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Rh substances with atomic weight of 35 and 37 respectively; he regards these substances as identical in chemical properties and inseparable by chemical reactions, and ordinary chlorine as a mixture of abou t 3 parts of (3 5 2 ) and one part of (37'). Mr. Aston, by the method of positive ray analysis, has discovered isotopes of boron, silicon, bromine, krypton, xenon and mercury.

The Charge of Electricity Carried by Caseous Ions and Elec- trons. The deflection of cathode and positive rays by electric and magnetic forces supplies a method for finding the value of e/m; for the determination of e, the charge on an ion, other methods have to be employed. One such method used by J. J. Thomson is based on the important investigations of C. T. R. Wilson on the effect of ions on the deposition of clouds and fogs from supersaturated air. If dust-free air saturated with water vapour is suddenly cooled by expansion, no cloud or fog is deposited unless the supersaturation due to the cooling is very large. C. T. R. Wilson found that if ions are present in the gas they act as nuclei round which drops of water are deposited with a supersaturation much below that required for gas free from ions. A beautiful application of this is the detection of the path of an a particle from a radioactive substance. The a particle produces by collision ions all along its path; if the damp gas through which the particle is passing is suddenly cooled by expansion, drops of water will deposit on the ions and thus mark out the path of the particle. One of Mr. Wilson's photographs of such a path is shown in fig. 17. Mr. Wilson found that less supersaturation is

required to deposit water on negative than on positive ions. This result can be applied to find the number of ions in a moist gas, for if the gas is suddenly expanded by an amount sufficient to deposit drops on ions, but not sufficient to produce condensation in their absence, then each ion may be made the centre of a drop, and the problem of counting the ions is reduced to that of counting the drops.

We can calculate the amount of water that will be deposited by any given expansion of the air; hence, since we know the volume of the water we can determine the number of drops if we know the volume of a single drop. Observation of the rate at which a drop falls under gravity will give the size of the drop, for Stokes long ago showed that the velocity of a rain drop falling under gravity is

given by the equation v = - g ; when v is the velocity of the drop,

a its radius, / the viscosity of the gas, g the acceleration due to gravity, and p is the density of the gas. It has been found that, with the exceedingly fine drops formed round ions where the radius of the drop is comparable with the free path of the molecules of the gas, the velocity is greater than that given by the above, equal in

the proportion of fl-f Jfel, when C is a constant, and p the

pressure. But though this correction makes the relation between a and v a little more complicated, it still enables us to determine a when v is known. Thus the radius, and therefore the volume, of the drop can be determined, and from this, as we have seen, we can deduce the number of ions.

_Let n be this number per unit volume; then if a current of elec- tricity is sent through the gas by an electric force X, the current

passing through unit area will be neU when U is the mean velocity of the positive and negative ions under the force X. We know that it is proportional to the force and for a force of one volt per centi- metre is 1-5 cm. /sec.; and hence when X is known U is known, the current net) can be measured, and hence ne deduced; as n has been found by the drops, the value of e can be determined immediately. This was the method used by J. J. Thomson; a simpler method used afterwards by H. A. Wilson was to get drops round the negative ions alone by using an expansion that would deposit moisture on nega- tive but not on positive ions. He then showed the rate of fall of these drops, first under gravity alone, and then under a vertical electrical force X, acting on the drop in the same direction as grav- ity. Thus, when the electric field is acting, the force on the drop is

Xe+ 4 *pa 3 g,

and when it is off the force is only ?r/>a 3 g. Thus, if v lt v are respec-

o lively the velocities of the drop when the field is on and off,

From v, the rate of fall when the field is off, we can calculate as before the radius of the drop, and from the preceding equation we can determine e. Millikau, who has made most extensive and accu- rate investigations on the value of e, used a modification of the pre- ceding method. Instead of producing water drops by expansion on the ions, he obtained, by means of a sprayer, minute drops of oil; he observed the motion of one of these under an electric field in a gas which was subject to some ionizing agent, and from time to time an ion would strike against the drop and alter the charge; this would alter the velocity, and from the alteration of the velocity he could by a formula similar to that just given calculate the charge communicated to the drop by the ion. The value obtained for e by this method is, in electrostatic units,

e = 477Xio- 10.

From the value of e we can obtain Avogadro's constant, the num- ber of molecules in a cubic centimetre of gas at oc and 760 mm. pres-

sure. For Townsend has shown that Ne = P^F: where P is the

AL>

pressure when the number of molecules is N, u the velocity of the ion when'the force is X, and D the coefficient of diffusion of the ion into the gas. Townsend measured D, and found that the value of Ne determined by this equation was, within the limit of errors of experiment, equal to NE as determined by experiments on the quantities of hydrogen liberated by electrolysis. E is here the charge carried by an atom of hydrogen in the electrolysis of liquids. Thus the charge on the gaseous ion is equal to that on the liquid ion. Since one coulomb deposits 1-11827 milligrams of silver, and the atomic weight of silver is 108 and the density of hydrogen 8-987 X io* at oc, NE = i-29oXio 10, and as e=4-77Xlo- 10 , N = 2-7Xio". The number of molecules in a gramme molecule of any substance is 6-06X10".

Thus the study of the electrical property of gases has given the most accurate values available of two of the most important constants connected with the constitution of matter. By study- ing electrified atoms and molecules, we have been able to de- termine their masses and their properties with an accuracy far beyond that attainable by any method which can be used when they are in the normal state. (J. J. T.) GASQUET, FRANCIS AIDAN (1846- ), Roman Catholic cardinal and historian, was born in London Oct. 5 1846. He was educated at Downside College, Bath, afterwards becoming superior of the Downside Benedictine monastery (1878-1884). He was created cardinal in 1914. He has produced various works on mediaeval church history and liturgies, among them being Henry VIII. and the English Monasteries (1888-9); A Short History of the Catholic Church in England (1903); Parish Life in Mediaeval England (1906) and The Bosworth Psalter (1908). GAUL, GILBERT WILLIAM (1855-1919), American painter (see 11.532), died in New York Dec. 21 1919. He was awarded a gold medal at the Appalachian Exposition, Knoxville, in 1910. GAUTSCH-FRANKENTHURN, PAUL, FREIHERR VON (1851- 1918), Austrian prime minister, was born at Vienna Feb. 26 1851. He was director of the Theresa academy from 1881 to 1885, and Minister of Education from 1885 to 1893 and from 1895 to 1897. He was three times prime minister: first from