156 



KNOWLEDGE. 



Al'Rii.. 1911. 



consists of an elongated ring of glass tubing with a branch tube 

 and tap sealed into the smaller sides of the ring ; one of the 

 two longer straight tubes is capillary. A mercury pellet in 

 falling down the tube of larger bore pushes the gas in front of 

 it through the tube of narrow bore. The time of fall is 

 proportional to the viscosity of the gas : the more viscous the 

 less easily can it pass along the capillary bore, and the longer 

 the time of fall of the mercury pellet. The great advantage of 

 the apparatus is that only small quantities of the gas under obser- 

 vation are needed. Dr. Rankine has observed the viscosity at 

 different temperatures, and in this way has obtained measure- 

 ments which enable him to calculate the relative attraction 

 constants, radii and volumes of the molecules of the various 

 gases. The gases of the argon group — helium, neon, krypton, 

 xenon show interesting relationships respecting such constants. 



THE DENSITY OF R.ADIUM EM.W'.ATION.— Just 

 as in the series of compounds of carbon and hydrogen -there 

 is a group of hydro-carbons which are "saturated" and cannot 

 combine with other atoms without replacement of those 

 already combined in the molecule, so in the " Periodic 

 Classification of the Elements " there is a group of elements 

 whose atoms are unable to combine with other atoms, or 

 which have zero \alency. These elements are the rare gases 

 above mentioned. 



There is room for one or two elements more in the group 

 according as to how the natural periodic classification of the 

 elements is interpreted. The atomic weight of the one would 

 be about one hundred and seventy-si.x. of the other about 

 two hundred and twenty-two. Radium emanation or Niton, 

 as Sir William Ramsay suggests should be its name, is the 

 first break-down product of the radium atom, and is a gas 

 behaving in every way as if it should belong to the argon 

 group, and fit into one of these vacant places. From density 

 determinations and observations of the boiling-point and 

 critical point of this highly radio-active gas, it appeared that 

 its density was such that its atomic weight should be one 

 hundred and seventy-six. But since only one a particle or 

 helium atom is expelled during the decay of radium into 

 radium emanation, it was more probable that the density 

 should be one hundred and eleven, or atomic weight, two 

 hundred and twenty-two, unless the disintegration theory 

 which explains the numerous transformations of radio-active 

 substances was in fault. It was very important then to settle 

 the point and determine the density of radium emanation. 



The difficulty of such a determination would seem almost 

 insuperable, for — to put it succinctly — it would require 

 ;f 15,000 worth of radium to get half a cubic millimetre of 

 the emanation. Sir William Ramsay and Dr. R. W. Gray 

 have, however, tackled the problem successfully. They 

 have been able to construct a balance weighing to less than 

 one-hundred-thousandth of a milligram, so as to be able to 

 weigh with sufficient accuracy one-tenth of a cubic millimetre 

 of the emanation. 



It required several years of experimenting to design a balance 

 of such sensitiveness. Dr. Brill, working under Sir William 

 Ramsay, had improved the Nernst balance. This works by 

 the torsion of a quartz fibre, one end of the beam acting as a 

 counterpoise and pointer, but the deflection due to torsion of 

 the fibre is not directly proportional to the weight. Dr. Brill's 

 work suggested the employment of quartz knife edges and very 

 light beams, which idea Dr. Gwyer improved and introduced 

 a system of weighing which Dr. Steele, in Australia, indepen- 

 dently arrived at. Sir William Ramsay and Dr. Gray were 

 able to slightly improve Dr. Steele's balance and modify it for 

 their purpose. A short description of the balance in a few 

 lines is unworthy of such a beautiful instrument. It consists 

 in the main of a fine silica rod beam resting on a specially- 

 ground and very small quartz knife edge ; from the ends of the 

 beam is hung, by means of silica fibres, the substance to be 

 weighed, silica counterpoises and a small siHca bulb containing 

 air. By altering the pressure inside the balance case, a small 

 change in buoyancy of the air in the bulb is caused, so by 

 finding the change in weight produced by a given change in 

 pressure very fine adjustments of weight can be made ; a 

 mirror is attached to the beam of the balance, and the altera- 



tion in pressure necessary to bring back to zero the spot of 

 light reliected from the mirror on to the scale, gives a measure 

 of the alteration in weight. 



