This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Magnesia has sometimes caused the solution of uric acid deposits when alkalies have failed to do so, and Mr. Brande pointed out that it could render the urine alkaline, more permanently, if more slowly, than potash or soda. Thus, 2 dr. of soda gave a maximum of alkalinity in a quarter of an hour, 1 dr. of magnesia only at the end of six hours, and 1/2 dr. in twelve hours ("Philos. Trans.," 1810). A deposit of triple phosphate occurred, but since earthy salts can be passed in only limited quantities in the urine (Neubauer and others), it is of interest to know precisely how magnesia rendered the secretion alkaline. Caulet concluded from recent researches that both it and lime do so only indirectly through the digestive organs - i.e., they neutralize a part of the acid of the gastric juice, and consequently more soda is excreted with the urine, and becomes the direct agent in determining its alkalinity. In support of this, he finds on analysis no increase in the amount of earthy salts in the urine (rendered alkaline under administration of magnesia), but marked excess of soda (Bulletin de Therapeutique, 1875). In further support of this observation, we have the fact that during normal digestion, when the acid of gastric juice is being neutralized and withdrawn from the system, acidity of urine becomes less, and in some stomach-disorders is even replaced by alkalinity (Roberts, Jones).
An observation from comparative anatomy is also of interest. If much lime or magnesia were to be excreted by the urine in conjunction with uric and phosphoric acid, the insoluble salts formed would render the secretion solid, or nearly so, as it is in birds and reptiles. Such a secretion would not readily pass through the narrow urinary channels of the human race, and therefore alkaline earths pass out rather through the bowel, while in herbivora, the urine of which must be fluid and yet contain much earthy salt, the acids are excreted by the bowel (Caulet).