O. Lummer. Verflüssigung der Kohle und Herstellung der Sonnentemperatur (Sammlung Vieweg. Heft 9/10. Braunschweig. 1914).

The question of the liquefaction of coal has a long history and is of very great importance for the physicist and the chemist. Lummer begins his interesting monograph with an account of this subject. In the second chapter the question of the temperature of the arc crater as a function of the current strength and of the length of the arc is studied, and a sharp-jet photometric method is described, which makes it possible to measure the surface brightness of the crater when its position in the arc is changed. As a result of the measurements, it is found that, when the current strength is changed,

current from 10 to 80 amperes, and when the arc burns in air, the temperature of the positive crater, determined by its brightness, does not change, whereas the temperature of the negative crater exhibits fluctuations. Lummer further investigates the connection between the radiation of incandescent lamps with carbon and platinum filaments and finds that carbon behaves as a gray body, i.e., as a body uniformly reflecting all rays of the spectrum. The same “gray” emission as that of incandescent lamps is also given by the positive crater of the arc at normal pressure. This circumstance enables Lummer to find the temperature of the positive crater, which he determines to be about \(4200^\circ\) abs.

Lummer further describes the first observations of the liquefaction of carbon in the positive crater at reduced pressure and gives a picture of the melting phenomenon as it may be observed under high magnification. The photographs appended to this chapter make it possible to orient oneself conveniently in the details of the process, although, according to the author’s descriptions, the process itself, when observed subjectively, gives far more detail than the photograph.

The observations initially made at reduced pressure were then extended by Lummer to a pressure of 2 atmospheres, provided that the temperature necessary for melting could be obtained.

In the following paragraph the author describes in detail the melting of various sorts of coal and of diamond both in air and in various gases (\(N, CO_2, O\)) and shows that the product of the melting of coal is graphite.

The study of melting at increased pressure leads Lummer to the question of the connection between the temperature of the positive crater and the pressure, and enables him, by applying optical methods, to observe the temperature of the crater as the pressure changes from 1 atmosphere to 26 atmospheres, when the temperature rises from \(4200^\circ\) abs. to \(5560^\circ\) abs. Since the latest determinations of the solar constant define the “black” temperature of the sun as \(5850^\circ\), we must conclude that Lummer’s experiments have yielded a temperature reaching that of the sun, and that they lay the foundations for the further experimental study of physico-chemical processes on the sun, which are of enormous theoretical interest.

P. Lazarev.