Decembek 4, 1914] 



SCIENCE 



799 



atom or molecule, or group of atoms or m.ole- 

 cules. In tte case of rocksalt, the indications 

 are that the crystal possesses a structure inter- 

 mediate between the very simple arrangement 

 just described and one in which the smallest 

 element is a cube having a similar group of 

 atoms or molecules at every corner and at the 

 middle point of each face. The arrangement 

 is called by crystallographers the face centered 

 cube. The substitution of the sodiumi for the 

 potassium atom must transform one arrange- 

 ment into the other. This can be done in the 

 following way, if we accept various indica- 

 tions that atoms of equal weight are to be 

 treated as equivalent. Imagine an elementary 

 cube of the crystal pattern to have an atom 

 of chlorine at every corner and in the middle 

 of each face, and an atom of sodium or potas- 

 sium as the case may be, at the middle point 

 of each edge and at the center of the cube. We 

 have now an arrangement which fits the facts 

 exactly. The weights of the potassium and 

 chlorine atoms are so nearly the same as to be 

 practically equivalent, and when they are con- 

 sidered to be so, the arrangement becomes the 

 simple cube of sylvine. But when the lighter 

 sodium replaces the potassium as in rocksalt 

 the arrangement is on its way to be that of 

 the face centered cube, and would actually be- 

 come so were the weight of the sodium atoms 

 negligible in comparison with those of chlo- 

 rine. Of course the same result would follow 

 were two or three, or any number of atoms 

 of each sort to take the place of the single 

 atom, provided the same increase were made 

 in the number of the atoms of both sorts. We 

 might even imagine two sorts of groups of 

 chlorine and metal atoms, one containing a 

 preponderance of the former, the other of the 

 latter, but so that two groups, one of each 

 kind, contain between them the same propor- 

 tion of chlorine and metal as the crystal does. 

 We must merely have two groups which diiier 

 in weight in the case of rocksalt, and are ap- 

 proximately equal in weight in the case of 

 sylvine. But it was best to take the simplest 

 supposition at the outset; and now the evi- 

 dence that the right arrangement has been 

 chosen is growing as fresh crystals are meas- 



ured. For it turns out that in all crystals so 

 far investigated, the number of atoms at each 

 point must always be the same. Why, then, 

 should it be more than one? Or in other 

 words, if atoms are always found in groups 

 of a certain number, ought not that group to 

 be called the atom? 



As soon as the structure of a crystal has 

 been found we can at once find by simple 

 arithmetic the scale on which it is built. For 

 we know from other sources the weight of indi- 

 vidual atoms, and we know the total weight 

 of the atoms in a cubic centimeter of the 

 crystal. In this way we find that the nearest 

 distance between two atoms in rocksalt is 

 2.81 X 10"^ cm., which distance is also the 

 spacing of the planes parallel to a cube face. 

 From a knowledge of this quantity the length 

 of any X-ray wave can be calculated at once 

 as soon as the angle of its reflection by the 

 cube face has been measured. In other words, 

 the spectrometer has now become a means of 

 measuring the length of waves of any X-radia- 

 tion, and the actual spacings of the atoms of 

 any crystal. 



From this point the work branches out in 

 several directions. It will not be possible to 

 give more than one or two illustrations of the 

 progress along each branch. 



Let lis first take up the most interesting and 

 important question of the " characteristic " 

 X-rays. It is known that every substance when 

 bombarded by electrons of sufficiently high 

 velocity emits X-rays of a quality character- 

 istic of the substance. The interest of this 

 comparison lies in the fact that it displays the 

 most fundamental properties of the atom. The 

 rays which each atom emits are characteristic 

 of its very innermost structure. The physical 

 conditions of the atoms of a substance and 

 their chemical associations are largely matters 

 of the exterior : but the X-rays come from the 

 interior of the atoms and give us information 

 of an intimate kind. What we find is marked 

 by all the simplicity we should expect to be 

 associated with something so fundamental. 



All the substances of atomic weight between 

 about 30 and 120 give two strongly defined 

 " lines " ; that is to say, there are found among 



