February 9, 1899] 



NA TURE 



351 



of aboiu 100 metres/secoml for that part of the film on which 

 the phc;toijraph was taken. 



The electric discharges were obtained from a battery of six 

 Leyden jars, having a total capacity of 0'033 microfarad, and 

 being charged from an induction machine constructed for us by 

 Mr. 11. C. Wimshurst. This machine has twelve plates of 

 62 cm. diameter, and gives sparks which are 13 inches long. 

 The electrodes were, as a rule, placed l cm. apart, and an 

 image of the spark was projected on the slit of the spectro- 

 scope, the distance of the slit from the electrodes being equal 

 to four times the focal length of the projecting lens, so that 

 the image was equal in si/.e to the spark. The prism use<l 

 was made by Steinheil, and had a refracting angle of 60'. 



We may now pass to the description of the results obtained 

 when the spectrum of a single spark is taken on a moving film. 

 A preliminary trial with various metallic electrodes had shown 

 us that the sharpest results were obtained with zinc, and we 

 therefore chose that metal for our first investigation. The 

 principal lines of zinc as they appear on our photographs are the 

 double line, the least refrangible of the two having a wave- 

 length 4924-8, and the blue triplet, the wave length of the 

 leading line being ^48107. All the lines are curved on the 

 photographs taken with the spinning disc, but the displace- 

 ments, especially near the poles, are subject to considerable 

 variations. This is probably due to the fact that the path 

 of the metallic particles is not always straight, and, if straight, 

 its image does not necessarily coincide with the slit. A very 

 slight error in measurement will also affect the results con- 

 siderably when the total displacement measured is small. 

 Our results do not for this reason allow us at present to give 

 any opinion as to the maximum velocity of the particles near 

 the pole ; but if these are considerable, they drop down very 

 quickly to speeds which, in the case of zinc, are not far oft 500 

 metres/.second. 



We have adopted two methods of comparison between 

 different photographs. We have in the first place measured 

 the displacements at a number of nearly equidistant points, 

 and from these measurements we have deduced ^the time taken 

 for a metallic molecule to pass from the pole to a point 2 mm. 

 away from it. If this method could be applied in every case, it 

 would form a rational and consistent basis of comparison. But 

 the curved lines which are to be measured are often very 

 diffuse near the pole, this, and the continuous spectrum, may 

 render it impossible to obtain satisfactory measurements at that 

 point. In order not to have to reject unnecessarily a large 

 number of measurements because tlie spectrum near the pole 

 was indistinct, we have adoptetl another method, which, though 

 less rational than the first, is found to give consistent results. 

 From all our measurements we may deduce certain figures for 

 the molecular velocities at different and generally e<|uidistant 

 points on the photographs, and may take the average of all these 

 figures as the mean velocity of the particle. In the following 

 tables, V, will always refer to the mean velocity between the 

 pole, and a point 2 mm. away from it, while \\ refers to the 

 average velocity taken for different distances, as just explained. 

 The influence of change of capacity and change in the length of 

 the spark was investigated in the case of zinc, and the following 

 tables exhibit the results. As the zinc lines are sharp near the 

 pole, the first of the above methods of measurement could be 

 applied. 



Tabi.k I. — Average Velocity {V-^ in metres I second of Zinc 



Molecules. 



The first striking result to be deduced from the table is the 

 uniformly higher velocity deduced from the double line 4925, 



NO. 1528, VOL. 59] 



as compared with that found when one of the lines of the triplet 

 is measured ; for we have ascertained that the two first lines 

 of the triplet are always displaced by the same amount, and the 

 third is so much mixed up with the air lines in its neighbourhood 

 that it cannot be measured. It was one of the objects of the 

 investigation to detect, if possible, differences of this kind, 

 which might be accounted for by the fact that the molecules 

 producing different lines of the same spectrum have not neces- 

 sarily the .same mas.s. We nevertheless hesitate to ascribe the 

 smaller apparent velocity derived from \ = 4925 to this reason. 

 This line, as has been mentioned, is one component of a double 

 line, and the doublet is not resolved on the photographs taken 

 with the moving film. Near the pole where the light is strong, 

 the edge of the least refrangible component of the doublet 

 would be considered to be the least refrangible edge of the 

 doublet ; but near the centre of the spark the light is weaker, 

 and the lines, owing to the motion of the wheel, are drawn out 

 towards the violet. The most intense portion of the image will 

 here be that part where the two lines are superposed, and, in 

 wishing to .set the cross wire on the edge of the line, we should 

 be tempted to set it on the edge of the most refrangible com- 

 ponent. There is reason to believe that this is the cause of the 

 greater deflection of the double line, and the photographs show 

 some signs that if this source of error is eliminated, the mole- 

 cule giving out the double line moves more quickly than that 

 giving rise to the triplet. We reserve the decision of this point 

 until we have been able to apply greater dispersion. 



Comparing the spark obtained with different capacities, it is 

 found that when the spark gap is small, there seems a very 

 curious diniinulion of velocity as the capacity increases ; this is 

 not what should have been expected at first sight, as with the 

 large number of jars we should expect higher temperatures, and 

 therefore greater velocity of diffusion. When the spark gap is 

 I cm., the experiments do not leveal any marked change due to 

 capacity. When the gap is increased still further the sparks 

 become very irregular and unsteady, and no certain conclusions 

 can be drawn from our measurements ; the numliers marked 

 with a query are specially doubtful. When six jars are used 

 practically identical numbers are obtained for all sparking dis- 

 tances, but with small capacity the centimetre spark seems to 

 give a lower result than in the two other cases. VVhile we 

 should not like at present to consider this as an established re- 

 sult, the table serves to show that the centimetre spark and the 

 highest capacity used gives the most consistent numbers, and 

 our experiments with other metals were all made under the.se 

 conditions, except in the case of bismuth, where clearer spectra^ 

 were obtained with only two jar.s. 



Comparing different metals with each other, we find in the- 

 first place that those having comparatively low atomic weights, 

 viz. aluminium and magnesium, have higherniolecular velocities. 

 With magnesium the metal vapour is scattered about to such an' 

 extent that no measureinents could be made, but the average- 

 velocity of the aluminium molecule was found to be over three 

 times as great as that of zinc, the numbers not laying any claim 

 to accuracy. Comparing zinc and cadmium with each other, we 

 obtain almost identical numbers, both for the corresponding 

 doublet and triplets. 



Hismuth gave remarkable results. In spite of its high atomic 

 weight some of the lines are but little displaced, indicating an 

 average molecular velocity of 1420 metres/second. For other 

 lines the velocity falls down to that of zinc and cadmium, while 

 one line (\ = 3793) has a still smaller velocity. 



We have not obtained satisfactory results with mercury ; the 

 best were those in which poles used were of zinc or cadmium, 

 which were covered with amalgam. Differences in molecular 

 velocities were obtained for different lines, but the result here is 

 not so certain as with bismuth. There is obviously no simple 

 law connecting these velocities with the atomic weight. 



Dr. Feddersen was led through his researches to the con- 

 clusion that the metallic particles after being once torn off from 

 the electrodes by the discharge took no further part in it, were 

 thrown irregularly into the space surrounding the electrodes 

 quite independently of the electric current. Although in some 

 cases, and especially with magnesium poles, there is some 

 evidence that this is partly true, we are led to take the following 

 modified view of the matter. 



The initial discharge of the jar takes place through the air ; it 

 must do so becau.se there is at first no metallic vapour present. 

 The intense heat generated by the electric current volatilises the 

 metal, which then begins to diffuse away from the poles ; the 



