August 23, 1888] 



NATURE 



407 



§ 4. — Spectra of Luminous Gases. 



It was first shown by Kirchhoff that glowing gases emit light 

 of the same wave length, and therefore also of the same period, 

 as that which they absorb. 



In the modern theory of gases it is assumed that the molecules 

 of a luminous gas move over a certain distance, the length of 

 the " free path," in straight lines, until they collide with other 

 molecules, or with the sides of the containing vessel, when they 

 move off rectilinearly in another direction. 



At every collision the molecule is subjected to an elastic 

 impulse in a direction passing through its centre, causing 

 internal elastic vibrations. The periods of these vibrations 

 could, on the analogy of a corresponding problem in the theory 

 of elasticity, be calculated from a transcendental equation, if the 

 interior of the molecule were uniformly filled with matter ; 

 according to Thomson's theory of molecular structure they are 

 determined a priori, being the critical periods of the molecule. 

 In fact, during the collisions the external shells only are in 

 contact, but the surrounding ether remains unaffected, and 

 therefore the external vibrations must be of such a nature that 

 I = o (§ 1), which is the condition determining the critical 

 periods. But according to § 1 these periods determine the 

 wave-length of the light absorbed. Thus Kirchhoff's law is a 

 consequence of the theory. 



It has hitherto been assumed that the vibrations in a molecule, 

 arising from the collisions, take place along a fixed diameter, 

 and therefore that the vibrations due to one encounter are not 

 disturbed by a later one in another direction. If the tempera- 

 ture or the density of the gas is so great that the encounters 

 follow one another very rapidly, the investigation of § 1 is no 

 longer applicable, and light-waves of other than the critical 

 periods will be emitted. If a second encounter takes place only 

 after the vibration due to the first has nearly subsided, the 

 period of the emitted light will only differ slightly from a critical 

 period. As the density and temperature increase, the bright 

 lines will therefore gradually increase in width. 1 If a molecule 

 receives impulses in different directions in rapid succession, very 

 few of the vibrations will have the critical periods, and therefore 

 the dark spaces between the bright lines will ultimately dis- 

 appear, and the spectrum become continuous, as is well known 

 to be experimentally true. 



§ 5. — Applications to the Theory of Heat, 



It will be of interest to see what explanation Thomson's mole- 

 cular hypothesis can give of the manner in which the velocity of 

 gaseous molecules can be increased by the action of heat, as has 

 been assumed in what precedes. 



The energy due to the internal molecular vibrations cannot 

 possibly exceed a definite maximum value, for the amplitudes 

 and therefore the velocities of the centres of the shells must have 

 fixed upper limits, since the shells must remain one within the 

 other. This maximum may be attained either for vibrations of 

 a single critical period, or of all the critical periods. Suppose 

 this maximum value to have been nearly reached, then any 

 further disturbance of the internal equilibrium, tending to 

 increase the amplitude of motion of one of the centres beyond 

 the maximum value possible while the centre of gravity remains 

 fixed, will necessarily displace the centre of gravity, whether 

 the disturbance be due to a wave of light or to a mechanical 

 impulse. 



This leads to the general and fundamental proposition that 

 " A molecule will begin to move as soon as the energy of its 

 internal vibrations has attained its maximum value, supposing 

 the external influences to which the attainment of the maximum 

 is due continue to act. 2 



The internal equilibrium of a molecule may be disturbed 

 I either by light or heat, the disturbance in the case of light being 

 due to its action on the critical periods of the molecule. A 

 medium will therefore be heated when traversed by light-rays ; 

 the rays of the critical periods set the molecular shells in vibra- 

 I tion, and when the internal energy has reached its maximum 

 value, the centres of gravity of the molecules will begin to move, 

 and this motion will be perceived as heat. 



This result may be expressed by saying that the characteristic constant 

 C { of the molecule is a function of the temperature. It is preferable to regard 

 the ideal spectrum, whether due to emission or absorption, as something 

 definitely fixed ; external circumstances merely assisting or hindering its 

 formation. 



2 Sir W. Thomson also points out (" Lectures," p. 280) that a considerable 

 ncrease in the internal vibrations of a molecule must set it in motion, and 

 th,rc r ric cav.se a produc'.ion of heat. 



