Chemistry and Physics. 513 



closed by pistons. One of the compartments thus formed is filled 

 with a dilute solution, the solvent of which alone can pass through 

 the septum. The other compartment is filled with the solvent 

 only. Osmose takes place, the solvent entering the solution 

 through the septum and increasing the pressure exerted by that 

 solution ; the flow continuing until the osmotic pressure is bal- 

 anced by the pressure within the vessel. By varying the pres- 

 sures upon the pistons, equilibrium may be produced, and the 

 osmotic pressure measured. The author proceeds to treat this 

 osmotic system, with reference to the forces in operation in its 

 various parts, in the same manner in which the problem would be 

 discussed with reference to the kinetic theory of gases. And he 

 finds in this way, that the resultant of all the forces in the two 

 compartments which produce pressure on the septum, i, e., the 

 osmotic pressure, is equal to the gaseous pressure which the dis- 

 solved substance would exert were it distributed as a gas 

 throughout the volume occupied by the solution, assuming that 

 the mean kinetic energy of a dissolved molecule is equal to that 

 of a gaseous molecule at the same temperature.— Zeitschr. physik. 

 Chem., vi, 474; J. Chem. Soc, lx, 389, April, 1891. G. f. b. 



3. On the Production of Electrification in the Preparation of 

 solid Carbon dioxide. — In order to obtain rapidly and conven- 

 iently a considerable quantity of solid carbon dioxide, Hauss- 

 knecht fastened over the jet of a wrought iron cylinder contain- 

 ing the liquid dioxide, such as is found in commerce, a bag of 

 coarse linen cloth. By inclining the cylinder, the liquid issues 

 under a pressure of 60-80 atmospheres, and in expanding into 

 the gaseous state, absorbs so much heat as to solidify a portion of 

 the escaping liquid, this solid collecting in the bag in the form of 

 a compact snow. If the bag be made of strong canvas and have 

 a capacity of from one to two liters, and if the experiments be 

 conducted in the dark, it will be observed that the bag is filled 

 with a greenish-violet light and that electric sparks from 10 to 

 20 cm long issue from its surface. If the hand be placed in these 

 sparks the same peculiar- prickling effect is noticed as when it is 

 brought near the collector of an electric machine. This appear- 

 ance of. electrification is especially noticeable where imperfections 

 occur in the compression pump or in the valves or manometers, 

 so that the carbon dioxide issues under great pressure. The 

 cause of this development of electrification is no doubt similar 

 to or identical with that which operates in the hydro-electric 

 machine of Armstrong. The liquid issuing with great force is 

 converted into gas at once when it reaches the air. But under 

 so great a pressure and at so low a temperature, the gas thus 

 forced through the openings in the bag is accompanied by fine 

 particles of the liquid, and probably of the solid also ; the 

 friction of these solid and liquid particles developing the electri- 

 fication. For the success of this experiment it is essential that 

 the carbon dioxide be absolutely free from air. The luminous 

 phenomena in the interior of the bag appear first when the solid 



