540 



NA TORE 



[April 5, 1900 



already obtained may be of sufficient interest to justify pre- 

 liminary publication. The experimental details will be more 

 appropriately f;iven with the completed experiments. 



Curie has shown that the rays from active barium compounds 

 are of two kinds. One kind is easily absorbed, and is not 

 deflectable by the magnet. The other kind is much more pene- 

 trating, and does suffer deflection in a magnetic field. It is to 

 the latter kind exclusively that the experiments refer. 



The intensity of the radiation was measured by the electrical 

 conductivity of air exposed to it. It was again measured after 

 partial absorption by a plate of the material under investigation. 



In the following table the first column gives the coefficients 

 of absorption A defined by the equation 

 r = r^ e~^'^ , 

 while ^Q, r, are the initial and final intensities of the radiation, 

 and d the distance traversed. 



It will be seen that, although the coefficient of absorption is 

 not accurately proportional to the density, yet the departure 

 from this relation is not very great, if the enormous range of 

 density be taken into account. Thus between solid platinum 

 and the compressed sulphur dioxide used, there is a three 



thousand-fold difference of density. The quotients il^I^P-'i^.'' 



density 

 are respectively 7*3 and 5 "45. It is interesting to compare these 

 results with Lenard's observations on the absorption of the 

 kathode rays ( Wied. Ann. vol. Ivi. p. 255). He found that 

 the above relation between absorption and density held to about 

 the same degree of approximation. The coefficients of absorp- 

 tion for the kathode rays are, however, some five hundred 

 times greater than for the rays investigated in my experiments. 



We may, I think, fairly consider that the approximate pro- 

 portionality between absorption and density is an additional 

 argument in favour of the view that the deflectable Becquerel 

 rays are of the same nature as the kathode rays. To account 

 for the enormously greater penetrating power of the latter, one 

 must suppose either that the particles constituting them are 

 much smaller, or that their velocity is much greater. 



R. J. St RUT T. 



Planets at their Greatest Brilliancy. 



Mr. Denning's able and lucid article upon the planet 

 Mercury (Nature, March i) induces me to send a few notes. 

 With inclined elliptical orbits it is a complicated matter to 

 determine when an interior planet is at its greatest brilliancy. 

 But if the orbits are assumed circular and coplanar, interesting 

 results are easily obtained. 



Theory shows that there is a certain elongation, at which the 

 interior planet, viewed from the exterior one, has a maximum 

 brightness. Now, for a given elongation, there are two 

 distances, a long and a short one, between the planets. Con- 

 sider only eastern elongations. It will be found that Mercury 

 has its greatest brilliancy (for mean distances and circular 

 orbits) when its elongation is 22° 19', and when its distance 

 (I 00) from the earth is the larger of the two distances possible 

 for this elongation. The illuminated phase is 0"6o. Thus 

 Mercury is brightest before its maximum eastern elongation of 

 22" 47'. 



1 Saturated vapour at 13° C. 



NO. 1588. VOL. 61] 



Venus has its greatest brilliancy at elongation 39° 43' ; but 

 its distance (0-43) from the earth must be the smaller of the 

 two possible ones. The phase is 0*27. Thus Venus is brightest 

 after its maximum elongation of 46' 20'. 



But, if from Venus we view Mercury, then (as in the case of 

 the earth and Venus) we must take the shorter distance for 

 maximum brilliancy. The elongation is 31° 36', distance 0'54, 

 phase 0'40. Thus Mercury, seen from Venus, is brightest after 

 its maximum eastern elongation of 32° 21'. 



That a planet should be brightest exactly at maxinium elonga- 

 tion involves, I find, the following relationship between the 

 radii vectores : the radius vector of the exterior planet should 

 be just v/5^ times that of the interior one. When the factor 

 exceeds ^'5. the interior planet is brightest before maximum 

 elongation. When the factor falls short of ^'S- 'he interior 

 planet is brightest after maximum elongation. Circular orbits 

 are assumed. For the pairs, Mercury- Venus, Venus-Earth, 

 Earth-Mars, Jupiter-Saturn, the factor is less than ^,/5. But 

 for Mercury-Earth it is greater ; hence Mercury is brightest 

 before maximum elongation east, a fact clearly brought out by 

 Mr. Denning's observations. On several occasions I have seen 

 Mercury with the unaided eye, and, generally, after greatest 

 eastern elongation, when the conditions are less favourable than 

 before it. C. T. Whitmei.i.. 



Leeds, March 5. 



P.S. — The American Ephenicris for 1900 shows that the 

 maxima of brightness for Mercury occur very irregularly. One 

 maximum occurs 6 days before greatest east elongation, another 

 only I i days after superior conjunction. Eccentricity accounts 

 for these irregularities. 



The Use of Silica in Thermometry. 



I HAVE just learnt from your last number (p. 521) that Mons. 

 A. Dufour has recently exhibited two silica thermometers in 

 Paris, and that he proposes to study the suitability of silica for 

 use in thermometers. 



As I had the honour of exhibiting silica tubes of various sizes 

 last June at the soiree of the Royal Society, and also then ex- 

 hibited, in conjunction with Mr. Evans, our process for making 

 such tubes, I am anxious at once to state that I have continued 

 to study the applications of silica in conjunction with Mr. 

 H. G. Lacell, and that we have at this moment the bulbs and 

 stems of four delicate silica thermometers ready to be joined 

 and filled as soon as their scales and some fittings are delivered. 

 In February last we filled one of these ungraduated ther- 

 mometers and tested it. It was shown to our colleague, Mr. 

 J. E. Pearson, but was afterwards cut in two in order to alter 

 the length of the degrees (20 mm.), as they were not quite as 

 long as we then wished them to be. 



f may add that the scales for these thermometers have been 

 ordered, through the Cambridge Instrument Company, of 

 Messrs. Zeiss, and that a special glass thermometer has been 

 constructed for use in studying their zero points, which has now 

 been in the hands of the Superintendent at Kew for some days. 



Clifton, April 2, 1900. W. A. Shenstone. 



The Natural History Museum— A Correction. 



In a paper of mine on Ilyopsyllns coriaceu^, which appeared 

 recently in the Natural History Transactions of Northntnber- 

 latid and Durham, I referred to certain dissections — which had 

 been described by Mr. Thomas Scott, and are now in the 

 Natural History Museum at South Kensington — as having " de- 

 teriorated so as to be useless," at the same time ascribing this 

 statement to Prof. T. Jeflrey Bell, who had kindly examined 

 the dissections at my request. The statement, so far as Prof. 

 Bell's authority is concerned, is not quite accurate, and at his 

 request I wish to be allowed to correct it in your columns. 

 What Prof. Bell told me was that the dissections consist of 

 " nothing but unrecognisable fragments," and that " Mr. 

 Pocock, who had charge of the Crustacea in 1893, says the tube 

 came there in the state it is in now." 



I think I need scarcely add that my words, as quoted above, 

 were not meant in any way to impute negligence or want of 

 care to the officials of the Museum. G. S. Brauy. 



The Durham College of Science, Newcastle-upon-Tyne, 

 March 29. 



