352 



NA TURE 



[February 9, 1899 



subsequent oscillations of the discharge take place through the 

 metallic vapours, and not through the air. We find confirmation 

 of this view in a striking experiment which is easily repeated. 

 If a coil of wire be inserted in the spark circuit of a Leyden jar, 

 which may be charged either by a Wimshurst machine or an 

 induction coil, the air lines disappear almost completely, the 

 metallic lines alone remaining. According to our view we 

 should explain the experiment by .saying that the coil which adds 

 self-induction lengthens the duration of the discharge, and allows 

 time for the nietallit molecules to diffuse properly into the spark 

 gap. A great part of the energy of the current may then do useful 

 work by heating up the metallic molecules instead of those of 

 ail. Mr. Hemsalcch is at present engaged in investigating the 

 changes in the metallic spectra which accompany the insertion 

 of self-induction 



The first spark passing through the air will give rise to a 

 sound wave which, during the complete time of the discharge, 

 will only travel a few millimetres. We may therefore consider 

 that the mass of metallic vapours suddenly set free is driven by 

 its own pressure into the partial vacuum formed by the heated 

 air. It would seem more correct to liken the process to that of 

 a gas under pressure flowing into a vacuum than to that of a 

 pure thermal difl'usion. There is not much difference between 

 these views, and we may take it that in our experiment we have 

 approximately measured the velocity of sound in the metallic 

 vapours. This gives a relation between their temperature and 

 density. If we neglect the diflferences in the ratio of specific heat, 

 we fina approximately 



V = So ^'T/pT 



where T is the absolute temperature and p the vapour density 

 referred to hydrogen. Thus for cadmium the average molecular 

 velocity found was 560, and substituting p = 56 we obtain 

 T = 2700, which seems a possible value. Hence we conclude 

 that the molecule of cadmium in the spark cannot have a mass 

 which is much smaller than that directly determined near the 

 boiling point of the metal. 



In conclusion we have also taken some photographs in which 

 the silt was directly focussed on the sensitive film without the 

 interposition of the film. The photographs show a straight 

 image of the slit followed by a number of curved bands extend- 

 ing from both poles into the spark gap. 



The straight image we consider to be the initial discharge 

 through air creating sufticient heat to fill the space with vapour 

 through which the oscillating discharges may then pass. Our 

 experiments point to the fact that the periodic time was rather 

 too small in our experiments to give the best results. The 

 metallic molecule before it has had time to reach through a 

 sufticient distance was possibly aftected in its motion by the sub- 

 sequent oscillation. We hope to remedy this defect by intro- 

 ducing still higher capacities than those used. Our experiments 

 allow us to give the following approximate numerical data. The 

 air rendered luminous by the first discharge remains luminous 

 for a time of about 5 x lo"' seconds, the metallic vapours then 

 begin to diffuse and reach the centre of the spark (the gap being 

 I cm. long) in a time which in the case of cadmium was about 

 6 X 10"" .seconds. The periodic time of the oscillations with 

 our .six jars and a circuit possessing as little self-induction as 

 possible was about 2 x lo"'' seconds. The metallic vapours 

 remain luminous in the centre of the spark for a longer period 

 than near the poles, the duration of the time during which some 

 luminosity can be traced with a discharge from six Leyden jars 

 is about I '5 x lo"' seconds. 



AfR. BALFOUR AND PROF. J EBB ON 

 TECHNICAL AND SECONDARY EDUCATION. 

 'T'WO important speeches on technical and .secondary cdu- 

 ■*■ cation were made during last week— one by Mr. Balfour 

 in opening a new hall which has been erected in connection 

 with the Baltcrsea Polytechnic, and the other by Prof, [ebb at 

 Cambridge. The Times reports of the remarks made on these 

 two occasions are abridged below. 



Mr. Balfoi'r on Tkchnkal Education. 



