28 



KNOWLEDGE. 



January, 1915. 



and e was determined by recognising the reflected ray by 

 the conductivity it imparts to a gas. On substituting the 

 values for X and $ in the equation, I is found for the distance 

 between the layers of atoms in the crj-stal measured in 

 definite directions. The results for / thus obtained, together 

 with the density of the cn,-stal and the mass of an atom of 

 copper, pro\'ide all the data from wMch to calculate the 

 details of the anangement of the atoms in the crj-stal. 



The .*-ray spectrometer employed in such investigation 

 was described in this column for December last (see Volume 

 XXXVII, page 436). 



APPLICATION OF INTERFERENCE TO THE 

 STUDY OF THE ORION NEBULA.— In the October 

 number of the Asirophysical Journal Buisson, Fabiy, and 

 Bourget give an account of a most remarkable research 

 in which the etalon is employed in the study of the great 

 nebula of Orion. The method consists in producing, with 

 monochromatic light from the nebula, photographs of the 

 nebula in light of one wave-length ; while the photographs 

 are marked simultaneously by the ring system of an etalon. 

 By studying this system the "exact wave-length of the light 

 employed can be found. The comparison of this with the 

 wave-length of the same line from laboratory experiments 

 enables the motion in the line of sight to be calculated ; 

 slight distortion of the ring system yields data from which 

 the local circulation of the material of the nebula can be 

 deduced, while the number of wave-lengths over which 

 interference takes place gives the temperature. By com- 

 paring these results for a known radiation with unknown 

 ones the atomic weight of the unknown sources can be 

 estimated. 



The apparatus for the production of the photographs was 

 mounted centrally in the opening of the tube of the large 

 Marseilles reflecting telescope, so that the hght proceeds 

 from the objective to the apparatus without undergoing 

 reflection. This disposition has the disadvantage that the 

 total hght is diminished; but by keeping the 6talon and its 

 mounting small, the loss from this cause was only a thirtieth 

 of the whole amount of light. The pictures must be taken 

 in monochromatic light ; now the nebula gives out light 

 giving a bright-line spectrum, so that any radiation could 

 be isolated by means of suitable colour filters. One set of 

 filters aIlo-i\ed only one of the hydrogen lines to pass ; 

 another allowed only a double line in the ultra-violet due to 

 an unknown source to pass ; while visual observations were 

 also made on a green line, likewise of unknown origin. The 

 filters were placed in the telescope tube bet\veen the objective 

 and its focus. The light then passes through a rectilinear 

 achromatic photographic lens of uviol glass whose focus 

 coincides with that of the objective of the telescope. The 

 rays emerge from this lens again paraUel, but angularly 

 magnified ; the light now behaves as if it proceeded from 

 a nebula still infinitely distant, but enlarged angularly 

 eighty- times. The light now goes through the etalon, and 

 then through a second achromatic lens, which is again of 

 uviol glass. This lens forms images of the rings and the 

 nebula on the same plate placed in its focal plane. The 

 picture of the nebula is eighty times as large as if it had 

 been photographed directly by this last lens. 



The chief results of the research are that the nebula is 

 receding from the Sun at a velocity of 15-S kilometres a 

 second, while irregularities in the velocity at various parts 

 may amount to ten Idlometres a second. The ultra-violet 

 double hue is due to an element, " nebulium," of atomic 

 weight about 3 ; the green line which was studied 

 visually is due to another unknown element whose atomic 

 weight is between 1 and 3. The temperature of the 

 source does not exceed 15000° C. 



RADIO-ACTIVITY. 



By Alexander Fleck, B.Sc. 



CHEMICAL ACTION DUE TO RADIO-ACTIVE 



RADIATIONS.— It is a well-known fact that there are 

 a number of chemical compounds wluch are split up into 



their constituent elements when they are subjected to 

 the rays from radio-active substances. For example, it 

 has been found that thorium salts always give off^carbon 

 dioxide, wliich comes from traces of carbonate that have 

 been retained from the manufacturing operations. Radium 

 salts in aqueous solution also decompose the water and 

 give off about 0-091 cubic centimetre of mixed hydrogen 

 and oxygen per week per milligrame of radium element in 

 solution. In a recent number of the Comptes Rendus 

 (Volume 1.59, 1914, page 423) Scheuer describes another 

 experiment, showing the power of the radiations to induce 

 chemical activity. A mixture of hydrogen and oxygen 

 in water-forming proportions was made, and subjected to 

 the action of the raj-s from a quantity of radium emanation. 

 After some time it was found that some of the gases had 

 combined to form water. It is generally beheved that all 

 chemical reactions are " reversible " ; that is, that none of 

 the reacting substances will e\-er completely disappear ; but 

 that in some reactions, e.g., 2H24-Oo = 2H20, when the 

 mixed gases are exploded, the reverse reaction cannot be 

 detected. The above experiments with radium emanation 

 suggest, however, that in these circumstances the reaction 

 2H2O = 2H2 -r O., proceeds wth a speed comparable to 

 that of the reaction 2H2J-0., = 2H20, and that with radium 

 emanation hydrogen and oxygen become analogous to 

 the usual text-book case of hydrogen and iodine, 

 Ho-}-I„ "^ r 2HI. It would be verj' interesting to find out 

 whether this property of altering the state of equilibrium 

 is general. It mav be mentioned that chemical activity can 

 be induced by both a- and ^-rays, but that the a-rays are 

 about twenty times as effective as the /S-rays. 



RATIO OF URANIUM TO RADIUM IN MINERALS. 

 — That radium is genetically connected with uranium was 

 sho\\n by Boltwood and Strutt, who analysed a large 

 number of minerals, and found that the ratio of the quantity 

 of radimn to uranium in rocks, which exhibit no appearance 

 of the action of water or other disturbing agencies, is very 

 approximately a constant value. An accurate knowledge of 

 the value of this constant is of great importance in practical 

 work on radio-activity, and a large number of determin- 

 ations of it has been made. The value given in Ruther- 

 ford's " Radio-active Substances " as the most probable is 

 3-4. This means that the amount of radium in eqiulibrium 

 with one gramme of uranium is 3-4x10-' grammes of 

 radium. Various experimenters have found values from 

 3-1 to 3-S. In a recent paper in The Philosophical Magazine 

 (September, 1914) Rutherford has been able to check former 

 determinations with, the International Radium Standard, 

 and to correct the value of 3-4 to that of 3 -23. Tliis value 

 seems to be the most accurate obtainable at the present 

 time. 



THE END-PRODUCT OF THE THORIUM DIS- 

 INTEGRATION SERIES.— A few months ago the question 

 of the atomic weight of lead from radio-active minerals 

 was mentioned in these Notes, and it is now desirable to 

 draw attention to the paper by Holmes and Lawson in the 

 December number of the Philosophical Magazine, in wliich 

 they discuss the question of the end-product of thorium. 

 Soddy and Hyman found that lead derived from thorite 

 had an atomic weight of 208-4, and this supported the view 

 that this lead was a stable element derived from the dis- 

 integration of thorium. Holmes and Lawson attack the 

 subject by the same method which led Boltwood to conclude 

 that lead was the end-product of the uranium-radium 

 series, i.e., by the relative quantities of the elements in 

 minerals. In general, thev consider the amounts of thorium, 

 uranium, and lead in a large number of minerals whose 

 geological age is known, and their results are : ( 1 ) The 

 ratio of uranium to lead is constant, and (2) the ratio of 

 thorium to lead is variable. In minerals, in which both 

 uranium and thorium are present, they show that, owing to 

 its slower rate of disintegration, the rate at which thorium 

 would produce lead (assuming for the moment that lead is 



