( IIEMISTRY. 



.i!i the wavo-longth of light Professor 

 Thomson, supporting Cuuchy's view, says that 



il <l\ii:uiiic8 leaves no alternative hut to 

 .i.lniit tint tin- diameter of a molecule, or the 

 .li-i.iiico from the centre of a molecule to the 

 din rn of a contiguous one, in pulpahly homo- 



ns bodies such as gloss or water, or any 



transparent liquid or solid, exceeds the 



."tisaixith of tho wave-length, or thetwo- 

 luiiidred millionth of a centimetre. The au- 

 thor nrxt ivt'rrs to his own experiments show- 

 ing that tho attraction hetween parallel plates 

 of one metal held at a distance apart small in 

 comparison to their diameters, and kept con- 



1 with a galvanic element, would experi- 

 ence an attraction amounting to two ten-thou- 

 saiul-inillionths of a gram weight per area of 

 the opposed surfaces equal to the square of the 

 distance between them. Let a plate of zinc 

 and a plate of copper, each a centimetre square 



a hundred-thousandth of a centimetre 

 thick, be placed with a corner of each touching 

 a metal globe of a hundred-thousartdth of a 

 centimetre diameter. Let the plates kept thus 

 in communication with one another be at first 

 wide apart except at the corners touching the 

 little globe, and let them then be gradually 

 turned round till they are parallel and at a 

 distance of a hundred-thousandth of a centi- 

 meter asunder. In this position they will at- 

 tract one another with a force equal in all to 

 two grammes weight. By abstract dynamics and 

 the theory of energy, it is readily proved that 

 the work done by the changing force of attrac- 

 tion, during the motion by which we have sup- 

 posed this position to be reached, is equal to 

 that of a constant force of two grammes weight 

 acting through a space of a hundred- thousandth 

 of a centimetre ; that is to say, to two hundred- 

 thousandths of a centimetre-gramme. Now 

 let a second plate of zinc be brought by a sim- 

 ilar process to the other side of the plate of 

 copper ; a second plate of copper to theTemote 

 side of this second plate of zinc, and so on till 

 a pile is formed consisting of 50,001 plates of 

 zinc and 50,000 plates of copper, separated by 

 100,000 spaces, each plate and each space one 

 hundred-thousandth of a centimetre thick. 

 The whole work done by electric attraction in 

 tho formation of this pile is two centimetre- 

 grammes. 



The whole mass of metal is eight grammes. 

 Hence the amount of work is a quarter of a 

 centimetre-gramme per gramme of metal. Now, 

 4,080 centimetre-grammes of work, according to 

 Joule's dynamical equivalent of heat, is the 

 amount required to warm a gramme of zinc or 

 copper by one degree centigrade. Hence the 

 work done by the electric attraction could 

 warm the substance by only 16 ] ga of a degree. 

 But now let the thickness of each piece of met- 

 al and of each intervening space be a hundred- 

 millionth of a centimetre instead of a hundred- 

 thousandth. The work would be increased a 

 million-fold unless a hundred-millionth of a 

 centimetre approaches the smallness of a mole- 



cule. The heat equivalent would therefore 

 be enough to raise the temperature of material 

 by 62. This is barely, if at nil, admissible, 

 according to our present knowledge, or, rather, 

 want of knowledge, regarding the heat of com- 

 bination of zinc and copper. But suppose the 

 metal plates and intervening spaces to be made 

 yet four times thinner, that is to say, the thic k- 

 ness of each to bo tho four hundred-millionth 

 of a centimetre. The work and its heat equiv- 

 alent will bo increased sixteen-fold. It would 

 therefore bo 990 times as much as that required 

 to warm tho mass by ten per cent., which is 

 very much more than can possibly be produced 

 by zinc and copper in entering into molecular 

 combination. Were there in reality any thing 

 like so much heat of combination as this, a 

 mixture of zinc and copper powders would, if 

 melted in any one spot, run together, generat- 

 ing more than heat enough to melt each through- 

 out ; just as a large quantity of gunpowder if 

 ignited in any one spot burns throughout with- 

 out fresh application of heat. Hence plates of 

 zinc and copper of a three hundred-millionth 

 of a centimetre thick, placed close together 

 alternately, form a near approximation to a 

 chemical combination, if, indeed, such thin 

 plates could be made without splitting atoms. 



Professor Thomson remarks that, in the blow- 

 ing of a soap-bubble, much is done by the 

 stretching of a film, which resists extension as 

 if it were an elastic membrane. This resist- 

 ance is to be reckoned as a certain number of 

 units of force per unit of breadth, in the soap- 

 bubble. Observation of the ascent of water 

 in capillary tubes shows that the contractile 

 force of a thin film of water is about sixteen 

 milligrammes weight per millimetre of breadth. 

 Hence the work done in stretching a water-film 

 to any degree of thinness, reckoned in millime- 

 tre-milligrammes, is equal to sixteen times the 

 number of square millimetres by which the 

 area is augmented. The author's own experi- 

 ments had proved that, during this process, 

 about half as much more energy in the shape 

 of heat must be given to the film to prevent it 

 from sinking in temperature. Hence the intrin- 

 sic energy of a mass of water in the form of 

 a film kept at constant temperature increases 

 by twenty-seven milligramme-millimetres per 

 every square millimetre added to its area. 



Suppose then a film to be given with a thick- 

 ness of a millimetre, and its area to be aug- 

 mented ten-thousand-and-one fold : the work 

 done per square millimetre of the original film, 

 that is to say, per milligramme of the mass, 

 would be 240,000 millimetre-milligrammes. 

 The heat equivalent of this is more than half 

 a degree centigrade of elevation of tempera- 

 ture of the substance. The thickness to which 

 the film is reduced on this supposition is very 

 approximately a ten-thousandth of a millime- 

 tre. Tho commonest observation on the soap- 

 bubble (which in contractile force differs, no 

 doubt, very little from pure water) shows that 

 there is no sensible diminution of contractile 



