Persuasion at a Distance. (Book Reviews: Shaping Written Knowledge. The Genre and Activity of the Experimental Article in Science.)

Stereoscopic photographs of beta-ray tracks excited by strongly filtered x-rays in moist air have been taken by the Wilson cloud expansion method. In accord with earlier observations by Wilson and Bothe, two distinct types of tracks are found, a longer and a shorter type, which we call P and R tracks, respectively. Using x-rays varying in effective wave-length from about 0.7 to 0.13 A, the ratio of the observed number of R to that of P tracks varies with decreasing wave-length from 0.10 to 72, while the ratio of the x-ray energy dissipated by scattering to that absorbed (photo-electrically) varies from 0.27 to 32. This correspondence indicates that about 1 R track is produced for every quantum of scattered x-radiation, assuming one P track is produced by each quantum of absorbed x-radiation. The ranges of the observed R tracks increase roughly as the 4th power of the frequency, the maximum length for 0.13 A being 2.4 cm at atmospheric pressure. About half of the tracks, however, had less than 0.2 of the maximum range. As to angular distribution, of 40 R tracks produced by very hard x-rays (111 kv), 13 were ejected at between 0 and 30o with the incident beam, 16 at between 30o and 60o 11 at between 60o and 90o and none at a greater angle than 90o The R electrons ejected at small angles were on the average of much greater range than those ejected at larger angles. These results agree closely in every detail with the theoretical predictions made by Compton and Hubbard, and the fact that in comparing observed and calculated values, no arbitrary constant is assumed, makes this evidence particularly strong that the assumptions of the theory are correct, and that whenever a quantum of x-radiation is scattered, an R electron is ejected which possesses a momentum which is the vector difference b e t w e e n that of the incident and that of the scattered x-ray quantum. I N recently published papers, C.T. R.Wilson1 and W. Bothe2 have shown the existence of a new type of P-ray excited by hard x-rays. The range of these new rays is much shorter than that of those which have been identified with photo-electrons, Moreover, they are found to move in the direction of the primary x-ray beam, whereas the photo-electrons 5 move nearly at right angles to this beam. 3 Wilson, and later Bothe,4 have both ascribed these new p-rays to electrons which recoil from scattered x-ray quanta in accordance with the predictions of the quantum theory 1. C. T. R. Wilson, Proc. Roy. Soc. A 104, 1 (1923) 2. W. Bothe, Zeits. f. Phys. 16, 319 (1923) 3. See, e.g., F. W. Bubb, Phys. Rev. 23, 1.37 (1924) 4. W. Bothe, Zeits. f. Phys. 20, 237 (1923)