February 25, 1909] 



NA TURE 



487 



LETTERS TO THE EDITOR. 

 JT/ie Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond -with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Brilliancy and Intensity of the Cupric Chloride 

 Flame Spectrum. 



In the account of an interesting investigation of the 

 Jlamc spectrum of cupric chloride communicated by Peter 

 Kien (_Zcits. f. wissensch. Photographic, 1908, vol. vi., 

 J37) there occurs a sentence to the following effect ; — 



" How difiicult it is to decide upon the brilliancy and 

 intensity of a spectrum by means of photography may be 

 shown by the following example : — 



" I'rof. Hartley has published a very beautiful small 

 photograph of the cupric chloride spectrum, the only one, 

 moreover, which up to the present has been published. It 

 is not in the least over-exposed, notwithstanding that 

 Hartley gave it an exposure of two hours. My photo- 

 graphs were over-exposed in ten minutes — even if, as 

 Hartley did, I brought cupric oxide into the oxygen and 

 coal-gas flame saturated with chloroform vapour." 



I think it is due to the author and others to point out 

 that he writes under a misapprehension, inasmuch as his 

 spectra and mine were taken each in a different manner 

 and with a different object in view. He desired to photo- 

 graph the best spectrum obtainable from the chloride for 

 the purpose of measuring the bands, and therefore burnt 

 the usual rolls t" cigarettes ") of filter paper containing 

 either cupric chloride or the oxide. The " cigarettes " 

 were pushed by a spring through a tube into the f^ame 

 at a speed regulated by a clock-work arrangement. 



When communicating a paper on some devices facili- 

 tating the study of spectra (Sci. Proc. Roy. Dublin Soc, 

 vol. .xi., p. 237, 1907J, I demonstrated the extreme delicacy 

 of the cupric chloride reaction in e.xplanation of the reason 

 that, although there may be no green coloration of the 

 flame by copper, nevertheless the blue flame and cupric 

 chloride bands are seen when salt is thrown into a fire of 

 glowing coals. 



The experiment was made in the following manner : — 

 a quartz fibre about a millimetre thick was placed in a 

 solution of a copper salt and heated in the flame of the 

 Meker (or Mecke) burner supplied with coal-gas, which 

 was burnt with a blast of air at a pressure of about 

 700 mm. of mercury. The fibre was heated until all the 

 copper salt had been decomposed, as shown by scarcely 

 any evidence of a trace of copper being visible in the 

 flame when looked at in a darkened room. On diverting 

 about one-third of the coal-gas through the flask contain- 

 ing sponge soaked in chloroform, the hydrochloric acid 

 produced by the combustion of its vapour yielded a large 

 and brilliant blue flame due to the cupric chloride, which 

 was steady and continuous for a long period. For the 

 illustration of the text of the paper the first exposure of 

 the copper oxide was limited to two hours, then, without 

 removing the fibre from the flame, the chloroform tap 

 "Was turned on, and a similar exposure made. 



An excess of hydrochloric acid prevents the spectrum 

 being visible at all, so that with the large volume of 

 nitrogen in the air, and the consequent reduction of 

 temperature arising from the hydrochloric acid in the coal- 

 gas flame, the proportion of chloroform vapour must be 

 limited, and the resulting quantity of cupric chloride 

 vaporised is correspondingly small. 



In the experiments made by Kien, the greater intensity 

 of photographic action is caused by the use of oxygen 

 under pressure along with coal-gas, whereby, in conse- 

 quence of the much higher temperature and greater 

 quantity of heat, he is able to feed the flame with a much 

 larger proportion of chloroform vapour, and consequently 

 to volatilise a very much larger quantity of cupric 

 chloride in the same period of time than is possible with 

 the air blast. Furthermore, by the use of the " cigarette," 

 he has a larger quantity of copper in the flame at any 

 given moment. 



As a rule, my flame spectra are obtained by using the 

 NO. 2052, VOL. 79] 



oxy-hydrogen blow-pipe, and when the hydrogen is mixed 

 with chloroform the photographic period of exposure, 

 according to circumstances, varies from thirty seconds to 

 five minutes. 



