A large quantity of acid would easily overcome the feeble reaction of the blood, and thus, by remaining free, Sulphuric and other acids are enabled to act as astringents on certain of the glands. Other changes of combination have already been shown to take place during the process of absorption. Alkalies and their carbonates are more or less neutralized by the stomach acid. Substances soluble in alkalies are probably absorbed in such solution. Calomel, Chalk, Magnesia, and metallic oxides, as well as other insoluble medicines, are taken up in the soluble form, in which only they are capable of acting. But we are now concerned with medicines in the blood. And here I must protest against the idea that all chemical affinities have free play in that liquid. If it were so, many of our most valuable medicines would be decomposed and rendered insoluble there. All the mineral salts would be precipitated by the free Soda in that fluid. Acetate of lead would be immediately decomposed by sulphates, and Nitrate of Silver precipitated by chlorides. It is probable that the vital forces, as well as the viscidity of the plasma, exert a retarding or controlling power over such chemical tendencies. Some substances are decomposed while passing out of the body. Decomposing matters in the intestines and in the saliva, cause the formation of Sulphuret of Iron, while chaly-beates are taken, which blackens the fæces; and of Sulphuret of Lead, when lead is taken for some time, producing the well known blue line on the gums. _Changes of Reconstruction._--The elements of a body may be disturbed in the system, and combined together anew, without any material or apparent alteration of its properties. Probably many changes of this kind occur, but only some isolated instances have been verified. Thus Tannic acid, acquiring Oxygen, changes into Gallic. Benzoic and Cinnamic acids are converted into Hippuric acid, which passes out in the urine. Turpentine changes into a volatile oil, which communicates to the urine an odour of violets. Ferridcyanide of Potassium changes in the system into Ferrocyanide. Some of these changes will be afterwards considered more at length. _Changes of decomposition._--By this I mean such a disarrangement of elements as shall neutralize or reverse the action of a medicine. There is free Oxygen in the blood, and the most important change to which all organic substances are liable there is _oxidation_. This probably occurs in many cases. It always takes place with the starchy elements of the food, and with those parts of the nitrogenous tissues that have done their work, and are preparing to be excreted from the body. By this oxidation Wöhler has proved that the alkaline salts with vegetable acids are changed in the blood into alkaline carbonates. From being first neutral, they become now alkaline in their reaction, and affect the urine in the same way as free alkalies. (_Vide Hæmatics, ord. Acida, Alkalia, Tonica, Solventia._)