CELESTIAL CHEMISTRY. 223 



planets, with their satellites, are composed of the self-same 

 elements. 



In this place, it is interesting to refer to a theoiy by which such 

 facts may be accounted for, and the reason of the similarity 

 shown. The nebular hypothesis assumes that our solar system 

 was at onetime a gaseous mass, extending beyond the orbit of the 

 faithest planet, Neptune. Its composition was necessarily uniform 

 throughout, for the tendency of gases to diffuse into one another, or 

 intermingle, would have free play. In this nebula the tempera- 

 ture was very high, for the elementaiy bodies were in a vaporous 

 state in it, just as they are at present in the sun. But as soon as 

 the mass commenced to lose its heat, there were established cur- 

 rents and a general movement of rotation, and on the exterior a 

 shell, or, rather, equatorial band of condensed materials began to 

 form. The cooling and consequent contraction still continuing, 

 the band was left behind, but it sooner or later broke, in one or 

 more places, and aggregated into one or more globular masses, 

 which continued their rotation as planets. 



Tlu3 same thino' occurring several times in succession, and rings 

 of molten matter being left behind by the contracting gaseous 

 mass, as it lost its heat, eventually all the planets, as we now see 

 them, were formed, and the remainder of the nebula is the sun, 

 still preserving the form partly of ignited gas, and partly, prob- 

 ably, of a liquid or solid. It is, however, even now radiating its 

 heat away and cooling, though slowly. After, perhaps, giving 

 off a few more planets, whose orbits will not exceed in diameter 

 his present size, the sun, according to the hypothesis, will be no 

 longer visibly hot, and life on the planets will come to an end. 



This celebrated hypothesis has been very freely discussed, and 

 has received much adverse criticism. Many strong objections have 

 been urged against it, but the spectroscope confirms it. The 

 reader will not be able to appreciate the full value of this support, 

 until the constitution of the nebulae visible in the heavens has 

 been spoken of. It will, therefore, be reserved for that place. 



But let us not confine ourselves in these observations to our own 

 solar system. Let us see whether this little insti-ument, which is 

 scarcely anything more than a small triangular piece of glass, 

 will not enable us to establish a relationship with more distant 

 bodies than the sun and planets, — with other solar systems far 

 away in the abysses of sjiace. 



To the naked eye, there appear scattered over the sky at night 

 a multitude of stars of various colors. Even in our best tele- 

 scopes they are only glittering points, and no glimpse of their 

 chemical constitution could be presented before the spectroscope 

 was applied to investigate them. We were satisfied that they 

 shone by their own light, that they were suns, that they presented 

 many analogies to our solar system, and also many dissimilari- 

 ties. 



The stars, both single and double, when examined by the spec- 

 troscope, are observed to contain substances well known to us. 

 One of them, Arcturus, closely resembles our sun, as has been 

 shown by Rutherford. At once we perceive a fellowship between 



