" By these considerations and views re specting the cause of the acid reaction of urine, I have been induced to prepare an arti ficial urine, which possesses the properties of natural urine, even although sulphuric acid be altogether excluded.
" If 40 grains of dry phosphate of soda (or 90 grains of the crystallised salt, P 2 f 1-1 Na 0 Aq.) be dissolved in one pound0 of water, a fluid will be obtained having an alkaline reaction ; if to this fluid we add 15 grains of uric acid, and 15 grains of hippuric acid, and the mixture is heated, both acids will completely dissolve, imparting a strong acid reaction to the fluid. The solution thus prepared does not deposit a trace of uric acid at a temperature from 37° to 38° (= 98°, 100° Fahrenheit = the heat of the blood); nay, it is even only several hours after complete re frigeration that a sediment is formed, consist ing of uric acid containing soda : this sediment is of an analogous form to that deposited by natural urine after standing at rest for a long time. Upon collecting this sediment, in one of my experiments, after the lapse of twenty four hours, I found that it weighed 72 grains, so that there remained still in solution 22,1, grains of the organic acids. Dilute mineral acids produce immediately, in the fluids fil tered off from the sediment, a precipitate of uric acid.
" Proust, Prout, and all the other chemists who examined the urine previous to, or about the same period as, Berzelius, ascribed its acid reaction to the uric acid or phosphoric acid ; hippuric acid was not known as a con stant attendant upon uric acid.
" It follows, from all we have hitherto stated, that the acid nature of the urine of carnivorous animals, as well as that of man, depends upon the nature of the bases par taken of in the aliments, and upon the parti cular form of their combinations. In the flesh, blood, and other parts of animals, as well as in the grains of the cereal and legu minous plants, there exists no free alkali. The alkali which these substances contain is in variably combined with phosphoric acid ; the acids formed in the organism by the vital process, namely, sulphuric acid, hippuric acid, and uric acid, share the alkali amongst them ; and this, of course, must give rise to the liberation of a certain amount of phosphoric acid, or, what comes to the same point, to the formation of a certain amount of acid phos phates of soda, lime, and magnesia. The
proportional amount of the liberated phos phoric acid varies with the temperature ; at a higher temperature the phosphate of soda dissolves a larger amount of uric acid and hippuric acid than at a lower temperature— at from 37° to 38° more than at 15°. It is owing to this that urine, upon refrigeration, sometimes deposits uric acid, or orate of soda in a crystalline state, which, of course, can only take place by the uric acid, at a lower temperature, restoring to the phosphoric acid the soda or potass which, at a higher temper ature, it had withdrawn from it. At the common temperature phosphoric acid decom poses urate of soda, whilst, at a higher tem perature, uric acid decomposes phosphate of soda. When urine, containing uric acid, and manifesting an acid reaction, forms no sedi ment upon cooling, it shows that the amount of the phosphoric acid and that of the uric acid exactly balance each other with regard to their affinity for soda. Had there been pre sent a larger proportion of uric acid, this would have separated upon cooling ; whilst, on the other hand, the presence of a prepon derating proportion of phosphoric acid would likewise have caused the precipitation of uric acid, because the affinity of the former for soda would then exceed that of the latter. This explains the circumstance that urine, in certain states, when from some cause or other its amount of sulphuric, hippuric, or other acid, becomes increased, precipitates a larger proportion of uric acid than urine in its normal state. The solubility of uric acid in urine must decrease in proportion as the amount of the other acids present in the urine increases, because those acids share the soda with the uric acid ; and, of course, the larger the amount of soda which combines with these other acids, the less comes to the share of the uric acid. It is likewise owing to this, that uric acid very frequently preci pitates from urine upon the addition of mineral or other acids, and that urine of a turbid, whey-like appearance, from the presence of uric acid, frequently manifests a far more strongly acid reaction than normal urine.