454 



NATURE 



[August 30, 1906 



but the experiments were not yet complete. Prof. S. P. 

 Thompson, in reference to the Cavendish experiments, 

 pointed out that it was well known that gases were 

 absorbed by the walls of vacuum tubes. The Rev. A. L. 

 Cortie, speaking from the astronomical standpoint, was 

 able to declare that radium had not been detected in extra- 

 terrestrial bodies, although helium, which is produced 

 during its decomposition, is discovered in the sun. The 

 idea of a primitive substance is very ancient; it is simply 

 the materia prima of Aristotle. A considerable part of 

 the discussion turned on the use (or misuse) of the term 

 atom, a term which Prof. Tilden, speaking as a chemist, 

 was unwilling to give up. Undoubtedly the term has lost 

 its original etymological signification, but its use has 

 become too fixed to expect a change to be readily made. 

 The discussion proved so interesting and stimulating that 

 Mr. Soddy's paper, which gave rise to it, has been directed 

 to be published in full in the report. A more immediate 

 consequence was that the programme for the day was 

 completely upset ; a large number of papers had to be 

 held over in order that the next discussion arranged might 

 be taken. The subject was the notation and use of 

 vectors, and Prof. Olaus Henrici opened it. He ex- 

 plained the various notations which have been proposed 

 for vector and scalar products, and proceeded to give 

 examples of their use. He showed how the operator V 

 might be defined without reference to analytical geometry 

 from the relation dU = dp. aV , where U— ^any scalar func- 

 tion of position, and dp ; = length of displacement of the 

 representative point. He then applied the properties of 

 the operator v to the deduction, with great simplicity and 

 elegance, of results connected with the theory of partial 

 differential equations. Dr. C. G. Knott followed, and 

 deplored the substitution of vectors for quaternions, and 

 objected that neither scalar nor vector product was really 

 a true product. He advocated a return to the methods of 

 Sir W. Hamilton. He pointed out that Hamilton does 

 not speak of a vector or a scalar product, but of the vector 

 of a product and the scalar of a product. With regard to 

 the change of the usual negative to the positive sign sug- 

 gested by certain vectorists, he explained that it had com- 

 pelled Gibbs to introduce a third kind of product, and more 

 recently Jahncke had introduced a third in order to be 

 able to treat of strains. Prof. W. M. Hicks criticised 

 Henrici 's use of brackets to denote vector and scalar pro- 

 ducts on account of liability to confusion. Prof. Henrici, in 

 an eloquent reply, showed how easily all quaternionic 

 results could be derived from vector analysis. 



On IVfonday, August 6, an important discussion took place 

 on radio-activity and the internal structure of the earth, 

 opened by the Hon. R. J. Strutt. From the examin- 

 ation of a large number of rocks, both igneous and 

 sedimentary, he had come to the conclusion that there is 

 much more radium in all of them than would be needed 

 to maintain the earth's internal heat if the earth were 

 constituted of rock throughout. Hence he concludes that 

 the interior of the globe does not contain radium, and 

 that in all probability its composition is quite different in 

 other respects also from that of surface materials. The 

 thickness of the radio-active crust is estimated at forty-five 

 miles at most, which corresponds to an estimated tempera- 

 ture of 1500° C. at its interior surface. The inside nucleus 

 would be at this temperature throughout just as a loaf of 

 bread which has been in an oven long enough takes up 

 a steady temperature equal to that of the oven. In reply 

 to the possible objection that a gram of radium diffused 

 through an enormous volume of rock may not develop 

 nearly so much heat as it would do if concentrated, it was 

 argued (i) that the rate of emission of alpha particles of 

 pitchblende (to which particles the heat is mainly due) is 

 exactly what might be expected on the view that the 

 radium atoms contained in the mineral are as energetic as 

 they would be if they were all collected together, and (2) 

 direct measurements made by Pegram on uranium and 

 thorium have shown that these feeblv active elements give 

 about the amount of heat which their activity would 

 lead one to expect. Prof. J. Milne, who followed, 

 directed renewed attention to the bearing on the problem 

 of the three phases of earthquake tremors. The first, for 

 stations connected by small chords, travels at a slow, 



NO. 1922, VOL. 74] 



nearly constant rate, but for chords penetrating to a depth 

 greater than twenty miles the velocity increases to about 

 12 kilometres per second, indicating that the wave is 

 carried by something more rigid than the outer crust. 

