During recent years the great attention which has been paid to the study of reversible chemical actions has led to a desire for some method of specially indicating in the equations the reversibility of these actions. This is generally done by replacing the sign of equality by two parallel and oppositely-directed arrows; the use of this symbol in such cases does not only imply the reversibility of the action, it has the same value as the sign of equality in an ordinary equation. Unfortunately, there are several drawbacks to the use of this symbol: as generally printed, it is rather clumsy in appearance; the arrows, being widely separated, do not suggest the sign of equality which they replace; and further, in conjunction with formulæ, arrows are frequently employed, especially in organic chemistry, to indicate merely the stages and methods by which a substance can be produced from some other substance as starting-point. In such cases, as no attention is paid to the bye products, there is nothing of the nature of an equation involved. Numerous examples are to be seen in Richter's Organisehe Chemie, and amongst them cases in which oppositely-directed arrows, resembling the symbol employed for the equations for reversible actions, are used to indicate that each of the two substances concerned may serve as starting point for the preparation of the other, without involving equality on the two sides of the symbols. Such a double use of a symbol is highly objectionable.