July 14, 192 1] 



NATURE 



^Zl 



at crop level if the distance between adjacent wires 

 does not exceed the height of the wires above the 

 crop. It has to be remembered, however, that when 

 there is an appreciable excess of ions of one sign in 

 the atmosphere the values of the electric forces will 

 be affected. 



It is well known that Hooke's law of propor- 

 tionalit}'^ of force applied and deformation produced 

 holds for solids only so long as the deformation is 

 not large. The same may be said with regard to the 

 corresponding law for the deformation of viscous 

 liquids. In order to discover some more satisfactory 

 form of relation between deformation and force in 

 either case, Dr. P. G. Nutting has made observations 

 of the shear of various materials between parallel 

 plates 5 cm. by lo cm. in area and 02 cm. apart ; 

 his results are given in the May issue of the Journal 

 of the Franklin Institute. He finds that in all 

 cases the deformation at a given temperature 

 is proportional to a power of the force which 

 varies for different materials from 074 to 35. 

 Further, it is proportional to a power of the 

 time of application of the force, which varies for 

 different materials from 02 to 0-91, the low value 

 being characteristic of solids and the high one of 

 liquids. Dr. Nutting finds that the new law is applic- 

 able in other than mechanical fields. In a dielectric, 

 for example, the electrical displacement is propor- 

 tional to a power of the applied electric field, which 

 varies from 054 for paper to 116 for xylene, and 

 also to a power of the time of application of the field, 

 which varies from 074 for bakelite to —0-2 for mica. 

 For the best technical insulating materials the power 

 of the force is nearly 10 and the power of the time 

 nearly zero. 



About six months ago Liippo-Cramer published 

 his discovery that phenosafranine has the remarkable 

 property of desensitising photographic plates without 

 interfering with the developable image that has been 

 impressed on them, as in the course of ordinary 

 exposure. We have already referred to this and to 

 the solution that Messrs. Ilford have put upon the 

 market that enables the most sensitive plates to be 

 developed with no more precaution as to the safety 

 of the light than would be necessary if the plates 

 were one two-hundredth, more or less, as sensitive as 

 they are. In the British Journal of Photography for 

 June 17 and 24 Messrs. A. and L. Lumiere and A. 

 Seyewetz give details of experiments they have made 

 on this subject. They have examined the desensitis- 

 ing action of a large number of other safranines, and 

 find that while several are comparable in this matter 

 to phenosafranine, none show any appreciable advan- 

 tage to it, except that cresosafranine is more easy to 

 wash out of a gelatine film. Many other organic 

 bodies show a notable, and even useful, degree of 

 desensitising effect, but for general purposes pheno- 

 safranine is superior to them all. There appears to be 

 no well-defined relation between the constitution of 

 dyes and their desensitising properties. Phenosafranine 

 does not act merely as a light-filter, for it transmits 



NO. 2698, VOL. 107] 



red and violet, for both of which it desensitises. But 

 if the plate is washed after treatment with the de- 

 sensitiser, as the dye disappears the original sensi- 

 tiveness is restored. It is therefore assumed that 

 the dye forms an adsorption complex of much lower 

 sensitiveness than the original silver bromide, and 

 that this complex is unstable enough to be gradually 

 decomposed by water. The authors have also 

 examined plates treated with various typical desensi- 

 tisers by exposing them in a spectrograph and esti- 

 mating the loss of sensitiveness to light of different 

 wave-lengths. 



The summer meeting of the Association of Science 

 Teachers was held at Cambridge on July 9. In the 

 afternoon Dr. Aston gave a lecture at the Cavendish 

 Laboratory on "Atoms and Isotopes." Early ideas of 

 the structure of matter, leading up to the formulatior> 

 by Dalton of the atomic theorv, were reviewed, and 

 it was shown that the progress made in chemistry 

 during the nineteenth century, which depended on the 

 exact work done in the determination of atomic 

 weights, had been inspired by Dalton 's postulates. In 

 order to explain fractional atomic weights, Crookes- 

 had suggested that an element might be a mixture of 

 atoms of varying weight, but this was regarded as- 

 unlikely until in 1910 Sir Ernest Rutherford's work 

 on radio-activity showed that various forms of lead 

 obtained by radio-active changes had slightly different 

 atomic weights, though their chemical properties were 

 identical. To these substances Prof. Soddv gave the 

 name of "isotopes." The method of positive-ray 

 analysis due to Sir J. J. Thomson was then utilised. 

 By this means it was found that neon — atomic weight 

 20-2 — was probably a mixture of two isotopes of 

 atomic weights 20 and 22, and after much labour a 

 gas was obtained differing in density by 07 per cent, 

 from the original, the experimental error being 02 

 per cent. This was not conclusive, but more exact 

 methods of positive-ray analysis have shown that neon 

 is made up of two constituents of atomic weight 20 

 and 22 in the ratio of about 9 to i. Similarly, chlorine 

 has been shown to consist of at least two isotopes of 

 weights 35 and 37, and quite recently they have been 

 separated. The work done shows clearly that the 

 important property of an element is the atomic 

 number or the positive charge on the nucleus of the 

 atom, and it is this alone which determines the 

 chemical properties of the element. 



The Journal of the British Science Guild for June 

 contains an article by Sir Richard A. S. Redmayne 

 on the world-position in relation to coal. Great 

 Britain has been unfortunate in her recent experi- 

 ences. Prior to the war she exported about 73,000,000 

 tons of coal plus 21,500,000 tons shipped as bunker 

 coal, making 94,500,000 tons, or 32 per cent, of her total 

 output. But in 19 19 this total was only 473 million 

 tons, 206 per cent, of the production. In the present 

 year the figures will doubtless be still mcwe unsatis- 

 factory. Other countries have also produced less 

 coal. The entry of China as a competitor in the coal 

 markets of the West is significant. Oil, it is stated, 



