86 



CHEMISTRY. 



force by reduction of the thickness to the ten- 

 thousandth of a millimetre ; inasmuch as the 

 thickness, which gives the first maximum 

 brightness round the black spot seen where 

 the bubble is thinnest, is only about an eight- 

 thousandth of a millimetre. 



The very moderate amount of work shown 

 in the preceding estimates is quite* consistent 

 with this deduction. But suppose now the 

 film to be further stretched, until its thickness 

 is reduced to a twenty-millionth of a millime- 

 tre. The work spent in doing this is two thou- 

 sand times more than that which we have just 

 calculated. The heat equivalent is eleven 

 hundred and thirty times the quantity re- 

 quired to raise the temperature of the liquid 

 by one degree centigrade. This is far more 

 than we can admit as a possible amount of 

 work done in the extension of a liquid film. 

 A smaller amount of work spent on the liquid 

 would convert it into vapor at ordinary at- 

 mospheric pressure. The conclusion is una- 

 voidable, that a water- film falls off greatly in 

 its contractile force before it is reduced to a 

 thickness of a twenty-millionth of a millime- 

 tre. It is scarcely possible, upon any con- 

 ceivable molecular theory, that there can be 

 any considerable falling off in the contractile 

 force as long as there are several molecules in 

 the thickness. It is therefore probable that 

 there are not several molecules in a thickness 

 of a twenty -millionth of a millimetre of water. 



The author then cites what is called the 

 Kinetic theory of gases, worked out by Hera- 

 path, Joule, Clausius, and Maxwell. By a 

 simple application of the calculus of probabili- 

 ties, Olausius proved that the average length 

 of the free path of a nwlecule of gas, from col- 

 lision to collision, bears to the diameter of each 

 molecule the ratio of the whole space in which 

 the atoms move, to eight times the sum of the 

 volume of the atoms. It is also known that 

 the average velocity of the molecules, say of 

 oxygen, nitrogen, or common air, at ordinary 

 atmospheric temperature and pressure, is about 

 fifty thousand centimetres per second, and the 

 average time from collision to collision a five- 

 thousand-millionth of a second. Hence the 

 average length of path of each molecule, be- 

 tween collisions, is about 1 6 6 1 6 of a centi- 

 metre. 



Reference is then made to the experiments 

 of Cagniard de la Tour, Faraday, Regnault, and 

 Andrews, as to condensation of gases, which 

 experiments do not allow us to believe that 

 any of the ordinary gases could be made forty 

 thousand times denser than at ordinary at- 

 mospheric pressure and temperature, without 

 reducing the whole volume to something less 

 than the sum of the volume of the gaseous 

 molecules as now defined. Hence, according 

 to the grand theorem of Clausius quoted above, 

 the average length of path from collision to 

 collision cannot be more than five thousand 

 times the diameter of the gaseous molecule ; 

 and the number of molecules in unit of vol- 



ume cannot exceed 25,000,000, divided by the 

 volume of a globe whose radius is that aver- 

 age length of path. Taking now the preceding 

 estimate, T -ornnro f a centimetre, for the aver- 

 age length of path from collision to collision, 

 we conclude that the diameter of the gaseous 

 molecule cannot be less than s 006 1 606()6 of a 

 centimetre ; nor the number of molecules in 

 a cubic centimetre of the gas (at ordinary den- 

 sity) greater than 6 x 10 21 (or six thousand 

 million million million). 



The densities of known liquids and solids are 

 from five hundred to sixteen thousand times 

 that of atmospheric air at ordinary pressure 

 and temperature ; and, therefore, the number 

 of molecules in a cubic centimetre may be 

 from 3 x 10 24 to 10 28 (that is, from three million 

 million million million to a hundred million 

 million million million). From this (if we as- 

 sume for a moment a cubic arrangement of 

 molecules), the distance from centre to nearest 

 centre in solids and liquids may be estimated 

 at from 10006600 to 466ooooo6 f a centime- 

 tre. 



Prof. Thomson presents the grand conclu- 

 sions of his argument thus: "The four lines 

 of argument which I have now indicated, lead 

 all to substantially the same estimate of the 

 dimensions of molecular structure. Jointly 

 they establish, with what we cannot but re- 

 gard as a very high degree of probability, the 

 conclusion that, in any ordinary liquid, trans- 

 parent solid, or seemingly opaque solid, the 

 mean distance between the centres of contig- 

 uous molecules is legs than the hundred- mil- 

 lionth, and greater than the two thousand- 

 millionth of a centimetre. 



"To form some conception of the degree of 

 coarse-grainedness indicated by this conclusion, 

 imagine a rain-drop, or a globe of glass as 

 large as a pea, to be magnified up to the size 

 of the earth, each constituent molecule being 

 magnified in the same proportion. The mag- 

 nified structure would be coarser grained than 

 a heap of small shot, but probably less coarse 

 grained than a heap of cricket-balls." 



In his opening address before Section B of the 

 British association, in September, Prof. H. E. 

 Roscoe says, that although every year has to 

 boast of numerous important and brilliant dis- 

 coveries in chemistry, we are really but very 

 imperfectly acquainted with the fundamental 

 laws which regulate chemical action, and that 

 our knowledge of the ultimate constitution of 

 matter, upon Avhich those laws are based, is of 

 the most elementary nature. In proof of this 

 he refers as follows to the different opinions ex- 

 pressed by leading chemists in recent discus- 

 sions on the subject of the atomic theory. 



The President (Dr. Williamson) delivered a very 

 interesting lecture, in -which the existence of atoms 

 was treated as " the very life of chemistry." Dr. 

 Frankland, on the other hand, states that he cannot 

 understand action at a distance between matter sep- 

 arated by a vacuous space, and, although generally 

 granting that the atomic theory explains chemical 

 facts, yet he is not to be considered as a blind be- 



