Jan. 20, 18S1] 



NA TURE 



265 



evaporation is always more encumbered, partly, no doubt, 

 because its evaporating surface is a fixture. The only 

 limit to the rise of temperature of a liquid is its boiling, but 

 if this be prevented it may get superheated ; and, [unless the 

 solid boil (i.e. disintegrate internally) it can become superheated 

 to any extent. The possibility of this internal disintegration we 

 will examine directly, but at present we will consider it practi- 

 cally nil. 



Let us grant then that a publiming solid always rises in tempe- 

 rature if heated at a sufficient rate, and Dr. Carnelley's propositiL n 

 follows. 



We have seen that no liquid can exist at temperatures below 

 its freezing- or above its boiling-point, so that if we wish to 

 prevent the possibility of its existence, we need only make these 

 two pointsjcoincide. This can always be done by diminishing 

 the pres-ure, for the boiling-point of all .substance-; is greatly 

 affected by changes of pressure, nhile the freezing-joint is only 

 slightly altered, and even^ in the case of ice, in- the opposite 

 direction. 



Start then with the solid below its melting-point, and reduce 

 the pressure on it till the boiling-point coincides with, or parses 

 below the melting-pnint. There is now no region where liquid 

 can exist, and the sulid must therefore sublime ; but, by our 

 supposition, a subliming solid if heated will get hot, hence the 

 solid may now assume any temperature you please ; and the 

 hotter it gets the more pressure may be lirought to hear upon it 

 without causing it to melt, i.e. the pressure may be allowed to 

 increase to anything short of the vapour-tension at the new 

 temperature. If heated sufficiently, then the whole atmospheric 

 pressure may be let in, and no melting will occur. All that is 

 necessary is that heat shall be supplied at a sufficient rate to 

 compensate for the rapid evap nation (which however will not 

 be so rapid as in the vacuum), and to prevent its temperature 

 falling to the boiling-point ; for if it reached this, part (or all) 

 would quickly liquefy, and the whole fall to (or towards) the 

 melting-point. 



Thus we have the remarkable proposition that if, by the pro- 

 cess of lowering the boiling-point to coincide with or pass beluw 

 the melting-point, we manage to get ice across the gap which 

 ordinarily separates the e two pomts, it may he heated to I20° 

 or to any other temperature ; and that when at 120^ it wdl be 

 permanent, and will not melt even under the whole pressure of the 

 atmosphere. To prevent its melting you must keep on heating 

 it : if allowed to cool to 100°, five-ei;4hths of it will lie instantly 

 crushed to water, and the whole will be at 0° (assuaiing, what 

 is not likely to be correct, that the specific heat of hot ice is \). 



There is still the question of the pcssibdity of internal melting 

 or sublimation to be considered. 



Now I suppose that if a solid is perfectly homogeneous, a 

 change of state in its interior would with great difficulty occur, 

 and the solid might readily be superheated. But an excess of 

 pressure at any point, such as v ould he produced by a bubble of 

 air, would readily determine a melting -centie. In Prof. T)n 

 dall's ice-tlower experiment the miclei are probably minute 

 bubbles of air, and the ice v\':;lls of the cavities so produced are 

 suliject to the pressure of this air in addition to that of the 

 vap:)nr ; and accordingly melting sets in and spreads. But Dr. 

 Carnelley's ice is formed in vacuo, so that no air-bubbles are pos- 

 sible, and the only nuclei that can properly exist are little 

 bubbles of enclosed vapour ; and the^e, I imagine, can scarcely be 

 absent. Let us inquire then what can happen in the case of one 

 of these bubbles when the temperature o*^ the ice is raised either 

 by radiation or conduction. Initially, while the temperature is 

 constant, the vapour is saturated ; but no liquid is formed beciuse 

 this temperature is below the melting-p 'int. When heat is 

 applied, the ice, being less diathermanous than the vapour, will 

 get heated first, and so long as ihe temperature keeps rising it will 

 always be a little hotter than the vapDur, which consequently is 

 not quite saturated, and the presure it exerts is less than the 

 " vapour-tension " (/.;■. the temperature is above the boiling point), 

 and no water can be formed. The cavity v\ill of course enlarge 

 by sublimation, but very slowly, much more slowly in fact than 

 outside, if a vacuum is there artificially maintained. 