Everybody interested in the least in the progress of education 



must watch with the profoundcst interest the great experiment 



now being carried on in this metropolis, and not the least in 



the building where I am now addressing you, in connection 



NO. 1528, VOL. 59] 



with technical education. If I 'understand the matter rightly, 

 the experiment differs frcm any other efforts in the same direc- 

 tion which have been made, either in this country, in other 

 great centres of population, or on the continent of Europe — 

 in Germany or in Switzerland, or in any other countries which 

 have been pioneers in this matter of scientific and technical 

 education. 



Every scheme of education has to be considered from two 

 distinct points of view. We have to consider its effect in quali- 

 fying the individual who receives the education for the particular 

 work in life which he has to do. That is the first aspect of it ; 

 but there is another aspect not less important, which certainly 

 ought never to be lost sight of, and which is not lost sight of 

 in this institution — namely, the general educational results at 

 which any sound system of education ought to aim. There is 

 the technical side and there is the general side. There is the 

 skill infused in the pupil for following that profession in life 

 which he has selected, or which circumstances have forced upon 

 him ; and there is that other and that broader aspect in which 

 all education of every kind is intended to co-operate — namely, 

 the development of the general faculties of mind, eye, and 

 body, and also to make a man or a woman a complete citizen, 

 with all their faculties developed to the highest possible point. 



Technical Instruction. 

 Taking these two .aspects in turn, and dealing, in the first 

 instance, with the industrial and technical aspect, I do not feel 

 my.self qualified to S])eak with any authority upon that part of 

 the work of this institution which has to do with handicrafts. 

 I understand that the aim of the institution in this connection 

 is to supply those who are engaged in these handicrafts with 

 more theoretical and general instruction in connection with 

 their special pursuit which is required to enable the people to 

 reach the highest results in that pursuit. I cannot imagine a 

 better object. I am not aware that in other places the same 

 object is pursued systematically and successfully in the same 

 way in which it is pursued in the London polytechnics. After 

 all, it has to be recognised that work is mainly to be learnt in 

 the workshops, and I am convinced that no wise teacher would 

 for a moment attempt to substitute either the lecture-room or 

 experiment-room for that which can be learnt only in the 

 workshop. But unquestionably there are branches of knowledge 

 connected with trades and handicrafts which have a theoretical 

 side which cannot with equal facility be learnt except in a 

 place devoted to that side, and I believe that the work done 

 in this institution in that connection is one of the greatest 

 value, not merely to the pupils, but to the trades and industries 

 which they have elected to follow. 



.Scientific Education. 

 But there isfanother side, and, from a national point of view, 

 perhaps a decidedly more important side than that, and the side 

 I mean is the complete scientific equipment of a student for 

 those professions in which a thorough grounding in science, 

 theoretical and practical, is absolutely necessary ifhe is to make 

 the most 'of himself and the most of the profession in which he 

 is engaged. I have ahvay.; been deeply interested in this aspect 

 of the question, which is the one specially con.sidered in Ger- 

 many and elsewhere, and the value of which we have perhaps 

 in this country until recent years unduly ignored and neglected. 

 It is an interesting question to ask ourselves how it comes 

 about, and why it comes about, that it is only in the latter half 

 of the nineteenth century that the absolute necessity of this 

 thorough scientific grounding has been recognised in connection 

 with great iniiustrial enterprises. And the real reason I take lo 

 be this— that it is only after science has developed to a certain 

 point, and after industry has developed to a certain point, that 

 you can, as it were, successfully and usefully combine the two, 

 and that there is forced upon you the necessity of recognising 

 that almost every advance in theoretical science is reflected in a 

 corresponding advance of industrial enterprise, and in like 

 manner industrial enterprise and the practical application of 

 science is itself from day to day giving birth to new scientific 

 conceptions and new improvements either in the machinery of 

 discovery or in the results of discovery. If anybody wishes to 

 have a concrete illustnition of this abstract truth I would ask 

 him to make the following com|)arison. Take for a moment the 

 career of the greatest man of science that this world has ever 

 seen — I mean Sir Isaac Newson. As far as I know — I speak 

 under correction — neither by Sir Isaac Newton himself, nor by 



