Of all the animal secretions urine is undoubtedly one of the most important. Its varying properties, in health as well as in disease, the frequency with which it is emitted, and the consequent facility with which it may be submitted to examination, render it invaluable to the physiologist and pathologist as a means of throwing light on the processes, either healthy or morbid, going on within the body. Its study has therefore engaged the attention of physicians since the earliest times, and of chemists from the period when chemical analysis was first employed in the examination of natural objects. Notwithstanding the labour bestowed on the subject by many eminent men during the past sixty years, it is still, however, far from being exhausted. There are, indeed, portions of the chemistry of urine concerning which our ignorance is .almost complete. It is one of these obscurer parts of the subject that I have endeavoured to clear up, and I hope to succeed in showing that I have added at least a few facts to the sum of our previous knowledge. Of all the properties of urine none is more obvious, even to the ordinary observer, than its colour. The variations in tint which it exhibits at different times are striking, even to the unpractised eye, and they some times serve as important indications to the physician. Nevertheless concerning the chemical nature of the substances to which its colour is due very little is known. Our ignorance on this subject may be ascribed to various causes. In the first place, some of these substances occur in the urine only occasionally, and in very minute quantities, so that the preparation of a quantity sufficient for chemical examination becomes difficult and even impossible, especially when the urine containing them is not abundant. Secondly, it has been found that some of them are very easily decomposed, so much so that the mere heat required for the evaporation of the urine seems to be sufficient to effect a change in their properties and composition. It therefore becomes doubtful, after a long process has been gone through for the purpose of separating any colouring-matter from the other constituents of the urine (a process in which, perhaps, strong chemical reagents have been employed), whether the substance procured was originally contained as such in the urine, or is not rather a product resulting from the decomposition of some other substance or substances. Thirdly, several of the bodies colouring the urine possess very few characteristic properties. They are amorphous and syrup-like, and they retain water with so much pertinacity that on attempting to dry them they undergo decomposition. Neither their compounds nor their products of decomposition exhibit any distinguishing characteristics. They belong to a class on which, for want of a better, the name extractive matter has been conferred. With some chemists, to call a body an extractive matter is to place it among a class which is held to he unworthy of minute examination. To others the name extractive matter is merely a convenient word for a mixture, sometimes occurring in nature, of certain definite, perhaps even crystallized substances, which, by appropriate means, may be resolved into its constituents, and thus be made to disappear entirely from the list of definite chemical bodies. As regards the extractive matter of urine, this view may to some extent be justified, when we recollect that from what was considered to be extractive matter sixty years ago, such well-characterized substances as urea, hippuric acid, and creatine have been successively eliminated; and it is therefore natural to expect that by further research it will be found to contain others of the same nature. I believe this view to be erroneous; and I shall succeed, I hope, in showing that, after having removed from the extractive matter of unne everything which can assume a definite form, there remains a residuum which cannot be further resolved without decomposition. Still, any one holding this view is not likely to undertake the investigation of extractive matters as such, unless it be for the purpose of obtaining something which may be supposed to be contained in them. Lastly, the properties of these colouring and extractive matters, however important they may be to the physiologist and pathologist, present so little that is interesting to the chemist, that the latter would probably not occupy himself with their examination unless for some particular purpose. For myself, I frankly confess that, had I not had a special object in view, this investigation would not have been undertaken. The information for the sake of which it was commenced having been obtained, I should then have abandoned all further inquiry, had I not found reason to suppose, in the course of my experiments, that a more extended investigation would lead to results interesting from a physiological point of view. My endeavours have, I think, been attended with some measure of success; and should physiologists, on becoming acquainted with the results, be of the same opinion, my labour will not have been quite in vain.
X.
Bakerian lecture
.—On the Relations between the viscosity (internal friction) of liquids and their chemical nature