This carbonate, easily decomposed by any acid, is equivalent to a free alkali. Thus these salts fulfil a double function. They may supply the respiratory process, as has already been explained. They also act on the blood, and on the secretions, as alkalies. Alkalies are used wherever there is an excess of acid in the system, whatever that acid may be. Alkali should naturally be present to neutralize it, for it is unnatural to have a large excess of acid in the secretions, or any excess in the blood. We thus administer alkalies on the restorative principle. They are not always excreted; but whether excreted or not, they tend to render the secretions neutral and alkaline by increasing the quantity of basic matter in the system. Alkalies being more stable than acids, and being more invariably secreted by the kidneys, it is more easy to render the urine alkaline than to make it acid. Alkalies are thus of use in a tendency to lithic acid deposit. The bicarbonates are probably the best for this purpose, for they may be given in large doses, being less irritating. The celebrated Vichy water contains bicarbonate of soda. Lime-water has been used as a solvent for stone. These all act by first diminishing the acid of the blood, and subsequently influencing the secretion of the urine, which secretion, acting as a counterpoise to the condition of the blood, may be generally taken as an index to it. Dr. Pereira recommends alkalies in cases of dyspepsia and pyrosis, when there is an excess of acid secreted by the stomach. He further thinks it likely that they may aid in the digestion of fatty substances when there is a deficiency of bile. This is quite hypothetical, and depends upon a theory which has been shaken by M. Bernard. The same eminent authority states that Ammonia invariably enters the blood as a salt, and thus can exert no peculiar action beyond the coats of the stomach. I shall state my reasons for doubting this when I consider Ammonia separately in Chapter IV. Ammonia will be again enumerated as a stimulant--Potash as a catalytic--and lastly, all the alkalies as diuretics. The alkalies are useful in gout and rheumatism in diminishing the acidity both of the blood and of the secretions. It is often advantageous to apply an alkaline wash locally around an affected joint, in the fluids about which the morbid process seems especially to have fixed itself. If we may receive a certain theoretical explanation of these two kindred diseases, still more light will be thrown on the advantage attending the employment of alkaline restoratives. To this I shall recur when I consider the Catalytics which are used to counteract these same disorders, not, as here, by restoring a wanting material, but apparently by determining the process in a different direction, and thus checking it altogether.