512 



NATURE 



\Sept. 29, 1 88 1 



above ; and the Soleil, in which the point of contact 

 between the carbon point and the copper is surrounded 

 by chalk or lime, which is rendered incandescent. 



Then there are the " incandescent lamps," specially so 

 called, in which a thread of carbon a few inches long is 

 inclosed in a vacuous space where, as there is no oxygen, 

 there will be no combustion, and the carbon does not 

 waste. Swan's light, which is of this class, occupies a 

 very conspicuous place in the Exhibition, and is used for 

 the illumination of the Salle des Si^ances, in which the 

 meetings of the Congress are held. Edison's two rooms 

 are nightly thronged by visitors, who come to see not 

 only his lights, but his numerous other inventions, which 

 are here exhibited. Lane-Fox's light and Maxim's (which 

 has been stopped by some accident) belong to the same 

 class. No opal or ground glass is necessary with incan- 

 descent lights, as they are less dazzling than arc lights. 

 They certainly give very beautiful illumination to a room, 

 and their convenience for lecture-room purposes was well 

 seen on the occasion of an illustrated lecture given by M. 

 Mercadier in the Salle des Seances at the meeting of the 

 Society of Telegraph Engineers on Thursday last. They 

 can be extinguished in a moment and re-lighted in a 

 moment. 



The Exhibition is open in the daytime from 10 till 6, 

 and in the evening from S till 1 1. The largest attendance 

 is in the evening, when the lights are in full action. 

 Besides those inside, which make the interior almost like 

 daylight, there are two very powerful lights above the 

 roof, which are furnished with reflectors, and throw beams 

 of light like comets' tails in various directions. 



The Congress commenced its sittings on the 15th inst., 

 when an opening address was delivered by M. Cochery, 

 the official presi'Jent, and the hours of meeting and other 

 details of organisation were arranged. The foreign 

 members were called upon to elect three vice-presidents 

 to join the three French vice-presidents (all official) who 

 had already been named. After a brief conference Sir 

 William Thomson, Prof. Helmholtz, and Prof Govi of 

 Naples were proposed ; nd unanimously elected. It was 

 agreed to divide the Congress into three sections, devoted 

 respectively to theoretical electricity, telegraphy with 

 telephony, and miscellaneous applications of electricity, 

 including the electric light ; the first section meeting at 

 9.30 a m., the second at 2, and the third at 4 p.m. Each 

 section has sat fcr about two hours daily, an interval of 

 two hours between the first and second being allowed for 

 dejeuner. 



M. Dumas was elected president of the first section, 

 with Prof. Kirchhoff and Dr. De La Rue as vice-presi- 

 dents. Prof. Mascart and M. Gdrard being secretaries. 

 The discussion of the subject of international electrical 

 units, the choice of which is regarded as the most impor- 

 tant work of the Congress, was then begun, and occupied 

 the rest of the sitting. Sir William Thomson introduced 

 the question in a very lucid speech, in which he described 

 the course which had been taken by the British Associa- 

 tion, and recommended a substantial adoption of the 

 British Association system. He was followed by Profess- 

 ors Wiedemann and Helmholtz, who favoured the adop- 

 tion of a mercurial unit of resistance; and a large com- 

 mittee, containing men of both views, was appointed to 

 draw up a Report. This Report was anxiously awaited, 

 and was presented on the 19th inst. It consisted of the 

 following seven recommendations, which had received 

 the unanimous consent of the Committee, and have now 

 been formally adopted by the Congress. 



1. The fundamental units for electrical measurements 

 to be the centimetre gramme and second (C.G.S.). 



2. The practical units ohm and volt to retain their 

 present definitions, 10' for the ohm and lo^ for the volt. 



3. The unit of resistance (ohm) to be represented by a 

 column of mercury of a square millimetre section, at the 

 temperature zero Centigrade. 



4. An international commission, to be charged with the 

 duty of determining by new experiments, for practical 

 purposes, the length of the column of mercury, of a square 

 millimetre section, at zero Centigrade, which represents 

 the value of the ohm. 



5. The name Ampere to be given to the current pro- 

 duced by a volt in an ohm. 



6. The name Coulomb to be given to the quantity of 

 electricity defined by the condition that an Ampi^re gives 

 one Coulomb per second. 



7. The name Farad to be given to the capacity defined 

 by the condition that a coulomb in a farad gives a volt. 



It will be observed that the " weber," a unit familiar to 

 British electricians, is not mentioned in these resolutions. 

 The reason, as stated by Prof. Helmholtz to the Congress, 

 is that Weber himself employs a unit of current derived 

 from the millimetre, milligramme, and second, and this 

 unit, which is one-hundredth of the C.G.S. unit, or one- 

 tenth of the weber, as commonly understood by British 

 electricians, is known as " the weber" in Germany. 



The reason for adopting a mercurial standard defined 

 by size was explained by Sir William Thomson to be the 

 desire to guard as much as possible against secular 

 change. 



It transpired in the discussions which took place in 

 committee that mercurial standards, as actually con- 

 structed, are glass tubes which must be refilled with 

 mercury every time they are to be used. The external 

 communications are made by means of platinum wires 

 attached to plates of the same metal, the latter being well 

 amalgamated before use. It is obvious that these opera- 

 tions involve much more labour and risk of error than 

 comparison with a standard coil ; and we therefore do 

 not anticipate that recourse will be had to the mercurial 

 standard except on rare occasions. Coils will as hereto- 

 fore continue to be used for all ordinary measurements of 

 resistance. The international committee which is to 

 make the new deterinination will be nominated by the 

 governments of the various countries concerned, and 

 independent determinations will doubtless be made by 

 different members of the committee in different labora- 

 tories. It will thus be seen what amount of consistency 

 is attainable in such measurements, and whether it is 

 sufficient to render the standard praclically accurate. The 

 German authorities assert that accuracy to one part in 

 two thousand can thus be ensured. 



THE CAUSE OF COLLIERY EXPLOSIONS 



ONE of the most instructive documents ever penned 

 on the subject of the cause of explosions in colUeries 

 has recently appeared, in a latel)--issued Blue-book, in the 

 form of a Report to the Home Secretary by Prof. Abe', 

 C.B., F.R.S., of Woolwich, who, at the request of the 

 Home Department, conducted a series of experimental 

 researches upon the cause of the terrible disaster at the 

 Seaham Colliery on September 8, 1880. In 1S45 Fara- 

 day and Lyell first directed attention to the influence 

 exerted by the presence of coal-dust in mines upon the 

 magnitude of an explosion of fire-damp. In 1867 and 

 1875, 'he subject was further advanced in France by 

 Messieurs Verpilleux and Vital, the latter of whom showed 

 that air charged with fine coal-dust, rich in inflammable 

 material, may explode when there is present a much 

 smaller proportion of true fire-damp than is of itself suf- 

 ficient to constitute the atmosphere an explosive one. 

 Still more recently Mr. W. Galloway has conducted a 

 valuable series of investigations and experiments, the 

 results of which have been communicated to the Royal 

 Society in three very important memoirs. In the first of 

 these he showed that a certain mixture of air and coal- 

 dust, not itself inflammable, became so when there was 

 also present a much smaller proportion of fire-damp than 

 any Davy lamp could detect. In the second he showed 



