Nov. 3, 1882.] 



♦ KNOWLEDGE ♦ 



371 



W 



A PROBLEM IN ATOMIC PHYSICS. 



By Prof. J. Tyndall. 



JE must refresh ourselves by occasional contact 



cith the solid ground of experiment, and an in 

 teresting problem now lies before us awaiting experi- 

 mental solution. Suppose 200 men to be scattered equably 

 throughout the lengtli of Pall Mall. By timely swerving 

 now and then, a runner from St. James's Palace to the 

 Athenseum Club might be able to get through such a 

 crowd without much hindrance. But supposing the men 

 to close up so as to form a dense file crossing Pall Mall 

 from north to south : such a barrier might seriously 

 impede, or entirely stop, the runner. Instead of a crowd 

 of men, let us imagine a column of molecules under small 

 pressure, thus resembling the sparsely distributed crowd. 

 Let us suppose the column to shorten, without change in 

 the quantity of matter, until the molecules are so squeezed 

 together as to resemble the closed file across Pall Mall. 

 During these changes of density would the action of the 

 molecules upon a beam of heat passing among them at all 

 resemble the action of the crowd upon the runner 1 



We must answer this question by .direct experiment. 

 To form our molecular crowd we place, in the first instance, 

 a gas or vapour in a tube SB in. long, the ends of which 

 are closed with circular windows, air-tight, but formed of 

 a substance which offers little or no obstruction to the 

 calorific waves. Calling the measured value of a heat- 

 beam passing through this tube 100, we carefully determine 

 the proportionate part of this total absorbed by the mole- 

 cules in the tube. We then gather precisely the same 

 number of molecules into a column lO'S in. long, the one 

 column being thus three and a half times the length of 

 the other. In this case also we determine the quantity of 

 radiant heat absorbed. By the depression of a barometric 

 column, we can easily and exactly measure out the proper 

 quantities of the gaseous body. It is obvious that one 

 mercury inch of vapour, [^in the long tube, would represent 

 precisely the same amount of matter — or, in other words, 

 the same number of molecides — as 3\ in. in the short one ; 

 while 2 in. of vapour in the long tube would be equjivalent 

 to 7 in. in the short one. 



The experiments have been made ■^ith the vapours of 

 two very volatile liquids — namely, sulphuric ether and 

 hydride of amyl. The sources of radiant heat were, in 

 some cases, an incandescent lime cylinder, and in others a 

 spiral of platinum wire, heated to bright redness by an 

 electric current. One or two of the mea.surements will 

 suffice for the purposes of ilustration. First, then, as 

 regards the lime light. For 1 inch of pressure in the long 

 tube, the absorption was 18 4 per cent, of the total beam ; 

 while for 3-5 inches of pressure in the short tube, the 

 absorption was 18-8 per cent,, or almost exactly the same 

 as the former. For 2 inclies pressure, moreover, in the 

 long tube, the absorption was 2.")-7 per cent.; while for 

 7 inches, in the short tube, it was 2.tG per cent, of the 

 total beam. Thus closely do the absorptions in the two 

 cases run together — thus emphatically do tlio molecules 

 assert their individuality. As long as their number is 

 unaltered, their action on radiant heat is unchanged. 

 Passing from the limo-light to the incandescent spiral, the 

 absorptions of the smaller equivalent quantities in the two 

 tubes were 23-5 and 23( per cent. ; while the absorptions 

 of the larger equivalent quantities were 32'1 and 32G per 

 cent, respectively. This constancy of absorption, wlien the 

 density of a gas or vapour is varied, I have called " tlio 

 conservation of molecular action." 



But it may be urged that the change of density, in these 

 experiments, has not been carried far enough to justify 



the enunciation of a law of molecular physics. The con- 

 densation into less than one-third of the space does not, it 

 may be said, (juite represent the close file of men across 

 Pall Mall. Let us therefore push matters to extremes, 

 and continue the condensation till the vapour has been 

 squeezed into a liquid. To the pure change of density we 

 shall then have added the change in the state of aggre- 

 gation. The experiments here are more easily described 

 than e-xecuted ; nevertheless, by suflScient training, scru- 

 pulous accuracy, and minute attention to details, success 

 may be ensured. Knowing the respective specific gravities, 

 it is easy, by calculation, to determine the condensation 

 requisite to reduce a column of vapour of definite density 

 and length to a layer of liquid of definite thickness. Let 

 the vapour, for example, be that of sulphuric ether, and 

 let it be introduced into our 38-in. tube till a pressure of 

 7 '2 in. of mercury is obtained. Or let it be hydride of amyl, 

 of the same length, and at a pressure of G-6 in. Supposing 

 the column to shorten, the vapour would become proportion- 

 ally denser, ard would, in each case, end in the production 

 of a layer of liquid exactly 1 millimetre in thickness.* 

 Conversely, a layer of liquid ether, or of hydride of amyl, 

 of this thickness, were its molecules freed from the thrall 

 of cohesion, would form a column of vapour 38 inches 

 long, at a pressure of 7 2 inches in the one case, and of 

 6 6 inches in the other. In passing through the liquid 

 layer, a beam of heat encoimters the same number of mole- 

 cules as in passing through the vapour layer, and our 

 problem is to decide, by experiment, whether in both cases 

 the molecule is not the dominant factor, or whether its 

 power is augmented, diminished, or otherwise overridden 

 by the state of aggregation. 



Using the sources of heat before mentioned, and em- 

 ploying diathermanous lenses, or silvered mirrors, to render 

 the rays from those sources pai-allel, the absorption of 

 radiant heat was determined, first for the liquid layer, and 

 then for its equivalent vaporous layer. As before, a 

 representative experiment or two will suffice for illus- 

 tration. When the substance was sulphuric ether, and 

 the source of radiant heat an incandescent platinum spiral, 

 the absorption V)y the column of vapour was found to be 

 66'7 per cent of the total beam. The absorption of the 

 equivalent liquid layer was next determined, and found 

 to be 67 '2 per cent. Liquid and vapour, therefore, 

 differed from each other only 5 per cent. : in other 

 words, they were practically identical in their action. 

 The radiation from the lime-light has a greater power of 

 pe]ietration through transparent substances than that from 

 the spiral. In the emission from both of these sources 

 we have a mixture of obscure and luminous rays ; but 

 the ratio of the latter to the former, in the lime-light, is 

 greater than in the spiral ; and, as the very meaning of 

 transparency is perviousness to the luminous rays, the 

 emission in which these rays are predominaiit must pass 

 most freely through transparent sul)stances. Increased 

 transmission implies diminished absorption, and, accord- 

 ingly, the respective absorptions of either vapour and liquid 

 ether, when the limelight was used, instead of being 66'7 

 and 67 2 per cent., were found to be — 



V.apinir 3;?-3 per cent 



Liquid 33 3 „ 



no difference whatever being observed between the two 

 states of oggregation. This same was found true of liydride 

 of amyl. 



This constancy and continuity of the action exerted on 

 the waves of heat when the state of aggregation is changed, 



The millim^tro is ^^th of on inch. 



