522 



NATURE 



[June 23, 192 1 



pected if the short sections of chain or beaded par- 

 ticles were oriented or lined up by the magnetic field ; 

 the transverse waves of light vibrating in a plane 

 intersecting the length of the chains would not be de- 

 flected on account of the extremely small diameter of 

 the particles composing them, but waves vibrating in 

 the plane of the length of the chains would be reflected 

 to the side, and this would account for their plane of 

 polarisation being what it is. Such waves would 

 behave as if reflected from short rods in line with the 

 plane of vibration, while the extremely small diameter 

 of the rods would not sufficiently intercept the light 

 vibrating in a plane transverse to their length. 



The continuation of the investigation with artificial 

 light and other varied conditions is anticipated. 



Elihu Thomson. 



Thomson Laboratory of General Etectric Co., 

 Lynn, Mass., Mav 23. 



Geometrical Isomerism in Monomolecular Films. 



In the course of investigations on these films by a 

 method differing only in details from that described 

 by Langmuir (Journ. Amer. Chem. Soc, 1917. 

 p. 1868) I have found striking differences between 

 the properties of films formed from the "cis" and 

 "trans" forms of some fatty acids containing an 

 ethylenic linkage, which indicate that of the two pairs 

 of acids, oleic and elaidic, erucic and brassidic, oleic 

 and erucic are the '"cis" forms and elaidic and 

 brassidic the "trans." The results appear to be con- 

 sistent with Langmuir's conception of the structure 

 of the films, and this stereochemical configuration 

 is that usually regarded as correct from chemical 

 considerations. 



According to the theory, the films are one molecule 

 in thickness. With saturated acids, such as palmitic, 

 the molecules are attracted to the water by the 



J) 



a! — f^ 



O c&vboxv 



.)l.vt«n.C l.r.Wi;. 



Fig. I. 



carboxyl groups, and are arranged as in Fig, ij L Un- 

 saturated acids are also attracted by their ethylenic 

 linkages, and when, as in the acids mentioned, these 

 are approximately in the middle of the chains, the 

 molecules in the film will take up the position in 

 Fig. I, IL or IIL The attraction of the double bond 

 for water is less powerful than that of the carboxyl, 

 and when a lateral compression is applied to the 

 film the area per molecule will diminish by some 

 or all of the molecules straightening out to the 

 position L 



Fig. I shows that a difference is to be expected 

 between the "cis " and "trans" isomers. The double 

 bond in the "cis" form can approach as closely as 

 desired to the water, but in the "trans" form the 

 saturated portion of the chain ABC must be forced in 

 among the water molecules. Although it is known 

 from stereochemical considerations that a hydrocarbon 

 chain is flexible, yet its radius of curvature cannot be 

 reduced below that of a ring of five carbon atoms 

 without encountering resistance ; there will, therefore, 



NO. 2695, VOL. 107] 



probabl)- be a considerably greater resistance to the 

 approach of the double bond to the water in the case 

 of the "trans " form than in the case of the "cis." 



The results obtained point clearly, I think, to a 

 greater tendency to occupy the larger area with oleic 

 than with elaidic acid, and a larger with erucic than 

 with brassidic acid. Oleic acid, when first put on 

 distilled water and a compression of about 14 dynes 

 per cm. applied to the film, occupies about 40x10-'* 

 sq. cm. per molecule; the area decreases steadily with 

 time, however. Elaidic acid occupies about 30 units 

 of area at the earliest moment when readings can be 

 taken, and the area diminishes rapidlv to about 

 22 units, when the film behaves like one of palmitic 

 acid. 



In the 22-carbon series there appears to be a smaller 

 tendency than in the i8-carbon series for the double 

 bond to approach the water. Erucic acid gives films 

 rather similar to elaidic acid, but brassidic acid 

 occupies the greater area for so short a time that the 

 curves of compression of the films are not very 

 different from those of a saturated acid such as 

 palmitic. 



It is hoped to amplify these experiments and publish 

 full details later. N. K. Adam. 



Trinity College, Cambridge, May 28. 



Sources and Sinks. 



LoRO Kelvin in a paper "On the Forces Experi- 

 enced by Solids Immersed in a Moving Liquid " 

 (Proc. Roy. Soc. Edin., 1870) compared two tubes, 

 with liquid flowing through each, with two hard steel 

 magnets and stated that the forces are opposite in 

 the two cases ; unlike poles attracting and like poles 

 repelling in the magnetic system, while in the hydro- 

 kinetic there is attraction between like 

 ends and repulsion between unlike. 



That two sources of like sign attract 

 and two of unlike sign repel, as here 

 stated, is generally accepted. An 

 examination, however, of the case of a 

 source and an equal sink appears to 

 contradict this. When source and sink 

 coincide the fluid medium is at rest, 

 but when they are separated it is in 

 motion and possesses kinetic energy. 

 Work, therefore, must be done to effect 

 the separation. This suggests that the 

 force between source and sink is one of 

 attraction. That this is actually the 

 case is shown by the following experi- 

 ment. 



Two glass tubes A and B (Fig. i) 

 are connected by short lengths of 

 rubber tubing to short tubes, which 

 pass about i cm. apart through a cork in the neck of 

 a Winchester bottle full of water. The tube A is con- 

 nected to a water-supply and its open end constitutes 

 an experimental source. The end ot the tube B is an 

 equal sink. The source and sink attract smartly and 

 the ends of the tubes remain in contact so long as the 

 water flows. A. F. Dufton. 



The Royal School of Mines, South 

 Kensington, May 25. 



Polarisation Phenomena in an X-ray Bulb. 



Hitherto the potential difference required to pro- 

 duce a discharge through a well-exhausted vacuum 

 tube has been considered to vary only with the pres- 

 sure of the gas. In the course, however, of some 

 experiments with an X-ray bulb (where the pressure 