 Prof. J. W. Gregory, speaking as a representative of the 

 Geological Section, considered that Strutt had struck a 

 blow at the theory of contraction by cooling. We are no 

 longer bound to believe in very high temperatures in the 

 past history of the earth. Arrhenius's theory may now be 

 dismissed. He suggested the importance of mapping a 

 small area completely in regard to the radio-activity of 

 the rocks comprised in it. Sir W. Crookes declared his 

 belief that radium inside the earth may not be so radio- 

 active as at the surface. Pitchblende in thick masses 

 behaves much the same as in thin layers. An experiment 

 in which 50 mg. of radium were sealed in a glass tube 

 and deposited in a cavity in ice, and an exactly similar 

 tube containing 50 mg. of silica was similarly deposited, 

 showed that neither sank as much as one-thousandth of 

 an inch during prolonged observation. Sir G. Darwin 

 directed attention to the work of Gilbeck, Putnam, jtnd 

 Hayford, of the United States Coast Survey, who had 

 fixed a limit of about seventy miles to the thickness of 

 the crust. Sir Wm. Ramsay suggested that Mr. Strutt 

 should make a special examination of sulphides with the 

 object of finding whether they contained radium. He 

 further queried whether alpha particles give out all their 

 energy as heat — a query which must most probably be 

 answered in the negative. 



Mr. R. D. Oldham (also representing the Geological 

 Section) gave distinct evidence, derived from earthquake 

 phenomena, that there must be a central core, the radius 

 of which is about 0-4 of the earth's radius, having rather 

 less resistance to compression than the main body. Prof. 

 H. Lamb threw out a warning against laying too much 

 stress on arguments based on observation of earthquake 

 velocities. Too little is known as to effects of pressure and 

 temperature. 



Mr. Soddy showed that another explanation of the 

 apparent absence of radium heat might arise from pro- 

 cesses of upbuilding going on which may depend upon a 

 possible concentration factor. Prof. Hicks emphasised 

 Mr. Soddy's suggestion, and pointed out that even 

 cooling might be produced by such huilding-up processes. 

 He also suggested that the reason temperature does not 

 usually affect radio-active changes is that time comes in 

 as a factor, and he would like to see experimentally 

 whether a very long application of a low temperature 

 would not produce some effect. Mr. Fearnsides indicated 

 that in the most radio-active rocks elements of high 

 atomic weight were associated with those of low atomic 

 weight. 



The last organised discussion was held on Tuesday, 

 August 7, the subject being the nature of the radiation 

 from gas mantles. Unfortunately Mr. Swinburne, who 

 was to open it, was unavoidably absent ; his paper was 

 therefore read by the recorder. It consisted of a spirited 

 outline of the various theories that had been proposed to 

 account for the high luminous efficiency of the Welsbach 

 burner, with a declaration in favour of the simple tempera- 

 ture explanation. Low emissivity allows the mantle to 

 approach the temperature of the flame ; a substance of 

 greater emissivity could not rise so high in temperature, 

 and consequently the radiation which the latter would 

 give out would not be so rich in luminous qualities. 

 *' Though this simple explanation may be ample it does 

 not follow that there may not be all sorts of curious 

 things, such as selective emission, luminescence, catalytic 

 action, resonance, unstable oxidation and other occurrences 

 whose names are as impressive as vague." Dr. H. 

 Rubens, of Charlottenburg, followed with an account of 

 the experiments which he has recently conducted, and 

 which have been described in Drude's Annalen. Ceria for 

 radiations in the immediate infra-red is a very poor 

 radiator, while for luminous and the extreme Infra-red 

 radiations it behaves much more nearly as a perfectly 

 black body. On the whole, the thoria-ceria mantle h.-is 

 poor emissivity, and its temperature approaches 1600° C, 

 while the nature of the radiations from the added ceria 

 confers additional richness on the proportion of luminous 



