TENDENCY to detonate is probably the most important factor in determining the usefulness of fuels for internal-combustion engines. Although it is possible, by various means, to measure more or less accurately the relative knocking-characteristics of fuels, no way has heretofore been found of rating fuels that does not depend upon some arbitrary non-reproducible conditions and measurements. The general methods adopted have consisted in comparing one fuel with another, but no absolute standard has been available.Knocking is a function of several variables, the knocking characteristics of which have been found by keeping a certain number of them constant while certain others are varied, thus rating them in terms of load, the spark-advance necessary to produce knocking, the position of the throttle at which knocking begins, and the like. A second method consists in keeping all operating factors constant and comparing the fuels in terms of some non-knocking fuel, such as benzol, or some antiknocking fuel, such as tetraethyl lead, which must be added to the poorer of two fuels to make its knocking characteristics identical with those of the better one.To draw detonation specifications for fuels, a reproducible primary standard fuel must be developed and methods of comparing fuels must be agreed upon. The composition of gasoline is so complex and the knocking characteristics of its different constituents are so varied that great difficulty has been encountered in finding one or more hydrocarbons, the purity of which could be definitely established by test and which would thus be absolutely reproducible.Two hydrocarbons are cited that seem to be ideally suited for standard reference fuels, namely, pure normal heptane C7H16 and pure octane C8H18, which is prepared synthetically from tertiary butyl alcohol. By mixing these two hydrocarbons in different proportions it is said to be possible to duplicate the knocking characteristics of any commercial fuel between the limits of 60 per cent of heptane and 40 per cent of octane and 40 per cent of heptane and 60 per cent of octane. These hydrocarbons are also said to be almost identical in volatility, chemical composition and other physical properties, and that, consequently, irregularities are not to be feared.To make possible the drawing of detonation specifications for automotive fuels, it is therefore necessary only to specify that the knocking characteristics of a fuel for a given purpose should be equivalent to those of a mixture of any desired proportions of the heptane and octane in question and to agree upon one method of testing or some one of a number of methods that may be satisfactory.The discussion following the paper centers on the chemical difference between the two standard fuels and what accounts for the difference in their knocking qualities; on a theory concerning the properties of motor fuel which indicate its tendencies to knock; and on the effects of mixtures of heptane and toluene, and octane and toluene.