October 24, 19 12] 



NATURE 



219 



with such small quantities are obtainable, but they 

 are costly, and it would be out of the question to 

 employ them in the numbers and in the manner 

 required, for instance, for opsonic determinations. 

 As the author says, " it is a technique for con- 

 ducting quantitative tests in uncalibrated capillary 

 tubes with minimal quantities of reagents." 



Briefly, the method consists in the use of glass 

 pipettes, formed by drawing out a piece of glass 

 tubing in the blow-pipe flame into a fine stem. 

 By adapting a suction apparatus in the form of 

 a rubber teat to the undrawn-out portion, and 

 making a mark somewhere on the drawn-out 

 stem, we have the means of taking up any number 

 of minute similar volumes of a fluid and of making 

 any mixtures of fluids and dilutions thereof 

 required with considerable accuracy. 



13y an adaptation of these principles. Sir A. 

 Wright has devised methods for estimating the 

 bactericidal, agglutinative, and opsonic powers of 

 the blood, for measuring the coagulation time of 

 the blood, and for estimating quantita'-ively its 

 alkalinity, content of magnesium and calcium 

 salts, and anti-tryptic power. The making of 

 blood-films and preparation and standardisation of 

 therapeutic vaccines are also dealt with. 



Full details are given for the manipulation of 

 the glass in the blow-pipe, the making of the 

 apparatus required, and the carrying out of the 

 various procedures. The descriptions are supple- 

 mented by a profusion of illustrations in the text 

 and five plates, four of which are coloured. Truly 

 no bacteriological or pathological laboratory can 

 rifford to omit this book from its working library, 

 and we fanc}' that the chemist and physicist might 

 gather some hints of value from it. 



R. T. Hewlett. 



Lines in the Arc Spectra of Elements. By 

 F. Stanley. Pp. 140. (London : Adam 

 Hilger, Ltd.) Price: cloth, 12s. 6d. ; half- 

 morocco, 155. 6d. 

 In this publication the wave-lengths of the chief 

 lines in the arc spectra of fifty-five elements are 

 given. These are arranged in one long table in 

 the order of the wave-length numbers. The in- 

 tensities of the lines in the spectrum of the un- 

 diluted element are also given on a scale of i to 10, 

 the latter denoting the brightest lines. In a 

 separate column and on the same horizon as any 

 particular line will be found the wave-length of 

 the next prominent line belonging to the corre- 

 sponding element. This is very useful in deter- 

 mining whether any element is present in a sub- 

 stance under investigation. The most persistent 

 lines of any given element — that is, the lines which 

 last longest as the proportion of the element in 

 question is gradually decreased — are specially 

 denoted. The wave-lengths, which extend from 

 A 7900 to \ 2200, are given to the nearest tenth 

 of an Angstrom unit, and have been taken from 

 the most recent and trustworthy measures avail- 

 able. The pages opposite the wave-length tables 

 are left blank for the insertion of notes. To prac- 

 tical workers in elementary spectroscopic analysis 

 the compilation will be decidedly useful, 

 vo. 2243, VOL. 90] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold hitnself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



X-rays and Crystals. 



Messrs. Friedrich, Knipping and Laue have re- 

 cently published {K. Bayer. Akad. der Wiss., 1912, 

 p. 303) some remarkable effects obtained by passing 

 a fine stream of X-rays through a crystal before in- 

 cidence upon a photographic plate. A curious 

 arrangement of spots is found upon the plate, some 

 of them so far removed from the central spot that they 

 must be ascribed to rays which make large angles with 

 the original pencil. 



The positions of these spots seem to depend on 

 simple numerical relations, and on the mode in which 

 the crystal presents itself to the incident stream. I 

 find that when the crystal (zincblende) is placed so 

 that the incident rays are parallel to an edge of the 

 cube in the crystal the positions of the spots are to 

 be found by the following simple rule. The atoms 

 being assumed to be arranged in rectangular fashion, 

 any direction which joins an atom to a neighbour at a 

 distance na from it, where a is the distance from the 

 atom to the nearest neighbours and n is a whole 

 number, is a direction which a deflected (or secondary) 

 pencil will take, and it will in doing so form one of 

 the spots. In other words, we have to seek for all 

 the cases in which the sum of three squares is also a 

 square, and we then recover the positions of all the 

 spots on the diagram. For example, secondary 

 j pencils take the directions (2, 3, 6) (4, i, 8), and so 

 I on. In a few cases the sum of the squares is one 

 I short of a perfect square, e.g. (5, 7, 11), but in no 

 case is it on the greater side ; and there is at least 

 one direction (2, 5, 14) which ought by the rule to 

 i be on the diagram and is not. Otherwise the rule is 



quite successful. 

 I LIntil further experimental results are available, it 

 j is difficult to distinguish between various explanations 

 I which suggest themselves. It is clear, however, that 

 I the diagram is an illustration of the arrangement of 

 i the atoms in the crystal. 



The rule has suggested itself to me as a consequence 

 of an attempt to combine Dr. Laue's theory with a 

 fact which my son pointed out to me, viz. that all 

 the directions of the secondary pencils in this position 

 of the crystal are "avenues" between the crystal 

 atoms. W. H. Bragg. 



Leeds, October iS. 



Glaciation and Striation. 



In your issue of September 26, Dr. A. Irving asks 

 whether in my plate 17 (Phil. Trans., Ser. B., 

 vol. ccii) I have not overlooked the fact that the 

 striations are on the original cortex of the flint nodule. 

 My answer is that I have not done so ; the striations 

 are not on the original cortex of the flint nodule — as is 

 clear both from my description of the specimen and 

 from the carefully drawn figure. 



Dr. Irving also asks whether I have overlooked the 

 probability that the markings shown in Fig. 2 are 

 the etched-out skeletons of some spongoid fossil. My 

 reply is that no such probability exists, and that, in 

 consequence, I have not overlooked it. 



Your correspondent appears to be under some mis- 

 apprehension. He has never seen the specimen referred 



