PHYSICS. 255 



during- escape of steam (seldom over ^™™), a vibratory motion of tliis 

 valve was at first supposed by the author ; but further study and experi- 

 ment led him to the hypothesis that the steam jets in lifting the valve 

 do not begin to move from its middle point but from the periphery of a 

 circle n out to the circumference of the valve of radius r; so that the 

 pressure of steam on the under surfiice of the valve is composed of two 

 parts, of which the inner or aerostatic is produced by the solid steam 

 cylinder of radius p and the outer or weak rn'rodynamic part by the ex- 

 ternal hollow cylinder of r — p thickness of wall. The phases of develop- 

 ment of steam tension were also investigated. — {Nature, xxi, 180, Decem- 

 ber, 1879.) 



Aitken has propounded the rather remarkable theory that dust is the 

 germ of which fogs and clouds are the developed phenomena. Steam 

 was mixed with air in two large glass receivers, one of which contained 

 filtered the other unflltered air. In the former the steam condensed in 

 its well-known cloudy form, while in the filtered air no cloudiness what- 

 ever appeared. From these and similar experiments the author con- 

 cludes : 1st, that whenever water vapor condenses in the atmosphere it 

 always does so on some solid nucleus ; 2d, that dust-particles in the air 

 form the nuclei on which the vapor condenses; 3d, that if there was no 

 dust there would be no fogs, no clouds, no mists, and ])robably no rain, 

 audthatthe supersaturated air would convert every object on the surface 

 of the earth into a condenser on which it would deposit; and 4th, that 

 our breath when it becomes visible on a cold morning, and every puif of 

 steam as it escapes into the air, show the impure and dusty condition 

 of our atmosphere. The importance of these results, meteorologically, is 

 very great. — [Nature., xxiii, 195, December, 1880.) 



2. Conduction and Radiation. 



Weber has made a series of experiments uj^on the conductibility of 

 liquids for heat, resting in principle u^ion Fourier's theory. Sup- 

 pose a thin liquid layer bouuded by two parallel metallic plates, the 

 whole possessing a uniform temperature. At a given instant the lower 

 plate is cooled to 0° and the whole system is inclosed in a space at 0°. 

 I By determining the law of the variation of the temperature at the 

 center of the upper plate, the coefficient of conduction may be calcu- 

 lated. The experiment was made as follows: Upon a plate of copper 

 0.05™ in thickness, three small slips of glass 2 to 3"^™ thick were placed 

 to sui^port the second plate of copper. This plate was i^erforated at 

 center by a small hole through which the liquid was introduced until it 

 formed a shghtly convex meniscus at the edges of the plates, care being 

 taken to leave no bubble of air. When the whole had acquked the tem- 

 perature of the laboratory one of the junctions of a thermoelectric cou]>le 

 was put in contact with the center of the upper plate, the other remain- 

 ing in the melting ice. An aperiodic oscillation galvanometer in the 

 circuit gave, by a simple reading, the electromotive force, and so 



