278 



NATURE 



[July 19, 1S83 



metre, or all the molecules in a slice of 1/10,000 of the 

 thickness of that cube. Think of a cube filled with 

 particles, like these glass balls, 1 scattered at random 

 through a space equal to 1,000 times the sum of their 

 volumes. Such a crowd may be condensed (just as 

 air may be condensed) to 1/1,000 of its volume ; but 

 this condensation brings the molecules into contact. 

 Something comparable with this may be imagined to be 

 the condition of common air of ordinary density, as in 

 our atmosphere. The diagram with size of each molecule, 

 which, if shown in it to scale, would be 1 millimetre (or 

 too small to be seen by you), to represent an actual dia- 

 meter 1/10,000,000 of a centimetre, represents a gas in 

 which a condensation of 1 to 10 linear, or 1 to 1,000 in 

 bulk, would bring the molecules close together. 



Now you are to imagine the particles moving in all 

 directions, each in a straight line until it collides with 

 another. The average length of free path is ten centi- 

 metres in our diagram, representing 1/100,000 of a 

 centimetre in reality. And to suit the case of atmo- 

 spheric air of ordinary density and at ordinary pressure, 

 you must suppose the actual velocity of each particle to 

 be 50,000 centimetres per second ; which will make the 

 average time from collision to collision 1/5,000,000,000 of 

 a second. 



The time is so far advanced that I cannot speak of the 

 details of this exquisite kinetic theory, but I will just say 

 that three points investigated by Maxwell and Clausius — 

 viz. the viscosity, or want of perfect fluidity of gases ; the 

 diffusion of gases into one another ; and the diffusion of 

 heat through gases — all these put together give an esti- 

 mate for the average length of the free path of a molecule. 

 Then a beautiful theory of Clausius enables us from the 

 average length of the free path to calculate the magnitude 

 of the atom. That is what Loschmidt has done,- and 

 I, unconsciously following in his wake, have come to the 

 same conclusion ; that is, we have arrived at the absolute 

 certainty that the dimensions of a molecule of air is some- 

 thing like that which 1 have stated. 



The four lines of argument which I have now indicated, 

 lead all to substantially the same estimate of the dimen- 

 sions of molecular structure. Jointly they establish, with 

 what we cannot but regard as a very high degree of pro- 

 bability, the conclusion that, in any ordinary liquid, 

 transparent solid, or seemingly opaque solid, the mean 

 distance between the centres of contiguous molecules is 

 less than the 1/5,000,000, and greater than the 

 1/1,000,000,000 of a centimetre. 



To form some conception of the degree of coaise- 

 grainedness indicated by this conclusion, imagine a globe 

 of water or glass, as large as a football, 3 to be magnified 

 up to the size of the earth, each constituent molecule 

 being magnified in the same proportion. The magnified 

 structure would be more coarse-grained than a heap of 

 small shot, but probably less coarse-grained than a heap 

 of footballs. 



SMOKE ABATEMENT 



A N important meeting was held in the Egyptian Hall 

 -**- of the Mansion House on Monday last, to take 

 further steps towards the abolition, or at all events the 

 reduction, as far as possible, of the smoke nuisance. The 

 Lord Mayor presided, and the following among others 



1 The piece of apparatus now exhibited, illustrated the collisions taking place 

 between the molecules of gaseous matter, and the diffusion of one gas into 

 another. It consisted of a board of about one metre square, perforated with 

 100 holes in ten rows of ten holes each. From each hole was suspended a 

 cord five metres ljng. To the lower end of each cjrd in five contiguous rows, 

 there was secured a blue c floured glass ball of four centimetres diameter ; and 

 similarly to each curd of the other five rows, a red coloured ball of the same 

 size. A ball from one of the outer rows was pulled aside, and, being set free, 

 it plunged in amongst the others, causing collisions throughout the whole 

 plane in which the suspended balls were situated. 



