[J] This fact has been established beyond doubt by the recent great work of Studer and Bannwarth, Crania Helvetica Antiqua, 1894. _Vide_ p. 13. Sergi's attempt to interpret the data otherwise (1898 a, p. 67) is entirely erroneous. Gross's data apparently refer entirely to the later period of Teutonic invasions in the iron age (1883, p. 106). _Cf._ Munro, pp. 537 and 541. [K] Popular Science Monthly, December, 1897, p. 151. [L] From Montelius, 1897. [M] _Cf._ maps and data in J. Geikie, 1894; Penck, 1884; and Niederle, 1893, p. 25. [N] Bertrand, 1876 a and 1876 b, gives a full account of it. The best recent authorities upon Scandinavian culture are Sophus Mueller, 1897, and Montelius, 1895 b. Other works of reference are those of Worsaae, Nilsson, Hildebrand, Madsen and Rygh, full titles being given in our supplementary Bibliography of the Anthropology and Ethnology of Europe. Comprising nearly two thousand titles, it will be provided with a detailed subject index. [O] Bertrand, 1876 b, p. 40. [P] Reinach, 1892, pp. 72-78, for severe criticism of Penka's hypotheses. LIQUID AIR. BY IRA REMSEN, PROFESSOR OF CHEMISTRY IN THE JOHNS HOPKINS UNIVERSITY. Water, the substance most familiar to us, is known in the liquid, in the solid, and in the gaseous state. Everybody knows that by heating the solid it passes into the liquid state, and that by heating the liquid it passes into the form of gas or vapor. So also everybody knows that when the vapor of water is cooled it is liquefied, and that by cooling liquid water sufficiently it becomes solid or turns to ice. In the same way many of the substances that are known to us as liquids, such as alcohol and ether, can be converted into the form of gas or vapor by heat. In fact, this is true of most liquids. The temperature at which a solid passes into the liquid state is called its melting point, and the temperature at which a liquid passes into the gaseous state is called its boiling point. The boiling point of water, for example, is 100° C. (212° F.) in the open air. But the boiling point varies with the pressure exerted upon the surface. The pressure that we ordinarily have to deal with is that of the atmosphere. If the pressure is increased the boiling point is raised, and if the pressure is decreased the boiling point is lowered. In dealing, then, with the conversion of a gas into a liquid, or that of a liquid into a gas, both the temperature and the pressure have to be considered.