There are many examples of chemical reactions which only occur between two bodies when a third is present, which may nevertheless be found unchanged at the end of the process. Notwithstanding the fact that the third body is found unchanged at the end of the process, it may have undergone changes during the continuance of the process. Thus alcohol is not converted into ether and water by boiling alone, but it does undergo this conversion by boiling with sulphuric acid. The acid is found unchanged at the end of the process, but is changed during it into ethyl-sulphuric acid, which, combining with alcohol, again yields sulphuric acid along with ether.

In other cases, however, we cannot show that the substance has undergone change. Thus starch is converted into dextrin and sugar and cane-sugar into grape sugar by boiling with acids, but we do not at present know that the acid has undergone any change during the process as it does in the preparation of ether. Peroxide of hydrogen is rapidly decomposed by finely divided platinum or silver, and finely divided platinum will, on the other hand, cause oxygen and hydrogen to unite rapidly. Such actions, where the third substance seems to act by its mere contact with the other substances, and without undergoing change itself, are called catalytic. They are probably due to an attraction of some kind bordering both on chemical and physical between the molecules.

Thus some organic substances would resist the oxidising action of the air for a considerable time, but they are readily oxidised by charcoal. It is usually said that the charcoal has the power of attracting oxygen and condensing this gas upon its surface. It does not unite with the oxygen chemically so as to form CO2, but merely attracts it, holds it for a while, and then gives it off readily to any oxidisable substance. Platinum, palladium, rhodium, and iron absorb hydrogen, palladium doing so to an enormous extent, especially when it is in a spongy form. The hydrogen is supposed by some to be simply condensed within the metal, while others think that the hydrogen and metal unite to form a hydride. The hydrogen is given off from the metal in a nascent form, and has very strong affinities.

Thus palladium-hydrogen readily reduces ferric to ferrous salts, the hydrogen taking oxygen from the ferric salt and forming-water. But when the hydrogen is liberated from palladium or rhodium in presence of oxygen, it appears to convert the oxygen into ozone, and greatly increases its oxidising power. Thus palladium-hydrogen with oxygen colours a mixture of potassium iodide and starch paste blue, and oxidises haemoglobin to met-hsemoglobin and ammonia to nitric acid. Spongy rhodium, or iridium saturated with hydrogen, cause formic acid to be oxidised to carbonate, calcium formate being changed into calcium carbonate. Exactly the same action is possessed by an organic ferment, and in the conversion of the formic into carbonic acid the ferment and the spongy rhodium or iridium are alike unchanged. Spongy platinum, palladium, rhodium, and iridium may thus be regarded as inorganic ferments.1

1 Hoppe-Seyler, Ber. d. deulsch. chem. Gesellsch., 1883, Feb. 12, p. 117.