1st. Those which increase the salivary excretion by _external_ application to the secreting vessels, by mastication, as the following acrimonious and pungent substances, _Anthemis Pyrethrum_; _Colchearia Armoracia_; _Daphne Mezereum_; _Nicotiana Tabacum_, &c. 2nd. Those whose _internal_ exhibition affects these organs through the medium of the circulation, of which _Mercury_ is the only true example; for all the preparations of this metal, when administered in certain quantities, produce salivation. The acrid Sialogogues, or Masticatories, by stimulating the excretory ducts, and increasing the secretion of saliva, sometimes relieve the pain of tooth-ache, and are commonly resorted to for that purpose; they are besides supposed capable of relieving other congestions, or inflammatory dispositions, in remote parts of the head, by the derivation they occasion from the neighbouring vessels, especially the branches of the external carotid. Mercury, in its metallic state,[167] is perfectly inert, and does not exert any influence whatever upon the living body: this fact is sufficient, if any serious refutation were necessary, to overturn the theory which attributes its sialogogue property to the gravity of its particles, by which “it is disposed to retain the _direct line_ in which it is propelled from the heart, and is therefore more certainly determined to the vessels of the head.” It has been also supposed to act by diminishing the _lentor_ of the blood, and disposing it to pass more easily into the salivary glands, so as to increase their secretion: equally gratuitous and improbable are the chemical hypothesis which have been offered to explain this curious and singular property; Dr. Cullen endeavoured to solve the problem, by supposing that “_Mercury has a particular disposition to unite with ammoniacal salts, and that such salts are disposed to pass off by the salivary glands more copiously than by any other excretion_.” Dr. Murray, however, very justly remarks that mercury has not any peculiar tendency of this kind; and that if it had, these salts are not more abundant in the saliva, than in some other secretions. Dr. Murray then proceeds to submit a theory which he considers better calculated to explain the phenomenon; he observes, that the urine appears more peculiarly designed to convey matter which has been received into the circulating mass, but which is still excrementitious, from the system. To pass, however, with this fluid, it is necessary that the matter conveyed should be soluble in it; and when it is so, we can discover it in the secretion by chemical tests. If there is any property connected with it, therefore, which shall prevent this solubility, it probably will prevent the substance from being secreted. Now, the _phosphoric acid_, abundant in urine, must in this mode counteract the secretion of mercury in any form of preparation, by forming with it a compound, insoluble, and to which the slight excess of