September 2, 1922] 



NA TURE 



3i3 



eluded with absolute certainty that this line is due 

 to the predominant constituent of the element. 

 Refined measurements indicate a value rather less 

 than this integer. The mean of some sixteen in- 

 dependent and very consistent comparisons with 

 lines due to mercury and compounds of carbon and 

 oxygen works out at 55-94 + 0-05. 



The accepted chemical mean weight, 55-84, suggests 

 the presence of a lighter isotope, and a general con- 

 sideration of elements already analysed points to the 

 value 54 as the most probable. A very faint line is, 

 indeed, visible in all cases where the 56 line is really 

 strong, but it is impossible to make certain that it 

 is due to iron. Further, if we accept the difference 

 of the above figures as exact and assume 54 to be the 

 only lighter constituent, this line should have about 

 one-twentieth the intensity of the 56 line ; actually 

 its intensity appears much less. 



Iron may therefore be taken as being almost, if 

 not entirely, a simple element of atomic weight, 

 approximately 56. It mav contain a small pro- 

 portion of an isotope 54, but this is by no means 

 certain. F. W. Aston. 



Cavendish Laboratory, Cambridge, 

 August 26. 



Density of Adsorbed Films. 



When a very small quantity of such a liquid as 

 castor oil is placed upon a clean surface of water 

 dusted with talc, it spreads over the water surface, 

 brushing the talc on one side, and forms an invisible 

 circular spot, the size of which depends upon the 

 amount of oil used. This adsorbed film has the 

 same surface tension as the surrounding water surface, 

 namely, 73 dynes per. cm. Hence Devaux, Lang- 

 muir, and others regard the film as being only one 

 molecule thick. 



If such an invisible film of castor oil be contracted, 

 the surface tension decreases until it falls to about 

 57-2 dynes per sq. cm. Further contraction does 

 not much reduce the tension. Marcelin was of 

 opinion that as the diameter of the film was decreased, 

 it thickened until it was two molecules thick, and 

 that when this occurred, the film had the same 

 surface tension as castor oil in bulk. But the amount 

 of contraction the surface undergoes in the case of 

 castor oil is only about 40 per cent., and this Devaux 

 points out is not sufficient to make the film two 

 molecules thick, and he suggests that these thicker 

 films are not two molecules thick, but are mono- 

 molecular films with closer packing of the molecules. 

 On this view the film of limited area surrounded by 

 water is stretched until its tension reaches that of 

 a clean surface of water. 



If the above explanation be correct, it is clear that 

 the density of the stretched film of oil having a surface 

 tension of 73 dynes per cm. must be considerably 

 less than that of the same film when its surface tension 

 is only 57-2 dynes per cm. 



Now when calculating the dimensions of the mole- 

 cules of various substances, by the surface tension 

 method, I gather that Devaux and Langmuir regard 

 the oil films, when they are stretched by the sur- 

 rounding water surface, as having the same density 

 as the liquid in bulk. Would it not be safer to 

 assume that the density of the film, when it has 

 the same surface tension as the oil in bulk, is more 

 nearly equal to that of the oil in bulk ? The point 

 is one of considerable interest and importance, and 

 well worthy of consideration. 



R. M. Deeley. 



Tintagil, Kew Gardens Road, 

 Kew, Surrey. 



NO. 2757, VOL. I IO] 



The Pigeon Tick. 



We wish to record the occurrence of the pigeon 

 tick, Argas reflexus, in Cambridge, where it is parasitic 

 on the pigeons which breed between the inner and 

 outer roofs of King's College Chapel. On August 

 4 we found a specimen, which Mr. C. Warburton was 

 kind enough to identify for us. This specimen is now 

 in the collection of the Molteno Institute of Parasit- 

 ology. On a later date we found six more individuals. 



The parasite seems to have been found previously 

 in the British Isles in Canterbury Cathedral only, and 

 not at all since 1908. This new locality, therefore, 

 may indicate that it will be found elsewhere if searched 

 for carefully. 



The adult tick is not permanently attached to its 

 host, but hides in crevices in masonry and woodwork, 

 leaving its hiding-places to feed at night. 



L. H. Matthews. 

 A. D. Hobson. 



Zoological Laboratory, Cambridge. 



An Ancient Wasp. 



I have just received from Mr. John P. Byram a 

 small collection of fossil insects which he obtained at 

 the head of Bear Gulch, 12 miles from Una, Colorado. 

 The formation is Green River Eocene, and Mr. Byram 

 states that the material comes from a lower stratum 

 than the insects previously obtained by us. One of 

 the specimens is a beautifully preserved wasp, with 

 wings outspread, belonging to the modern genus 

 Hoplisus. It is 12 mm. long, with a wing-spread of 

 about 19 mm. ; the head and thorax are black ; ab- 

 domen fusiform with narrow base, and the hind 

 margins of the segments broadly pale-banded, as in 

 living species ; the legs are colourless, probably 

 yellow originally ; the anterior wing shows a pallid 

 stigma, and a strong dusky cloud including the basal 

 part of the marginal cell and the whole of the second 

 submarginal ; the venation is essentially that of the 

 modern Hoplisus quadrifasciatus, except that the 

 marginal cell is more slender, and in the hind wing 

 the cubitus practically meets the nervulus. 



The only fossorial wasp from the Eocene previously 

 described is Scudder's Didineis solidescens, winch is 

 evidentlv quite different from the present species, 

 but is too poorly preserved for the accurate deter- 

 mination of the genus. No older wasps are known. 



This Eocene Hoplisus, which may be called Hoplisus 

 archoryctes, doubtless preyed on the Homoptera, 

 which are so numerous in the same rocks. It is, I 

 think, the most impressive instance of the persistence 

 of type which I have ever seen, when we consider 

 that it belongs to a highly specialised group of insects, 

 and proves that within this group there has, at least 

 in one line, been no change of form or colour in the 

 many millions of years which we now believe to have 

 elapsed since the Eocene Even the cloud on the 

 wings is as in living species. Could the species be 

 restored to life, H. archoryctes would fall into our 

 system, merely forming another species to be added 

 to the many similar ones existing. 



T. D. A. COCKERELL. 



Universitv of Colorado, Boulder, Aug. 1. 



Black Coral. 



Prof. Hickson's very interesting article on the 

 therapeutics of Black Coral (Nature, August 12, 

 p. 217) is sure to stimulate further study, and it is 

 therefore worth while remembering that in alchemical 

 terminology corals are sometimes used as a pseudonym! 

 for antimony. M. Xierexstein. 



University of Bristol, August 13. 



K 2 



