File:X rays and crystal structure (1915) (14797939523).jpg

Identifier: xrayscrystalstru00braguoft (find matches)
Title: X rays and crystal structure
Year: 1915 (1910s)
Authors: Bragg, William Henry, Sir, 1862-1942 Bragg, William Lawrence, Sir, 1890-1971
Subjects: Crystallography X-rays
Publisher: London : G. Bell
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: MSN

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e length are plotted against the logarithmsof the atomic numbers of the absorbers a secondseries of straight lines are obtained, which are shownin Fig. 59. The slope of these lines shows thatthe absorption varies very nearly with the fourthpower of the atomic number ; but the constant ofthe proportion changes as one of the critical valuesis passed through. Both propositions maybe summedup in the statement that the atomic absorptioncoefficient, viz. Kiv, where w is the actual weight ofthe atom, K being the ordinary mass absorptioncoefficient The value of the constant C is about 1.79 x lo* onone side of the critical point for each wave length, *Proc. Roy. Soc, 1912, p. 426. INTENSITY OF X-RAY REFLECTION 181 and C.235 X 10*^ on the other: the ratio of the twovalues being nearly 8 to i, a relation which hasbeen noticed before (p. 44). The series of absorption coefficients of the platinumrays pass through two critical points : the few valuesobtained by Peirce—values which are of the same
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^0 6 ao4 20 z zoo 21 8 21 6 21 4 212 21 0 Fig. 59. order as have been found by the older methods—seem to show that the constant experiences anotherdrop in the ratio 4 to i. It may be noticed that the critical value of thewave length when palladium is the absorber liesbetween the two wave lengths emitted by silver.The high absorption of the shorter wave could not 182 INTENSITY OF X-RAY REFLECTION be measured accurately with the palladium foil avail-able, as its thickness was too great. Apart from this direct attack upon the absorptionquestion there are certain indirect methods whichgive very interesting information respecting particularfeatures of the problem, and have a direct bearingon the subject of this chapter. We have alreadymentioned (p. 43) the existence of certain criticalpoints in the relation between quality of X-ray andatomic weight of absorbing screen. The existenceof these discontinuities is made manifest in severalstriking ways. It will be remembered that a given atom

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