By bringing Ammoniacal gas in contact with the vapour of Cyanic acid, we produce Cyanate of Ammonia, a poisonous salt. This, when exposed for some time to the air, changes into Urea, which is isomeric with it, but comparatively innocuous. Urea, when heated, gives off Ammonia, and becomes Cyanuric acid. On again heating this, hydrated Cyanic acid sublimes. This, when brought in contact with water, becomes Bicarbonate of Ammonia. Two equivalents of Cyanic acid, uniting in one compound, produce Fulminic acid. This, if combined with the oxide of Silver or of Mercury, forms a compound which is caused by the slightest friction to explode with terrific violence. If to the elements of this dangerous acid be added those of two equivalents of Ammonia, we again have Urea, a substance which is continually forming in the body by the oxidation of some of the nitrogenous tissues.--Urea with water changes lastly into Carbonate of Ammonia. These several compounds, alike or identical in their ultimate composition, are possessed of very different properties, whether regarded as medicinal or as chemical agents. A similar set of changes may be produced among the elements of Uric acid, also an animal product. Changes in some respect similar to these which are produced by the Chemist are no doubt continually going on, or capable of being set up by various influences, in the circulating blood, which is at a heat fit for such processes; in which also is a considerable quantity of free oxygen, as well as soda, ammonia, and other elements, in a state fit for combination; and to all these is superadded the agency of the vital principle, the object and effect of which is continual alteration, destruction and reproduction. By supposing the establishment of a set of changes in the blood, we may possibly gain some insight into the cause of the powerful effects of some apparently insignificant medicines belonging to this division of Hæmatics. Neurotic medicines appear to act by their mere presence, contact and excitation. This may not always be the case with Catalytics. Their power cannot in all cases be clearly accounted for by a simple and direct influence on the blood, the muscles, the nerves, or any of the tissues. It seems sometimes as if it were on the processes that their presence sets going, and on the products thus generated, that their influence and power depend.