I90 



NATURE 



[December 8, 1910 



of flame for which this would be substantially true can be 

 drawn from Callendar's experiments, because he was look- 

 ing along a thin row of flames in which there was but 

 little lateral extension. 



The flames met with in a gas-engine cylinder or in 

 explosion vessels differ from open flames such as can 

 readily be produced in the laboratory, both in respect of 

 the lateral extension which has just been mentioned and 

 also in respect of density. In both these particulars the 

 difference is rather great, the least dimension of the mass 

 of flame in a gas-engine cylinder being only in the smallest 

 sizes comparable with the diameter of the Meker burner 

 flame, while the density of the gas just after firing in the 

 gas engine is from twenty to thirty times that of the 

 burner flame gases. It does not seem possible from 

 theoretical considerations to determine the effect of these 

 two factors with sufficient accuracy to enable any quanti- 

 tative inference as to radiation in the gas engine to be 

 drawn from laboratory experiments on flames, but it is 

 useful to discuss their probable qualitative effects. 



In Fig. 1, P is the point of observation at which the 

 pyrometer is placed, as in Callendar's experiments, and 

 the portion of the flame from which the radiation is 

 measured is that intercepted by the small cone. If a 

 second similar flame B is placed behind h at a consider- 

 able distance, but so that it is intersected by the cone, 

 then the radiation recorded by the pyrometer will be 

 increased, say, by 50 per cent., showing that of the radia- 

 tion emitted by B and falling on A 50 per cent, is absorbed 

 and the remainder is transmitted to the pyrometer. The 

 absorbed energy is, of course, not lost, but must result 



in ' slightly increased radiation from A in all directions. 

 The flame .\ appears to be a little hotter because of the 

 proximity of B. Thus the increase of radiation absorbed 

 at the pyrometer is due, not only to the radiation trans- 

 mitted from B, but also to an increase in the intrinsic 

 radiance of A. If the two flames are a considerable 

 distance apart, the latter part is negligibly small, since 

 the flame A does not then receive much radiation from 

 B, and what it does receive is dissipated in every direc- 

 tion. But when flame B is pushed close up to A into the 

 position of B' (Fig. 2) this effect may be considerable, 

 and it is obvious that it will be greatly enhanced if the 

 two flames are extended laterally as in Fig. 3. For in 

 such case flame A must get rid of the energy which it 

 is receiving by radiation from B', mainly by an enhanced 

 radiation in the direction of P. It may therefore be 

 expected that the effect of lateral extension will be to 

 make the flame apparently more transparent. 



To a first approximation it may be expected that the 

 radiating and absorptive powers of a gas at a given 

 temperature will be proportional to its density. That is 

 to say, two geometrically similar masses of flame, in 

 which the temperatures at corresponding points are the 

 same, and the densitties in inverse proportion to the 

 volumes (so that the total masses are the same), will 

 radiate in the same way and to the same total amount. 

 \t would seem that this must be so, so long as the vibra- 

 tions of the radiating molecules are the same in character 

 and amplitude in the two cases. For there will then be 

 the same number of molecules vibrating in exactly the 

 same way and arranged in the same way in the two cases. 



NO. 2145, VOL. 85] 



The only difference is in the scale of the arrangement, and 

 this can only affect the matter if the distance between 

 molecules is comparable with the wave-lengths of thp 

 radiation emitted, which is not the case. It is only, how 

 ever, within moderate limits that the molecular vibratio: 

 are independent of density. Angstrom found that tl 

 absorption of the radiation from a given source in a tiii 

 of COj at ordinary temperature and atmospheric pressm 

 was reduced by increasing the length and diminishing th. 

 pressure ' in the same proportion so as to keep the mas>. 

 of gas constant. Schafer found that on increasing the 

 pressure the absorption bands of this gas were widened, 

 so that the curve connecting intensity of radiation ant 

 wave-length did not remain of the same shape. ^ Th' 

 experiments were made at low temperatures, and at tl 

 higher temperatures, in which the committee are more 

 particularly interested, there has been but little work. 

 There is no reason to doubt, however, that the charact' 

 and amount of the radiation from CO, and steam 

 high temperatures will change with the density. 



From the point of view of the molecular theory, such 

 a change might be anticipated from either of two causes. 

 An increase of density implies a proportionate increase in 

 the frequency of molecular collisions, and this wou]<l 

 result in greater facility of interchange between the trans- 

 lational and atomic types of energy. It is possible thai 

 the equilibrium proportion of the two types might b'- 

 different in consequence. The denser gas may conceivabl\ 

 possess, with a given amount of translational enerfjv . 

 more atomic energy, and therefore radiate more stronj:' 

 at a given temperature. It is certain that there would 1 

 a more rapid attainment of equilibrium in the gas aft' - 

 an explosion or a rapid expansion. Another pKjssible cau'-'- 

 is a direct interaction between the molecules, apart from 

 collisions. Two molecules at a sufficient distance apart 

 will vibrate practically independently, each behaving as 

 though the other was not there, except that there will be 

 a tendency for them to vibrate in the same phase. But 

 if the two are close together they react on one another 

 so that the natural period or periods of the two together 

 will not be the same as those which each would have if 

 it were isolated. 



Such direct measurements as have been made of the 

 radiation after a closed-vessel explosion suggest that the 

 flame is more transparent than might be inferred from 

 the experiments on open flames. According to information 

 given to the committee by Prof. Hopkinson, \V. T. David 

 has found that the radiation received by a bolometer placed 

 outside a fluorite window in the cover of a cylindrical 

 explosion vessel 30 cm.x^o cm. is greatly increased bv 

 highlv polishing that portion of the oooosite cover which 

 can be " seen " by the bolometer. This implies that a 

 thickness of 30 cm. of flame in these circumstances cart' 

 transmit much of the radiation which it emits. The 

 density of the gas in this case was atmosphpric, and the 

 30 cm. thickness in the explosion vessel would be equiva- 

 lent to perhaps 150 cm. of open flame if absorntion were 

 simplv proportional to density. .According to Callendar's 

 experiment, such a thickness would be almost completely 

 opaque. It is possible that the lateral extension is 

 sufficient to account for this result. The open flame 

 should be a cylindrical mass of dimensions 150 cm.X 

 150 cm., instead of a long strip with a cross-section of 

 3 cm., in order to make the two cases strictlv comnarable. 

 It will be remembered that in the discussion above it 

 appeared that the laterally extended flame would seem to 

 be more transparent. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



CAMBRIDGE. — The Walsingham medal for iqio has beeh 

 awarded to A. V. Hill, of Trinity College. A second me«j 

 has been awarded to J. C. F. Fryer, of Gonville and Ca 

 College. 



The followinef have been elected to the Clerk Max\ 

 scholarship : — R. D. Kleeman and R. T. Beatty, both 

 Emmanuel College. 



At a meeting of the Fitzwilliam Museums syndicate, 



' Ark. for Mat. Astron. oc^ Fys^V. StocUho'.rr, vol. iv., No. 30, p. i. 

 2 Ann. der Physik, vol. xvi. (1905), p. 93. 



