physics. 593 



a limiting temperature is finally reached, at which both the liquid and 

 the vapor have the same weight in the same volume. At this moment 

 they cannot be separated; the vapor cannot escape to the top, the liquid 

 cannot sink to the bottom. The meniscus first disappears, the surface 

 of separation ceases to be distinct, then the entire mass is mingled 

 together, showing undulating and moving streaks, evidence of a mix- 

 ture of different densities, and finally the whole assumes a homogeneous 

 state which is supposed to be gaseous. The "critical point" is reached 

 and it may be defined as the temperature at which a liquid and its sat- 

 urated vapor have the same density. In a subsequent paper Jamin 

 discusses the laws of compressibility and condensibility of gases as 

 illustrating this view. (C. B., xcvi, 1448; J. Phys., II, h, 389, 393; 

 Phil. Mag., July, 1883, V, xvr, 71.) 



Gerard- Ansdell has studied the critical point of mixed gases, using for 

 the purpose a mixture of carbon dioxide and hydrogen chloride. The ex- 

 periments were made in a graduated Cailletet tube 50 cubic centimeters 

 in capacity, the capillary portion having a diameter of 2 millimeters. 

 After determining the critical point, the maximum tensions of the vapor 

 at different temperatures, and the volumes of the liquid and gaseous 

 portions, the apparatus was opened in distilled water, and the hydrogen 

 chloride, which dissolved, was determined by analysis. The critical 

 point of the carbon dioxide was 31°, and of the hydrogen chloride, 51.25°. 

 But the critical point of the various mixtures was not found to vary pro- 

 portionally to the percentage, but departed therefrom by as much as 

 3.6°. (Proc. Boy. Soc, xxxiv, 113 ; J. Phys., January, 1883, II, n, 45.) 



3. Conduction and Badiation. 



To demonstrate to an audience the relative conductibility of copper 

 and iron, Petrouchewski uses two air-thermoscopes of identical dimen- 

 sions, the reservoir of the one being a vertical tube of copper and of the 

 other a similar tube of iron. The heat from a gas flame is conducted to 

 the thermoscopes by means of large wires of copper and of iron, respec- 

 tively. A few minutes after the lamp has been lighted the level of the 

 liquid in the copper thermoscope has fallen 25 to 35 centimeters, while 

 that in the other has fallen only 7 or 8. A similar apparatus for show- 

 ing the low conductibility of water in comparison with mercury has 

 been devised by the same experimenter. (J. Soc. Phys. Ghim. Busse, 

 xiy, 154 ; J. Phys., December, 1883, II, n, 576.) 



Violle has measured the radiation from silver at its melting point, as 

 preliminary to measuring that of platinum, proposed by him as a photo 

 metric standard. A bath of melted silver was placed below a thermo- 

 electric battery connected with a mirror galvanometer, so that the radia- 

 tion from the metal fell normally on the face of the battery through an 

 opening one square centimeter in area made in the double wails of a suit- 

 able screen and closed with a plate of quartz. The screen was cooled by a 

 current of water. As the metal cooled the radiation at first decreased; 

 H. Mis. C9 38 



