibit no obvious selective absorption. Of these pigments, urobilin is certainly one, and when not seen on direct examination of the untreated normal urine, its band not infrequently appears on standing, or on the addition of a mineral acid. Yet the quantity present is at best extremely minute, and wholly ་ * Die Quantitative Spectralanalyse.' Tübingen, 1876, p. 78. inadequate to account for the coloration, and I am therefore convinced that the statement that urobilin is the chief colouring matter of normal urine is entirely incorrect. Indeed, as far as normal urine is concerned, urobilin can hardly be reckoned as one of its colouring matters at all, for even a very faintly tinted solution of this pigment yields a well-defined absorption band, far darker than is ever seen in normal urine. In some morbid urines, on the other hand, it affects the colour profoundly. The second pigment referred to is hæmatoporphyrin, which, as I have elsewhere shown,* can usually be detected by appropriate means even in normal urine; but here, again, the amount present is so infinitesimal that it can have no appreciable effect upon the colour. The occasional deposition of pink urate sediments, apart from any noticeable deviation from perfect health, shows that uroerythrin must also be reckoned among the pigments of normal urine; and if further confirmation is needed, it is obtained, as Riva† and Zoja have shown, by the examination of the extracts obtained by shaking specimens of urine with amylic alcohol. Since, however, the above-mentioned pigments, with the possible exception of uroerythrin, can have no material influence upon the colour of normal urine, we are driven to the conclusion that there must exist in the urine another much more abundant colouring matter, of a yellow tint, which even in concentrated solution yields no absorption bands, or that the colour is due to the presence of more than one such substance. There are not wanting records of investigations directed to the isolation of such a pigment, or mixture of pigments, and products have been obtained by several observers, which they have looked upon as the substance in question, but the various products have differed in their properties, and no one of them has met with general acceptance. The literature of the subject will be found admirably epitomized in papers by Thudichum and Schunck,§ published in 1864 and 1867 respectively, and to these epitomes there remains little to be added, seeing that during the twenty-seven years which have since elapsed, no fresh observer has, as far as I am aware, published any investigations upon the subject. Referring my readers to these epitomes for records of the earlier work of Proust, Berzelius, Lehmann, Harley, and others, I only propose to allude here to the results obtained by Tichborne, Thudichum, and Schunck. C. Tichborne* (1862) threw down most of the colouring matter of a large quantity of concentrated urine upon a basic copper precipitate, and extracted the pigment from the precipitate by means of cold dilute sulphuric acid and alcohol. In this way he obtained a solution which, on evaporation, left a brown residue, very hygroscopic and smelling of stale urine, solutions of which yielded, according to the degree of concentration, the various tints of normal urines. The pigment was soluble to almost any extent in water, was insoluble in ether, sparingly soluble in absolute alcohol, and more readily in rectified spirit. It was precipitated from solution by basic lead acetate. The results of elementary analysis led Tichborne to think that it was derived from hippuric acid by the subtraction of water, the percentage composition obtained being C, 67.80; H, 4·23; N, 8·56; O, 19:41. It is extremely doubtful whether combustion analyses of such substances are calculated to materially advance our knowledge, in the absence of any of the ordinary guarantees of the purity of the substance analysed; and so simple a process as that employed by Tichborne could only be expected to yield a product of a moderate degree of purity. Thudichum† (1864) obtained from normal urine by a variety of processes a substance to which he gave the name of urochrome, and his researches which have extended over a long period form the most elaborate contribution yet made to the subject. In the second edition of his work on the urine,‡ in which his later researches are embodied, he gives four methods for the isolation of urochrome in which phosphomolybdic acid and the neutral and basic lead acetates are employed as precipitants, and sulphuric acid, sulphuretted hydrogen, &c., for the extraction of the pigment from the precipitates. Great pains were taken to obtain the pigment in the highest attainable degree of purity. Thudichum describes urochrome as forming yellow crusts when its solutions are evaporated, as dissolving very readily in water, fairly readily in ether, and least easily in alcohol. It was precipitated from its solutions by lead acetate, silver nitrate, acetate and nitrate of mercury, &c. On heating with mineral acids the aqueous solution became red, and resinous flakes were thrown down from which three definite substances could be obtained, which were minutely studied, and subjected to ultimate analysis. These substances were a red pigment, Chemical News,' 1862, vol. 5, p. 171. soluble in ether, with a port wine colour, and showing an absorption band to the more refrangible side of D (omicholic acid), a portion soluble in alcohol, showing a band extending from E to beyond F (uropittine), and a residue scarcely soluble in water or alcohol, but readily dissolved by alkalies (uromelanine). Thudichum assigns to urochrome a faint absorption band at F, and was led to regard the pigment as a feeble alkaloid, on account of its precipitation by phosphomolybdic acid. Schunck* (1867) employed the acetates of lead as precipitants, and extracted the colouring matter with cold sulphuric acid or sulphuretted hydrogen. He came to the conclusion that the urine owes its colour to two distinct yellow pigments, one soluble and the other insoluble in ether. The pigment soluble in ether (urian) yielded, on heating with mineral acids, a brown resinous substance, readily soluble in alcohol (uroretine), whereas the pigment insoluble in ether (urianin) yielded a brown flocculent substance scarcely soluble in alcohol (uromelanine). He made numerous combustion analyses of these products, and his results differed widely from those of Tichborne, especially in the much smaller percentage of carbon found. For urian, the pigment soluble in ether, Schunck obtained the percentage composition C, 51-23; H, 5-38; N, 1.26; 0, 42:13. Whereas urianin gave C, 46-44; H, 5.66; N, 3.16; 0, 44.74. In more recent years, Thudichum has on various occasions upheld the claims of urochrome to be regarded as a definite chemical entity,t in reply to the criticisms of Maly and others. Dr. Lewis Jones, who, some years ago, made some investigations on this subject, has favoured me with an account of his results, which were never published. He arrived at the conviction that the yellow colour of urine could not be due to urobilin, which, in solution, has a redder colour than urine. Moreover, the yellow pigment is insoluble in chloroform, in which urobilin dissolves freely. Urobilin, even in very dilute solution, has a very distinct absorption band at F, whereas normal urine shows no band at F unless viewed in deep layers, and then shows only a diffused obscuration about the region of the F line, quite unlike the sharp band of urobilin. He found that an extract obtained by the lead acetate method, evaporated in vacuo, with proper precautions, yielded a yellowish crust, from which chloroform dissolved out any traces of urobilin. The remainder, when dissolved in water, reproduced the colour of the original urine when diluted to the same bulk. From normal urine the amount of urobilin obtained was very minute; more could be |