Organic chemistry has been largely developed by the discovery from time to time of special reagents and special types of reactions which have shown themselves to be capable of extensive application. Such, for example, was Frankland’s discovery, in 1852, of zinc-ethyl—the first of the organo-metallic compounds, and the type of a series of substances of great theoretical importance, and of great practical value by reason of their reactive powers. They led to the synthesis of the paraffins, the secondary and tertiary alcohols, and ketones. A few years later Wurtz introduced the use of metallic sodium as a condensing agent, and showed thereby how the hydrocarbon _butane_ could be produced from ethyl iodide: 2C2H5I + Na2 = C4H10 + 2NaI. Use was made of the same agent by Fittig, in 1863, in effecting the synthesis of the homologues of benzene by the action of an alkyl iodide upon bromobenzene: C6H5Br + CH3I + Na2 = C6H5.CH3 + NaI + NaBr. In like manner Kekulé, in 1866, obtained benzoic acid by the action of carbon dioxide upon bromobenzene: C6H5Br + CO2 + Na2 = C6H5COONa + NaBr. The readiness with which magnesium can now be obtained, mainly as the result of Sonstadt’s efforts to develop its metallurgy, has led to its application, at the suggestion of Barbier, in 1899, in place of zinc. The particular form of magnesium compound now employed as a reagent was prepared by Grignard in 1900, and is known by his name. It is obtained by bringing an ethereal solution of an alkyl iodide or bromide into contact with magnesium, when the metal is dissolved, forming, in the case of methyl iodide, MgCH3I.(C2H5)2O. Grignard’s reagent has shown itself to be extraordinarily reactive, and a great number of condensations—of hydrocarbons, alcohols, aldehydes, acids, ketones, amides, and additive compounds—have been effected by means of it. Other condensing reagents of value are aceto-acetic ester, sodium amalgam, sodamide, sodium ethoxide, dimethyl sulphate, zinc chloride, aluminium chloride, fused caustic potash, hydrogen chloride, phenyl-hydrazine, hydrogen peroxide in presence of a ferrous salt (Fenton’s reagent), ammonia, and various amines. The application of these reagents has led to the discovery of a variety of new compounds, the mode of origin of which has served to elucidate their constitution. The great majority of organic syntheses, especially when they start by the use of inorganic materials, consist in passing from simple to complex molecular groupings by condensation processes. An interesting example of the reverse process is seen in the production of _carbon suboxide_, or _carbon carbonyl_, C3O2, obtained from various malonyl compounds, but most conveniently prepared by the action of phosphoric oxide on malonic acid under diminished pressure, or by treating an ethereal solution of dibromomalonyl chloride with zinc: