70 Scientific Intelligence. 



7. Jahresbericht uber die Fortschritte der Chemie und verwand- 

 ter Theile anderer Wissenschaften. — Begrtindet von J. Liebig und 

 H. Kopp. Herausgegeben von F. Fittica. 1891, Viertes Heft, 

 November, 1897; 1896, Erstes Heft. Braunschweig (Fried rich 

 Vieweg & Sohn). —The fourth number for 189] of this most valu- 

 able publication has recently been issued ; it contains pages 

 1441-1920, with table of contents. With this has appeared also 

 the first number for 1896, pages 1-324. 



8. The change of alternating currents into direct. — This has 

 been accomplished by various mechanical devices, and by employ- 

 ing an alternating current motor to run a direct current dynamo. 

 L. Graetz calls attention to a very interesting method of accom- 

 plishing this purpose, which depends upon a remarkable property 

 of aluminium. It has long been known that in an electrolytic 

 cell in which one of the electrodes is of aluminum, a remark- 

 able weakening of the current sent through the cell is produced 

 if the aluminum constitutes the anode and if it is in the presence 

 of oxygen, while no weakening of the current results if the 

 aluminum is made the cathode. This phenomenon is variously 

 explained. By some it is supposed to be due to a layer of badly- 

 conducting oxide on the anode, by others to a resistance devel- 

 oped during the passage of the current, by others to a species of 

 condenser working between the electrode and the fluid, and to a 

 kind of dielectric polarization, which is not the common elec- 

 trolytic polarization. The reason of this last hypothesis is based 

 upon the fact that currents of less electromotive iorce than 22 

 volts cannot flow through the cell, and currents of higher electro- 

 motive force have their tension reduced to this amount. 



The fact that after stopping the current no residual polariza- 

 tion, approximating to the value of the apparent polarization, can 

 be found, is opposed to the idea of electrolytic polarization. A 

 dielectric condenser effect, however, would explain this fact. The 

 other electrode in the cell plays no important part. It can be 

 made out of platinum or other metal. The author uses carbon, 

 and any liquid which will develop oxygen on the electrode: he 

 has especially employed alum solutions. If an alternating cur- 

 rent is sent through a series of such cells, and if the number is 

 sufficiently great, so that the anode polarization exceeds or at 

 least is equal to the electromotive force of the alternating current, 

 the positive portion of the current would be stopped by the cells 

 and only the negative portions would pass through. The cells 

 thus separate the positive currents from the negative, and by 

 means of a simple arrangement of the cells an alternating current 

 can be changed into a direct. Under certain conditions as much 

 as 95 to 96 per cent of the energy of the alternating current can 

 be changed into the energy of direct currents. — Wied. Ann., No. 

 10, 1897, pp. 322-327. J. T. 



9. Observation of air waves. — Helmholtz has shown, that if a 

 warmer and lighter layer of air moves over an under layer of 



