October 2, 1898.] 



KNOWLEDGE. 



195 



surface of the plate. It will bo seen that the height 

 of the curve seems to correspond with the intensity 

 of the photographic action, and to confirm the theory 

 with which we set out." In one total solar eclipse 

 photograph taken during the American eclipse of 18G9 

 there was a curious spot of light upon the dark body 

 of the moon, which was spoken of by one writer as 

 probably duo to a comet between us and the moon. It 

 was situated immediately beneath a bright prominence on 

 the sun's limb, and was evidently due to the reflection 

 of the light of the prominence from the inner surface of a 

 drop of water on the back of the plate during the exposure. 

 The wet collodion process was used in those days. Now 

 that dry plates are used, comets on the moon are less likely 

 to be discovered. 



Figs. 2 and 3 have been made from photographs kindly 

 sent me by Mr. M. Glover, of Stephen's Green, Dublin. 

 They represent the image of the sun setting over the 

 roofs in the neighbourhood of his studio, and show the 

 ring of light reflected from the back of the photographic 

 plate encroaching upon the dark objects on the horizon. 

 Owing to the size of the sun's disc the inner edge of the 

 nebulous ring is not quite as sharply defined as in the case 

 of nebulous rings about the over-exposed images of stars 

 and artificial points of light. 



THE CONSTITUTION OF GASES. 



By J. J. Stewart, B.A.Cantab. 

 (Contimu'd from paye 55.) 



WHEN a solid is raised to such a temperature 

 that it becomes luminous, and the light which 

 it gives out is viewed through a spectroscope, 

 a continuous spectrum is observed which 

 consists of an uninterrupted band of light, 

 and which extends, if the luminous body is hot enough, 

 from the dark red rays at one end to the violet at the 

 other. When a gas, on the other hand, is raised to a high 

 temperature it gives a very different sort of spectrum. 

 This consists of a series of biright lines, or sometimes 

 bands, separated by intervals of darkness, the position of 

 the lines being characteristic of the particular gas. In 

 some cases the luminous bands are very numerous, as in 

 the spectrum of nitrogen ; in others the spectrum is of a 

 very simple character, as that of sodium vapour, which, 

 under certain conditions, consists of only two narrow 

 yellow lines close together. 



The spectra of hot gases are explained by the vibrations 

 set up in the free gaseous molecules. When the tempera- 

 ture of a gas is raised the velocity of its particles increases, 

 and the encounters between them become more numerous 

 and energetic. After a collision two molecules may be 

 imagined rebounding from each other in a state of vibra- 

 tion due to the shock. The molecules of each gas are 

 capable of vibrating in certain definite modes, which are 

 characteristic of the particular gas, and these \ibrations, 

 being communicated to the luminiferous ether, give rise to 

 the definite rays in the spectrum of the gas. 



It is a remarkable fact that when the rays from a bright 

 source of light, such as incandescent lime, are made to 

 pass through a layer of cooler gas, the spectrum produced 

 has dark lines occupying the same places as would be 

 fiUed by the bright lines occurring in the spectrum of 

 the gas in question when heated. The dark Imes in 

 the solar spectrum are an example of this, the layer of 



* A further acco\int of tlicse experiments will be found iu the 

 Memoirs of the Soi/al Astronomical Hociet^, Vol. XL VI., pp. 2.30 — 2.33. 



cooler gas here being found in the region outside the sun's 

 photosphere. The occurrence of these dark lines is due to 

 the absorption by the cold gas of a part of the energy in the 

 luminous waves passing through it. The molecules of the 

 gas are capable of absorbing those vibrations which they 

 are able to give out when they themselves are heated. 

 An analogous phenomenon is the resonance of a musical 

 string. The stretched string can be made to vibrate when 

 the note which it is capable of giving is sounded in its 

 neighbourhood, and a set of such strings can take up from 

 a mixture of notes those vibrations which they themselves 

 would yield, and thus they absorb a portion of the energy 

 of the vibrating air. Even when the layer of gas is 

 sufficiently hot to give out light, the dark lines are seen 

 when the source behind it is hotter than the gas, as the 

 rays given out from the gas, being less intense than the 

 neighbouring rays which are not affected by it and pass 

 through it, appear dark in comparison. 



On the fact of the possession by each gas of a 

 characteristic spectrum the methods of spectrum analysis 

 dei^end. When a compound under investigation can be 

 vaporized, the presence of various elements in it may be 

 detected by observing the position of the lines in the 

 spectrum and comparing these with the known position of 

 the lines in the spectra of the different elements. Such 

 methods are being increasingly used in astronomical 

 investigations, as giving an insight into the constitution 

 of the fixed stars and nebulae. 



The molecules of a gas during the greater part of their 

 course move freely and independently of neighbouring 

 molecules, but in the case of liquids a molecule never gets 

 beyond the influence of its neighbours so long as it remains 

 part of the liquid. The particles of the liquid move with 

 various velocities, and when the liquid has a fi-ee surface 

 it may happen that some of the molecules on or near that 

 surface have such a high velocity that they are shot right 

 out of the liquid and move about freely in the space above 

 it. This is what takes place when evaporation is going on. 

 When the temperature of the liquid is raised the rapidity 

 of movement of its molecules is increased, and there are a 

 greater number in the surface layer which have a high 

 enough velocity to get outside the sphere of influence of 

 their neighbours — that is, evaporation takes place more 

 rapidly. Amongst the molecules of the vapour thus 

 produced above the liquid, some in the course of their 

 movements approach so near the liquid surface that they 

 become entangled amongst the liquid particles and are 

 re-absorbed into the liquid. This entering and leaving of 

 the molecules is constantly going on at every exposed 

 liquid surface, and evaporation only takes place when the 

 number of molecules which escape from the liquid is 

 greater than that of those which re-enter it. When the 

 number absorbed is in excess, condensation occurs. But 

 even when the mass of the liquid remains unaltered and 

 neither evaporation nor condensation seems to be taking 

 place, this movement of molecules outward and mward is 

 still going on, only now as many molecules re-enter as 

 leave the liquid ; what has been called a state of movable 

 equilibrium has been set up. 



The solution of a gas in a liquid is a similar phenomenon. 

 When the gas-molecules reach the surface of the liquid, 

 they are retained there, on account of the attraction of 

 the molecules of the liquid on those of the gas. Some of 

 the molecules which have thus entered the liquid, on 

 returning to its surface with a high velocity are able to 

 leave it, and, as before, equilibrium ensues when the 

 number entering and leaving is the same. When the 

 pressure of the gas alters, the number of molecules striking 

 the liquid surface will change in the same proportion, and 



