Rays of Positive Electricity and Their Application to Chemical Analyses/Spectra Produced by Bombardment with Positive Rays
The spectra produced when the positive rays strike salts of the alkali metals are very interesting. The salts give out the lines of the alkali; for example LiCI give out the red lithium line and sodium salts the D line. It is remarkable the lines due to the metal are more easily excited in the salts the than in the metal themselves. Thus if the liquid alloy of sodium and potassium is bombarded by positive rays the specks of oxide on the surface glow brightly with the sodium light while the clean surface remains quite dark. Some observers have noticed what seems a similar effect with hydrogen, viz. that the hydrogen lines are more easily excited in water vapour than in pure hydrogen. The fact that in the positive ray photographs, the parabolas corresponding to a certain type of ray, for example the carbon or oxygen atom with two charges, is more easily developed from compounds than from the molecules of the gases themselves is probably connected with this effect
The production of spectra by bombardment with cathode rays has been investigated by Gyllensköld ("Ark. f. Math. Ast. oet. Fys.," 4, No. 33, 1908), and by Stark and Wendt ("Ann. der Phys.," 88, p. 669, 1912) who have shown that the colourless salts of the alkalies and alkaline earths and also of thallium, zinc, and aluminium give out the series lines of the metal when struck by the positive rays and that the lines given out do not depend upon the character of the salts. According to Stark and Wendt the seat of the emission is not the surface of the salt itself but a layer of gas, less than 1 mm. thick, close to the surface. This layer is analogous to the velvety glow which covers the surface of the cathode where an electric discharge passes through a gas at a low pressure.
To develop the spectrum of the metal the positive rays must have more than a certain critical amount of energy depending on the nature of the salt. The values of V, this critical energy, measured by the number of volts through which the atomic charge must fall to acquire it, have been recombinations measured by Stark and Wendt and are given in the following table :—
Metal Salt Light given out V Lithium chloride red 600 " oxide red 600 " " λ 6710 <800 Sodium chloride yellow light 750 Potassium chloride and oxide λ 580 <2400 Rubidium sub-oxide λ 572 <3500 Caesium chloride λ 566 <4500 Magnesium chloride λ 518 <1200 Calcium fluoride red violet light 150 carbonate " 1500 sulphate " 1500 oxide " 1400 Strontium chloride λ 496 <2500 Barium " λ 554-493 <2500 Thallium sulphate λ 535 4500 Aluminium oxide λ 396 <4500 Zinc " λ 475 <4600
It must not be supposed that the amounts of energy given in the last column represent the minimum amount required to excite the particular kind of light given in the third column. When energy has to be transferred from a charged atom to a corpuscle, the latter only receives a very small fraction of the energy of the atom, thus a very small fraction of the energy of the positive rays may be transformed into a kind available for light production.
Gyllensköld observed that in addition to the D lines sodium chloride gives out a series of bands in the blue and Stark and Wendt have shown that for this to occur the energy of the rays must exceed a critical value which in most cases is less than that required to excite the line spectrum.