The energy of internal motions therefore accounts for a portion 

 of the internal work of the mechanical theory of heat. ' 



The external work is effected by the motion of the centres of 

 gravity of the atoms, and this takes place in different and known 

 ways in solid, liquid, and gaseous bodies. Heat may act on a 

 medium either by radiation or conduction. Radiant heat differs 

 from light only in its action on our senses, so that what has been 

 said about light will apply also to radiant heat. In the case of con- 

 duction of heat the process is exactly the reverse. The external 

 work of the medium emitting the heat will, be transmitted 

 directly to the medium receiving it by contact— that is, by collisions 

 of molecules. - 



The disturbance of the internal equilibrium of the molecules is 

 here merely a secondary effect, but in this case also the internal 

 energy will gradually increase to the maximum value. 3 



The emission of light by a sufficiently heated solid is explained 

 as in the case of gases, but the spectrum in the case of the solid 

 is continuous. 



Just as the action of heat may produce such violent molecular 

 motion as to cause the emission of all possible kinds of light, so 

 the action of light may produce a molecular motion giving rise 

 to a special kind of light. This will only happen, however, 

 when the molecule (owing to specially favourable values of the 

 constants c t \ and in l ) is specially susceptible to some among its 

 critical periods. In this way the phenomenon of fluorescence 

 may be explained. G. W. DE Tunzelmann. 



(To be continued.) 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society, June 21. — " On the Determination of the 

 Photometric Intensity of the Coronal Light during the Solar 

 Eclipse of August 28-29, 1886. Preliminary Notice." By 

 Captain W. de W. Abney, C.B., R.E., F.R.S., and T. E. 

 Thorpe, Ph.D., F.R.S. 



Attempts to measure the brightness of the corona were made 

 by Pickering in 1870, and by Langley and Smith, independently, 

 in 1878, with the result of showing that the amount of emitted 

 light as observed at various eclipses, may vary within compara- 

 tively wide limits. These observations have been discussed by 

 Harkness (" Washington Observations for 1876," Appendix III.) 

 and they are again discussed in the present paper. Combining 

 the observations, it appears that the total light of the corona in 

 1878 was C072 of that of a standard candle at 1 foot distance, 

 or 3 "8 times that of the full moon, oro - ooooo69 of that of the sun. 

 It further appears from the photographs that the coronal light 

 varied inversely as the square of the distance from the sun's limb. 

 Probably the brightest part of the corona was about 15 times 

 brighter than the surface of the full moon, or 37,003 times fainter 

 than the surface of the sun. 



The instruments employed by the authors in the measurement 

 of the coronal light on the occasion of the solar eclipse of August 

 28-29, 1886, were three in number. The first was constructed 

 to measure the comparative brightness of the corona at different 

 distances from the moon's limb. The second was designed to 

 measure the total brightness of the corona, excluding as far as 

 possible the sky effect. The third was intended to measure the 

 brightness of the sky in the direction of the eclipsed sun. In 

 all three methods the principle of the Bunsen photometric method 

 was adopted, and in each the comparison-light was a small glow- 



1 The discrepancies occurring in the determination of the atomic weights 

 of gases may therefore be explained by assuming that internal work is done 

 by the motions of ths atoms, instead of assuming, as would otherwise be 

 necessary, that the internal work is only done by the motions of the 

 molecules and a decrease in the attractive force between them. For 

 "motion of the atoms" we should have to substitute "motion of the inner 

 spherical shells." 



2 For the method of deducing the differential equation of heat-conduction 

 from these considerations, see F. Neumann, " Vorlesungen tiber die Theorie 

 der Elasticitat," § 59. 



3 Dulong's law of atomic heat gives some information respecting the 

 relative value of this maximum. This law states that the quantity of 

 internal work due to heating is approximately the same, at any rate when in 

 the gaseous state, for elementary bodies which are ordinarily sjlid or liquid, 

 a given number of atoms always requiring the same quantity of heat to 

 produce a given rise of temperature. It follows, then, that for these 

 elements the maximum internal energy is very nearly the same. Carbon, 

 silicon, sulphur, and phosphorus behave exceptionally in this, as in many 

 other respects, and the law is not generally true for the elements which are 

 ordinarily gaseous. Since the maximum value of the internal energy de- 

 pends on the diameter of the molecule, as well as on the constants t , and m? 

 it may perhaps be concluded that the diameter of the molecules of these 

 elements are approximately equal. 



