121 



the vacuum glass tliat was tilled witli dry air ^) and in which such 

 a filter was placed again at (he entrance of the nariow part. This 

 oxygen now gave the same ring. On the other hand a piiolograph 

 of ice (see § 2) surrounded by liquid air taught ns that noneoftlie 

 interference lines of ice coincide with the I'ing of oxjgen. 



The diffraction image of water shows still an interesting detail 

 (see fig. 3 of the plate). Immediately following on the intense diffrac- 

 tion ring the lilm shows a very considerable almost uniform blackening 

 with a rather sharp outline at (f = 46°. 



For some other liquids too we found weak indications of a similar 

 blackening. 



For oxygen and argon the best films show beside the ring given 

 in table I still a weak second ring, for oxygen at '/ = 46°, for 

 argon at <p =: 49°. 



^ 4. The intense difiraction ring is due to the cooperation of 

 neighboiirimj molecides. As was shown by Ehrenfkst*) and at (he 

 same time by Debijk and Scherrf.k (I.e.) a diffraction ring like that 

 of § 3 may be due to the interference of Röntgen-iays diffracted by 

 arbitrarily orientated systems each of two (or more) particles, 

 which have a definite mutual distance (f.i. the two atoms in a 

 diatomic molecule, where each of the atoms is regarded as one 

 single diffracting centre.) Between the angle <p and the distance a 

 of the two diffracting particles we have then (see Ehkenfest I.e.) 

 the following relation 



7,72 A 

 0'=- (I) 



A ■ 'f 



4 jr sin — 

 2 



where 7. is the wavelength of the Ron tgen rays. 



o 



The values of a calculated in this way (with / = 1,54 A) are 

 given in table I. 



In the first place the fact, that also argon has a similar diffrac- 

 tion ring, involves that, at least for argon, this diffraction ring is 

 not due to the cooperation of atoms in the molecule.*) 



That this is neither the case for oxygen is to be expected by the 



') By a small window v in the vacuumglass we could state tlial the liquid was 

 perfectly clear. 



') P. Ehrenfest. These proceedings Vol. XVII, p. 1184. See also P. Debije 

 Ann. d. Phys. (4) 46, p. 809, 1915 



*) Unless argon should be more-atomic in the liquid state, which is not made 

 probable by the following. 



