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plate. It was at first thought that the positive rays were not deflected by a magnet, as magnetic forces which produced large deflections of cathode rays had no appreciable effect upon posi- tive ones. Wien showed, however, by using very strong magnetic fields, that they could be deflected, and that the direction of the deflection indicated that they carried a charge of positive elec- tricity; they can also be deflected by electric forces.

By measuring the deflection provided by electric and magnetic fields we can determine the value of ejm for the particles which con- stitute the rays. The result is of great interest. Instead of, as in the cathode rays, ejm having the same value for all the carriers, we find that elm has many different values separated by finite inter- vals; and instead of e/m being equal to 1-78 x io 7 , as in the cathode rays, we find the greatest value of e/m is 10*, which is the same as its value for a charged hydrogen atom. The values found for elm depend on the gases in the discharge tube; the outstanding result is that all these values of m correspond to masses of atoms or molecules of the chemical elements or compounds. Thus while the determination of e/m for the cathode rays shows that in a gas at a very low pressure the carriers of the negative electricity are all of the one type, being electrons whose mass is exceedingly small com- pared with that of any atom, the determination of e/m for the posi- tive rays shows that the carriers of the positive electricity are of many different types; and that all these types correspond to atoms or molecules of the chemical elements or compounds. It has already been shown that if charged particles, after passing through electric and magnetic fields, are received on a screen or photographic plate, all particles, for which e/m is the same, strike the plate on a parabola, and that for each different value of e/m there is a separate parabola.

These parabolas are shown in fig. 15, which is a reproduction of a photograph made by allowing the positive rays in a tube containing gases liberated by heating a certain mineral to strike against a photographic plate; taken from the top downwards they corre- spond respectively to the atom of hydrogen, the molecule of hydro- gen, the atom of helium, the atom of carbon with two charges, the atom of nitrogen with two charges, the atom of oxygen with two charges, the atom of carbon with one charge, the atom of nitrogen, the atom of oxygen, the molecule of water, the molecule of CO and that of N 2 (these form one parabola), the molecule of oxygen, the molecule of CO 2 and the atom of mercury. We find that many of the atoms can carry more than one charge, for when we find a parabola corresponding to one value of e/m we frequently find another corresponding to twice this value; thus carbon, nitrogen, and oxygen occur very frequently with two charges, other atoms such as argon with two and three charges, while mercury atoms have been detected with i, 2, 3, 4, 5, 6, 7 charges. It is significant that the atom of hydrogen never occurs with more than one charge. Multiple charges generally occur on atoms but not on molecules; there are, however, some molecules such as CO on which double charges have been found. Some of the positive particles, after passing through the hole in the cathode, lose their positive charge and become uncharged, and some of these neutral particles acquire a negative charge; thus mixed with the positively electrified parti- cles there are some negatively electrified ones. This power of acquiring a negative charge is confined to certain atoms; thus while the atoms of hydrogen, carbon, oxygen, fluorine occur with a nega- tive charge, the atoms of nitrogen, helium, argon and neon do not. It is exceptional for a molecule to acquire a negative charge, the molecules of oxygen and carbon, however, can do so. The equation

of a parabola formed by a particle on the photographic plate has already been given

where z is measured parallel to the displacement due to the magnetic field and y to that due to the electrostatic. C is a quantity which depends on the strength of the electric and magnetic fields and on the position of the photographic plate. If, as in fig. 16, we draw a

FIG. 16

line parallel to the axis of z, the intercept made by a parabola on this line will be proportional to (e/m)i; thus, if the top parabola is due to the atom of hydrogen, the next to the molecule of hydro- gen, the third to the atom of helium and the fourth to that of_oxy- gen, the intercepts AH, AH2, AO are in the proportion of I, 1/^2,1/4. Thus by comparing the intercept made by any parabola X with that made by the parabola due to the hydrogen atom we can find the molecular weight of the substance producing the parabola X.

Positive Rays as a Method of Chemical Analysis. Since from the measurement of the positive ray photographs we can de- termine the molecular weight of the gases in the discharge tube, we can analyze a gas by putting a small quantity of it in a dis- charge tube and taking a photograph of the positive rays. It is thus a method of chemical analysis, and its application has al- ready led to the detection of several new substances. In fact, though it has only recently been introduced, more substances have been discovered by this method than have ever been dis- covered by spectrum analysis. The method has many advan- tages. In the first place only a very minute quantity of the gas is required ; a small fraction of a cubic centimetre of gas at atmos- pheric pressure is all that is required to fill the discharge tube at the pressure at which the positive rays are produced. Aga.in, the method is very sensitive, as it will detect the presence of a gas which only forms a small percentage of the gas in the tube. The method not only detects the presence of the gas, but at the same time determines its molecular weight. It indicates, if the gas is an element, whether it is monatomic or diatomic; for if it is diatomic it will give rise to two parabolas, one due to the atom, the other to the molecule. The absence of double or negative charges will suggest that it is a compound and not an elemen- tary gas. The only ambiguity is that it does not distinguish between two substances of the same molecular weight; thus CO2, and NzO give the same parabolas, as also do CO and N2: we can often, however, remove this ambiguity by putting sub- stances in the tube which would absorb one gas and not the other, and testing whether or not this has removed the parabola.

Use of Positive Rays to Determine Atomic Weight. The meas- urement of the parabolas give, as we have seen, the atomic weight of the elements producing them; they can therefore be used to determine the atomic weight of elements which can be introduced in a gaseous state into the discharge tube. This method has the great advantage that the presence of impurities does not affect the result. Mr. Aston has lately, by the use of a positive-ray method for determining atomic weights, found the very important fact that, if oxygen is taken as 16, the atomic weights of the elements with the exception of hydrogen are rep- resented by whole numbers. Thus in working with chlorine he found no substance with an atomic weight of 35-4, but two