118 



KNOWLEDGE 



April. 1915. 



A M \\ Ml I HOD OF COUNTING TI1K MOLECULES 

 OF A GAS In ihe Astrophysicai Journal for December. 

 h»l4, Fowle describes a new method i>>i determining the 

 number of molecules v in a cubic centimetre of a gas at 

 norma] temperature and pressure. The method consists in 

 determining the transmissibility oi light of various wave- 

 lengths through the dry air vertically above Mount Wilson, 

 and computing the results by a formula due to Rayleigh, 

 This formula gives the coefficient of transmission in terms of 

 the refraction and oi the number of the particles in unit 

 volume Rayleigh was the first to regard the molecules of 

 .in .!•- capable ol acting as scattering particles, and considers 

 that, while as seen from the Earth's surface, much of the 

 light from the sky is tine to comparatively gross suspended 

 matter, yet an appreciable proportion is attributable to the 

 ulesol the air themselves, and that at high elevations 

 where the blue is purer the latter part may become pre- 

 dominant Fowle's result is w„ = (2-70± -02) x 10 19 , which 

 is in close ai < ord with the best determinations of this con- 

 stant by other methods. 



FRICTIONAL ELECTRICITY.— Very little is known 

 about electrification by friction, so that a paper on this 

 subject by Morris Jones is very welcome (Phil. Mag., 

 February, 1915). Measurements were made of the charge 

 produced on a surface by friction and of the frictional 

 work spent in generating it. The specimens were insulated, 

 and pressed against a rotating wheel covered with various 

 materials. The charge generated was measured by a 

 quadrant electrometer, and the work done could also be 

 determined. It was found necessary to give the insulators 

 a rest alter each rub, lor, if an insulator was rubbed a number 

 of times in succession, the deflexions rose to the maximum, 

 though the work done was the same for each rub, and less 

 than that required to give the maximum for a first rub. 

 When the normal pressure between the specimen and the 

 rubber was maintained constant, it was found that for each 

 pair of materials the charge reached a maximum as the quan- 

 tity of work was increased. The sign of the charge was plus 

 on quartz, glass, fluor-spar, Iceland spar, and heavy spar 

 when rubbed on flannel, silk, and chamois leather, but 

 negative on ebonite, amber, and sealing-wax when rubbed 

 on the same materials. When specimens of various metals 

 were rubbed against silk, the results were more complicated, 

 the charge varying both in magnitude and sign with the 

 state of the surface ; the maximum charge proved to be 

 independent of the pressure and velocity. A tentative 

 mathematical theory of the effects is given based on the 

 assumption that the friction removes electrons from either 

 the rubber or the specimen at a speed proportional to the 

 rate of working. 



QUARTZ GLASS. — This material is surpassed only by 

 invar in respect to its invariability of size with temperature. 

 It was discovered by Professor C. V. Boys, who used threads 

 of this material to support the deflected masses in his 

 classical research on the Newtonian constant of gravitation. 

 Boys showed how to prepare these threads by the bow and 

 arrow and other methods, and also prepared small pieces 

 of rods and tubes before the oxy-hydrogen blowpipe. 

 When once quartz has been fused it is very easy to work, 

 as it does not crack with sudden change of temperature. 

 This is partly owing to its low coefficient of expansion, but 

 also probably to its great mechanical strength. Shenstone 

 still further unproved the methods of making fused silica, 

 and the material is now a commercial article, being obtain- 

 able in two grades : one perfectly clear and colourless, the 

 other being silky and translucent in appearance. The 

 methods of manufacture have been so far perfected that 

 complicated apparatus of quartz glass can now be easily 

 made, and a preliminary difficulty as regards sealing the 

 leading in wires for electrical purposes has been overcome 

 completely. This wonderful and beautiful product has 

 already been applied to some commercial purposes, such as 

 for making electric lamps, and the extension of its uses 

 to various other fields is certain. 



RADIO-ACTIVITY. 

 By Alexander Elf.ck, B.Sc. 



ANNUAL REPORT— The report issued annually by 

 tin Chemical Society always devotes a section to the 

 progress made in radio-activity during the past year. 

 Most of the questions raised in the section of the report 

 which has ]ust been issued have been mentioned from time 

 to time in these columns The three following subjects, 

 dealt with more fully in the report are, however, of some 

 importance, and may briefly be referred to now. 



NEW WORK ON THE DISINTEGRATION SERIES. 

 — Fajans and Towara have announced, in the Natiowissen- 

 schaflen (1914, page 685), that they have discovered a 

 radio-active substance which they have not been able to 

 separate from bismuth, and which gives a-rays. The activity 

 of the bismuth which has thus been obtained is five times 

 as active as an equal quantity of uranium ; and, deducing 

 its period from the range of its a-rays, the period of average 

 life of this " radium-H " is between one hundred thousand 

 and one million years. 



THORIUM ACTIVE DEPOSIT.— It has been generally 

 accepted that when a radio-active element underwent dual 

 disintegration we had a homogeneous collection of atoms 

 which had the faculty of choosing in which of two ways 

 they could disintegrate. If the experiments of Wood 

 (Proceedings Physical Society, Volume XXVI, page 248) 

 are confirmed, a large modification either of our views on 

 dual disintegration or on the evolution of the radio-elements 

 through the Periodic Table will be required. The 

 experiments in question dealt with the volatilisation of 

 thorium active deposit, and seemed to show that the 

 substance hitherto referred to as thorium-C is composed 

 of two elements, one of which commences to volatilise about 

 200° C. below the other. 



ATOMIC- WEIGHT DETERMINATIONS OF RADIUM 

 AND URANIUM. — Despite the most careful atomic-weight 

 determinations, there is still a greater discrepancy between 

 the values obtained for these elements than there should 

 be if the three o - particles emitted between uranium-I 

 and radium had the same atomic weight as helium derived 

 from the atmosphere, namely, 3-99. The writer of the 

 report — Professor Soddy — therefore suggests that it is 

 an open question whether the law of the conservation of 

 mass accurately holds in radio-active change. To the 

 present writer it seems that a more probable explanation 

 would be that radio-active helium is an " isotope " of 

 atmospheric helium, but of a slightly higher atomic weight, 

 just in the same way as Ashton's meta-neon had an atomic 

 weight of 22 compared to that of 20 for atmospheric neon. 



ACQUIRED RADIO-ACTIVITY.— Sir W. Crookes has 

 recently published a paper (Philosophical Transactions of 

 the Royal Society, Volume CCXIV, page 433) describing the 

 behaviour of diamonds when buried in a radiferous salt or 

 exposed to radium emanation. As is to be expected, the 

 gems, just like any other material similarly placed, are coated 

 with the active deposit, and so possess the " acquired 

 activity." The author, however, finds this difference, that 

 the material producing the activity so obtained can easily 

 be dissolved from glass ; while, when it is obtained on dia 

 mond, it can withstand the most vigorous chemical agents. 

 The activity is, so far as can be ascertained, constant over 

 a period of years. 



In the paper referred to Sir W. Crookes does not attempt 

 to explain the phenomena observed, although it would be 

 easy to suggest a number of explanations capable of expert 

 mental verification. 



