238 CURRENT TOPICS. [J. F. I.

Industrial Employment of Very High Pressures. GEORGES CLAUDE. (Comptes Rendus, October 13, IgIg.)-At present 200 atmospheres is about the maximum pressure used, and this chiefly in the Haber process and in the liquefaction of air. The author maintains that pressures of 1000 atmospheres can be more easily employed than those of lower amounts. Leather collars work all the better at higher pressures. It is not difficult to provide walls sufficiently strong to withstand high pressures, but it is difticult to secure tight joints. Owing to the smaller volume of containers at high pressure for the same mass of gas it is easier to make the joints tight for high pressures. As the cost of compression rises in propor- tion to the logarithm of the pressure the expense of compressing to 1000 atmospheres is one and a half times as much as of attaining 100 atmospheres. G. F. S.

Some Characteristics of the Spark Discharge and Its Effect in Igniting Explosive Mixtures. C. C. PATERSON and NORMAN CAMPBELL. (Proceedings Physica. Society of London, vol. xxxi, Pt. iv, IgIg.)-For the ignition of mixtures of petrol and air a quanti)ty of energy as small1 as .ooo~ joule was found to suffice. When the spark potential is lowered the quantity of energy requisite in- creases with great rapidity. A train of similar sparks has no greater igniting power than a single spark.

In the ignition of hydrogen and air it was found that reduction of the radius of curvature of the electrodes augmented the igniting power.

Experiments on an aeroplane engine showed that satisfactory ignition could be obtained by the use of quantities of energy much smaller than those supplied by a magneto or by a battery and coil system. G. F. S.

The Speed of Sound Pulses in Pipes. A. L. FOLEY. (Physical Review, August, 1919.)-A sound pulse of cylindrical form was pro- duced by an electric spark. It spread out from the line source, in part travelling in the open air and in part passing through various tubes set with their axes perpendicular to the line of the spark. This pulse was illuminated for an instant by a second spark and its shadow, due to its having a density different from thlat of the sur- rounding air, was caught and recorded on a photographic plate. The distance each portion of the pulse had come could thus be observed. It was found that the speed is greater or less in short tubes than in the free air, according to position with respect to the source, but that in longer tubes the speed, at first greater than in free air, gradually lessens and finally becomes smaller. The intensity of the pulse when it enters the pipe seems to be that which determines the speed. The greater the intensity the greater the speed. G. F. S.