2 Sitzungsbericltte of the Vienna Academy, Oct. 12, 1865, p. 395. 



3 Or say a globe of 16 centimetres diameter. 



were present : — The Duke of Northumberland, the Duke 

 and Duchess of Westminster, Sir William Siemens, 

 Sir Frederick Abel, Sir Lyon Playfair, M.P., Sir 

 Frederick Pollock, Sir T. Spencer Wells, Mr. George 

 Cubitt, M.P., Dr. Farquharson, M.P., Col. Makins, 

 M.P., Capt. Galton, Mr. Edwin Chad>vick, C.B., Mr. 

 Ernest Hart, Mr. C. Waring, the Hon. Rollo Russell, 

 General Lowry, C.B., Mr. George Shaw (chairman of the 

 City Commission of Sewers), Mr. W. R. E. Coles, Mr. W. 

 Chandler Roberts, of the Royal Mint, and Mr. Gregory, 

 Master of the Clothworkers' Company. 



The proceedings were opened by the reading of a 

 Report, which has been carefully prepared by the Council, 

 detailing the steps which have already been taken, and 

 giving particulars of the exhibitions of last year in London 

 and Manchester. The Report also deals with the w ork 

 which has been done regarding the chemical composition 

 of smoke by Prof. Chandler Roberts, and the many tests 

 of coal made by Mr. Clark. In this important investiga- 

 tion, attention was called to the fact that a very great 

 discrepancy exists between the heating efficiency of 

 various types of grates, stoves, furnaces, and the like. 

 In some forms of grate, for instance, only 22 per cent, of 

 the total heat is utilised, whilst others require nearly 

 three tons of coal to do the same work which other stoves 

 manage to get out of one ton. 



The Council desired to report that, so far as they had 

 been able to ascertain, the most marked benefit resulting 

 from the movement had been in the increased use of gas 

 and coke for heating purposes. The improvement in gas- 

 heating apparatus had been considerable, and the use of 

 coke had been greatly facilitated by its being supplied to 

 the public in more convenient sizes than formerly, and by 

 the introduction of firebrick or other slow-conducting 

 substances used in the fireplaces for burning it. The use 

 of smokeless coal had also been extended in the metro- 

 polis ; but the Council found that the description of such 

 coal supplied was in a large number of instances unsuit- 

 able or inferior, and from that cause, coupled with the 

 fact that smokeless coals were not generally supplied by 

 coal merchants, there had not been, so far, any very 

 marked increase in its consumption. Marked improve- 

 ment had, however, been made in open grates and 

 stoves for burning that description of coal, and one firm 

 of manufacturers, who brought out a cheap stove at the 

 South Kensington Exhibition, had sold upwards of 14,000 

 during the pa=t two years ; and they remarked that the 

 public seemed ready to burn non-smoky coal if proper 

 stoves for using it were offered at a reasonable price. 

 Appliances for improving the draught of chimneys had 

 also been introduced lately, and that tended to facilitate 

 the use of smokeless coal. The Council had examined 

 the present state of the administration of the law for the 

 suppression of smoke, and they considered that in view 

 of the enormous extension of buildings and factories in 

 London and large towns, and in view also of the evidence 

 that smoke could be to a great extent, if not entirely, 

 avoided, the scope of legislative enactments for abating 

 smoke should be extended and their provisions duly en- 

 forced. 



One part of the Report deals with a matter to which 

 we attach the greatest importance. It is suggested that 

 there should be some place which the public can visit 

 and where they may examine any apparatus that is 

 approved of, or which they may wish to purchase ; but 

 above this it is pointed out that a place is requisite where 

 scientific, chemical, and other tests may be made for the 

 information of the public generally, but especially for the 

 help of inventors and manufacturers who may wish to 

 try new suggestions. The Report also suggests that in 

 connection with this there should be some place for dis- 

 cussion and public lectures, for the general advancement 

 and diffusion of knowledge touching smoke abatement. 

 The third proposal is certainly the most doubtful one, but the 



