ADDRESS. 23 



highest temperature, and for this reason does not reveal itself by absorp- 

 tion in the solar spectrum. 



It is not unreasonable to assume that the two brightest lines, which 

 are of the same order as the third line, are produced by substances of a 

 similar nature, in which a vibratory motion corresponding to a very high 

 temperature is also necessary. These substances, as well as that repre- 

 sented by the line D3, may be possibly some of the unknown elements 

 which are wanting in our terrestrial chemistry between hydrogen and 

 lithium, unless indeed D3 be on the lighter side of hydrogen. 



In the laboratory we must have recourse to the electric discharge to 

 bring out the spectrum of hydrogen ; but in a vacuum-tube, though the 

 radiation may be great, from the relative fewness of the luminous atoms 

 or molecules or from some other cause, the temperature of the gas as 

 a whole may be low. 



On account of the large extent of the nebulte, a comparatively small 

 number of luminous molecules or atoms would probably be sufficient to 

 make the nebute as bright as they appear to us. On such an assumption 

 the average temperature may be low, but the individual particles, which 

 by their encounters are luminous, must have motions corresponding to 

 a very high temperature, and in this sense be extremely hot. 



In such diffuse masses, from the great mean length of free path, the 

 encounters would be rare but correspondingly violent, and tend to bring 

 about vibrations of comparatively short period, as appears to be the case 

 if we may judge by the great relative brightness of the more refrangible 

 lines of the nebular spectrum. 



Such a view may perhaps reconcile the high temperature which the 

 nebular spectrum undoubtedly suggests with the much lower mean tem- 

 perature of the gaseous mass, which we should expect at so early a stage 

 of condensation, unless we assume a very enormous mass ; or that the 

 matter coming together had previously considerable motion, or consider- 

 able molecular agitation. 



If the hydrogen shown by the spectroscope in the nebulae and in the 

 atmospheres of the stars is retained by these bodies, we should be able to 

 assign approximately an inferior limit for the force of gravity at their 

 surfaces ; provided that we assume that the gas is in the uncombined 

 state, and always exists in some greater proportion than in the free space 

 about them. 



The inquisitiveness of the human mind does not allow us to remain 

 content with the interpretation of the present state of the cosmical masses, 



but suggests the question — 



' What see'st thou else 

 In the dark backward and abysm of time ? ' 



What was the original state of things ? how has it come about that by 

 the side of ageing worlds we have nebulse in a relatively younger stage ? 

 Have any of them received their birth from dark suns, which have col- 



