762 GILBERT N. LEWIS. By combining these results with those derived from Jahn’s measure- ments of the electromotive force at 18’ of cells Ag + AgCl, HCl(cJ, HCl(cr), AgCl + Ag the series of free-energy values has been extended from 0.033 to 0.00167 molal (Table IX). And with the aid of calori- metric data and an electr.omotive force determination of the cell Ha, HCI, HgzCL + Hg by Lewis and Rupert, the absolute free energy of HC1 (referred to the free energies of the elements as zero) at 18, 25, and 35’ in solutions from 0.00167 molal to 4.5 molal has been calculated and tabulated (Table X). From these free-energy data, with the aid of the assumption that at the lowest concentration (0.00167 molal) the ion activity is equal to the ion concentration, a series of absolute activity coefficients for hydrochloric acid covering the concentration range 0.00167-4.5 molal has been com- puted (Table XI). These have then been compared with the activity coefficients of potassium chloride derived from the electromotive-force measurements of MacInnes and Parker (Table XII) and the osmotic pressure calculations of Bates (Table XIII). The results lead to the conclusions that the activity coefficients both of hydrochloric acid and potassium chloride decrease up to 0.1 molal far more rapidly than do the conductance ratios. The difference at this concentration amounts to 9% for hydrochloric acid and 15% for potas- sium chloride, so that in using the conductance ratio as a measure of ion activity in mass-action expressions, as is commonly done, a corresponding error is involved. At a concentration of about 0.5 molal the activity coefficient of hydrochloric acid reaches a minimum, and then increases very rapidly with increasing concentration, becoming at the highest concentratios (4.48 molal) 2,23 times as great as at zero concentration. The results show further that the activity coefficients of potassium chloride derived independently from electromotive force and from freezing-point measurements are in remarkable agreement, affording evidence that in the case of this salt the conductance ratio is a true measufe of ion concentration (though not of ion activity), since this assumption is made in the calculation from the freezing points, but not in that from the elec- tromotive forces. Bos’roN, MASS. [CONTRIBUTION FROM THE CHEMICAL LABOR~TORY OF THE UNIVERSITY OF CAGIFORNIA. I TRE ATOM APQD THE MOLECULE. BY GILBSRT N. LBWIS. Received January 26, 1916. In a paper entitled “Valence and Tautomerism”l I took occasion THIS JOURXAL, 35,1448 (1913); see also the important article of Bray and Branch, IEd., 35, 14-40 (19x3).763 TH& ATOM AND THE MOLECULE. to point out the great importance of substituting for the conventional Classification of chemical substances, as inorganic or organic, the more general classification which distinguishes between polar and nonpolar substances. The two classifications roughly coincide, since most in- organic substances are distinctly polar, while the majority of organic substances belong to the nonpolar class; thus potassium chloride repre- sents the extreme polar type and methane the nonpolar. Nevertheless, there are many inorganic substances which, under ordinary circum- stances, are predominantly nonpolar, and many organic substances which, at least in a certain part of the molecule, are strongly polar. This article was apparently unknown to Sir. J. J. Thomsonl when he wrote, in 1914, an extremely interesting paper on the “Forces between Atoms and Chemical Affinity” in which he reached conclusions in strik- ing accord with my own, and discussed in considerable detail the theories of atomic and molecular structure which led him to these conclusions. To enable us to appreciate the importance and the usefulness of a dis- tinction between the polar and nonpolar types of chemical molecules no hypotheses are necessary, but in a more minute examination of the nature of such a distinction some theory of atomic structure is indispensable. Such a theory I have employed for a number of years in the interpreta- tion of chemical phenomena, but it has not hitherto been published. I shall present this theory briefly in the present paper, for, while it bears much resemblance to some current theories of the atom, it shows some radical points of departure from them. As an introduction it will be de- sirable to review the characteristics of polar and nonpolar compounds. Polar and Nonpolar Types. The very striking differences in properties between the extreme polar and the extreme nonpolar types are summarized in the following table quoted from my previous paper: POLAR. Mobile Reactive* Condensed structure Tautomerism Electrophiles Ionized Ionizing solvents High dielectric constant Molecular complexes Association Abnormal liquids NONPOLAR. Immobile Inert Frame structure Isomerism Non-electrophiles Not ionized Not ionizing solvents Low dielectric constant No molecular complexes No association Normal liquids 1 PhiL. Mag., n7, 757 (1914). * In my former paper the words “inert” and “reactive” were inadvertently trap, posed and appear in the wrong columns.764 GILBERT h;. LEWIS. All of these properties with respect to which fundamental distinctions haw been made between the two types, and which seem so unconnected, are in fact closely related, and the differences are all due to a single cause. Even before making any more special hypothesis we may very safely as- sume that the essential difference between the polar and the nonpolar molecule is that, in the former, one or more electrons are held by sufficiently weak constraints so that they may become separated from their former positions in the atom, and in the extreme case pass altogether to another atom, thus producing in the molecule a bipole or multipole of high elec- trical moment. Thus in an extremely polar molecule, such as that of sodium chloride, it is probable that at least in the great majority of mole- des the chlorine atom has acquired a unit aegative charge and there- fore the sodium atom a unit positive charge, and that the process of ion- ization consists only in a further separation of these charged parts. If then we consider the nonpolar molecule as one in which the elec- trons belonging to the