But if cooling be permitted the ice will cool the fastest ; and 

 the vapour at once becomes over-saturated and condenses. The 

 temperature is now below the boiling-point, and liquefaction 

 instantly sets in and rapidly spreads, the ice consuming its own 

 heat in the process. 



Internal disintegration therefore will not occur while the tem- 

 perature is rising, but it w ill set in at a great pace if it be allowed 



to become stationary or to fall, unless there be an utter absence 

 of nuclei. If the temperature rises very high the pressure of the 

 internal vapour will of course be great, and ultimately might 

 even.be able to burst the ice, but this would scarcely occur under 

 several atmospheres. 



It would be interesting if Dr. Carnelley would kindly try the 

 following experiments : — 



1. Heat ice in vtuuo with a pressure gauge, and, still heating 

 it, stop the passage to the condenser so that the pressure is 

 allowed to accumulate, and note the pressure and temperature 

 when collapse occurs. 



2. Heat ice up to any temperature, and, still maintaining a 

 I good vacuum, remove the supply of heat, and see if the ice does 



not collapse. 



3. Heat the ice up to 120°, and, still heating it, let in the atmo- 

 sphere gently (but make the air come in through hot pipes, I'r it 

 will melt the ice), and see if the ice does not last rather longer 

 than it would have done in the vacuum, because the evaporation 

 will be more obstructed. Bat if the second experiment succeed, 

 the temperature must never be allowed to fall much or to remain 

 stationary long. 



Finally, it is important to point out explicitly that the Carnelley 

 experiment has no bearing on the change of the melting-point of ice 

 with pressure. Our knowledge on this point remains as it was-, viz. 



that the value of -- about zero centigrade is - "oo?! ; that is to 



dp 

 say, the melting-point rises and falls about '0071° centigrade per 

 atmosphere of pressure decrease or increase. 



Of course this number is not absolutely constant, but its varia- 

 tion with pres ure is very slight, and moreover has no bearing 

 on the Carnelley experiment, as was naturally but err.ineously 

 .supposed by Prof. Pcttersson in the Beiichte (18), and 1 believe 

 also by Prof. Ayrton at the Chemical Society, though I had not 

 the pleasure of hearing his remarks. 



University College, London Oliver J. Lodge 



Note. — With reference to the above second experiment and 

 the reasoning which suggested it, it is important to remark that 

 I have all along assumed that the vapour-tension of ice at any 

 temperature is jirecisely the same as that of water at the same 

 temperature. But Prof. Foster considers it possible that the 

 vapour-tension of ice may be less than that of water, and would 

 hence explain the permanence of vapour inside an ice-cavity 

 without attending to whether the temperature were rising or falline, 

 provided it were not falling too fast. This would be a most 

 important fact to discover and verify ; but I think the Carnelley 

 experiment in its present form does not inform us concerning its 

 truth or falsity. 



Another thing it may be interesting to note is the rate of 

 variation of boiling-point with pressure at difltrent temperatures, 

 which can be calculated on thermodynamic principles (aftrr 

 Prof. James Thomson) from empirical data for the latent belt 

 of steam, and for the density of saturated steam at any tempe- 

 rature. 



It is, at the temperature fl and the pressure />, — 



d6 _ fl2 



dp 273 X -cooS X (796-2 - -6956) />' 

 a fraction which has the value 28 at 100° C, and 2180 at 0° C. : 

 these numbers represent the rate of rise or fall of boiling- 

 P"int in centigrade degrees per atmosphere increase or decrea e. 

 Integrating this equatioii, we get the value of the vapour- 

 tension/ of water in atmospheres (mejadynes per square centi- 

 metre) at any absolute temperature 8, viz : — 



log / = 9172S \ -695 log '^ -f 796-2 Z' i- - iM 

 ' e V373 e/ ) 



the logarithms being to the base e. 



On the Spectrum of Carbon 

 I HAVE a great respect for Dr. Watts's spectroscopic work, 

 nevertheless the experiments he has described in Nature, 

 vol. xxiii. p. 197, appear to me singularly inconclusive ivx the 

 purpose for w hich he has adduced them. How could any one 

 expect to get a tube of g.as free from hydrocarbons when the 

 joints were of india rubber and melted paraffin ? I have long 

 since found it necessary to forego rubber joints if I would 

 exclude hydrogen. Salet has shown that the hydrocarbons from 

 the blowpipe-flame used in sealing in wires, &c. , and the last 

 traces of dust, can only be removed from tubes by burning them 

 out in a current of oxygen. But more than this, 1 have found 