The emanation was collected and sealed in a very fine tube 

 which was counterpoised, then broken so as not to lose any 

 particles of glass, the balance case evacuated so as to expel 

 the emanation and replace it with air. then counterpoised 

 again and the change in pressure noted. 



Numerous corrections had to be made: the weight of the 

 air entering the density bulb, after breaking it to let out the 

 emanation ; the change in the buoyancy due to the fact that 

 the density bulb is glass and the counterpoise silica ; the 

 volume of the emanation that penetrates into the glass walls 

 owing to its high activity, were among these corrections. The 

 \()lume of the emanation was determined and the amount of 

 its decrease before the actual determination by its change in 

 activity. 



The density came out to be two hundred and twenty-three 

 as a mean of five experiments. 



The results then confirm the disintegration theory of 

 radioactive transformations which demands that since the 

 atomic weight of radium is 226-5 that of the emanation 

 should be 222-5. Further than this, they demonstrate 

 the manipulative genius of Sir William Ramsay, and the 

 success of these delicate experiments in the face of such huge 

 difficulties reflects great credit on his collaborator. 



MESOTHORIU.M.— .\s an illustration of the disintegration 

 theory, the products of change of thorium might be mentioned. 

 Thorium first breaks down into mesothorium I., which in eight 

 years has half broken down into mesothorium II.. which again 

 is half gone in nine hours, changing into radiothorium and 

 throwing off /3 and 7 rays; radiothorium then throws off an a 

 particle (an electrically-charged helium atom) and becomes 

 thorium X. Thorium X breaks down into thorium emanation, 

 w hich is a gas ha\-ing a half life period of only seventy-seven 

 seconds, and breaks down with loss of an a particle into 

 thorium A which is followed by other products B.C. and D., 

 the final product being as yet unknown. Owing to the fact 

 that considerable quantities of thoria are used in commerce 

 for the manufacture of incandescent gas mantles, it has lately 

 been possible to extract appreciable amounts of mesothorium 

 and radiothorium. 



The monazite sand is treated in some way devised by Dr. 

 Hahu, so as to separate the mesothorium and so on from the 

 thorium, and such preparations have lately been put on the 

 market by the firm of Kncifler & Co.. and will doubtless be 

 used largely for medical purposes. 



Professor Soddy has found that mesothorium and radium 

 are identical in chemical properties — a very remarkable fact. 

 He points out, too, that thorium, ionium, radiothorium form 

 one group of radioactive elements, and mesothorium, radium, 

 thorium X form a second group which are chemically identical 

 and inseparable elements, although their atomic weights differ 

 by two units in each case ; while the last member of the first 

 group and the first member of the last group (radiothorium 

 and mesothorium) possess the same atomic weight and yet 

 are chemicalh- different. 



THE NATIONAL PHYSICAL LABORATORY.— The 

 annual meeting of the General Board at the Laboratory, 

 Bushey House, Teddington, was held on Friday, March 17th, 

 and in the afternoon a number of guests had the privilege of 

 going over the Laboratory. They were received in the 

 building containing the National Experimental Tank by Sir 

 •■Xrchibald Geikie. who is the Chairman of the General Board. 

 .Ml the departments w-ere thrown open and members of the 

 staff were at hand to explain all the w-ork which is carried out. 



THE ROYAL AUTOMOBILE CLUB'S SUGGESTED 

 LABOR.-^TORY. — We learn from the Secretary of the Royal 

 .Automobile Club that the Expert and Technical Committee 

 has been asked to consider the expediency of establishing a 

 central research laboratory for the scientific investigation of 

 motor car problems and to report .-is to the equipment and 

 maintenance to such a laboratory. 