That salt is decomposed and hydrochloric acid formed 

 by the action of water vapour when salt is thrown into 

 a coal fire is certain. It is proved by the fact, which I 

 found out when studying this spectrum in 1887, that the 

 characteristic blue flame is not obtainable when salt is 

 thrown into a fire of charcoal. In 1890 Salet proved the 

 origin of the blue flame to be cupric chloride {Comptes 

 rendus, ex., p. 282), and not in any way connected, as 

 had been suggested, with the spectrum of carbon, carbon 

 monoxide, or hydrocarbon flames, nor due to the element 

 chlorine or to hydrochloric acid. My interest in the matter 

 thus came temporarily to an abrupt termination, because, 

 having by this time become aware that minute quantities 

 of copper are to be found in most metalliferous and many 

 other minerals, also in acids, it was easy to account for 

 the blue flame being frequently seen by reason of the 

 extraordinary delicacy of the cupric chloride flame reaction. 

 Coal ashes always contain copper, the origin of which is 

 commonly pyrites, and in the fire this is speedily burnt to 

 oxide. Sulphur dio.xide, steam, and air, even below a very 

 dull red heat, convert salt into sodium sulphate and hydro- 

 chloric acid, and hence the formation of cupric chloride in 

 presence of an excess of hydrochloric acid. Kien '3 paper 

 gives an admirable historical account of the subject, which 

 is particularly interesting owing to the extraordinarily 

 illusive and elusive character of this spectrum. Much of 

 this may be read in the Phil. Mag. (4), vol. xxiv., 417-9, 

 and the pages of Nature during 1S76 and 1879. 



A very beautiful engraving of the cupric chloride bands 

 is given in Lecoq de Boisbaudran's " Spectres lumineux," 

 published in 1874. W. N. Hartley. 



Royal College of Science, Dublin, February it. 



On the Radio-active Deposits from Actinium. 



In the course of some experiments which Mr. W. T. 

 Kennedy has been making at Toronto during the past few 

 months, he has found a marked similarity in the active 

 deposits obtained on positively and negatively charged 

 electrodes placed within an air-tight vessel and subjected 

 to the influence of the active emanation issuing from a 

 sample of actinium. 



In his experiments the electrodes consisted of two small 

 circular brass discs provided with guard rings of the same 

 metal, and placed parallel to each other at a distance of 

 2 mm. apart. The discs during an exposure were placed 

 with their planes vertical and directly over an open metal 

 tube 1-5 mm. in diameter, with the edges of the guard 

 rings almost in contact with the edges of the upper end 

 of the tube. The salt used was carried in a small tray 

 which could slide freely up and down the tube, and by 

 means of a clamp be supported at any required distance 

 from the discs. 



In carrying out a set of experiments on the effect of 

 varying the pressure of the air in the vessel containing the 

 discs and the salt, it was found at high pressures that the 

 active deposit appeared almost entirely on the negative 

 electrode. As the pressure was decreased, however, the 

 active deposits on both electrodes increased, and ultimately 

 at certain definite pressures, which were different for the 

 two electrodes, reached maximum values. When the 

 pressures were still further lowered, the amounts of the 

 deposit received on both electrodes rapidly decreased, and 

 finally approached equality. Up to the present the lowest 

 pressure used is J mm. of mercury, and at this pressure 

 the deposit on the negative electrode was found to be only 

 about 3 per cent, greater than that obtained on the positive. 

 From the rapid character of the decrease in the amounts 

 of the deposit obtained at the lower pressures, it seems 

 highly probable that, with the arrangement of apparatus 

 used, and the relative distances between the parts adopted, 

 both electrodes would fail to show any activity, or at 

 greatest a very small one, if the air were entirely removed 

 from the exposing vessel. 



In a particular experiment with the salt at a distance 

 of I cm. from the disc electrodes, a maximum activity was 



