EVOLUTION OF THE CHEMICAL ELEMENTS. 531 



identical with, that which is now presented by a large number of 

 irresolvable nebulae whose spectra show them to be composed of 

 gaseous matter in an incandescent state. This gaseous or nebulous 

 condition, though exceedingly rare relatively to the solid forms of 

 matter familiar to us, is nevertheless a state of a high degree of aggre- 

 gation as compared with the forms of matter by which it is surrounded 

 and with its wholly unaggregated state. Before the operations which 

 may be designated as molar can commence, a degree of aggregation 

 must be reached far exceeding that which exists in tiiose molecules 

 which are the vehicles of luminiferous i-adiations. The particles con- 

 stituting the ethereal matter of interstellar space must be supposed to 

 be so minute and relatively far separated as not to exert any ap23reci- 

 able influence upon one another tending to produce molar motion or 

 organization ; a condition which is explained on the same grounds as 

 the fact that one system in space exerts no appreciable influence upon 

 another system. 



If the so-called chemical elements are simply so many stable mo- 

 lecular aggregates, whose differences are due to different modes and 

 degrees of aggregation, then the gases of our earth are simply the 

 most diffused state in which masses of these aggregates can be ob- 

 tained. A gas is a diffused mass of homogeneous molecules, and this 

 definition is as true of the compound gases, steam, carbonic acid, or 

 vapor of alcohol, as it is of the simple ones, such as hydrogen, nitro- 

 gen, or vapor of mercury. It might, then, be naturally supposed that 

 the nebulae would contain a number of such gases, and as it is scarcely 

 to be presumed that all the modes of forming stable aggregates are 

 represented on our planet, so, in addition to some of those found here, 

 it is reasonable to expect that nebulae will contain some not known to 

 us. In so far as the spectroscope to which, indeed, we owe all our 

 positive evidence of the existence of true nebulae is able to inform 

 us, this view is confirmed. Two of our commonest gases, hydrogen 

 and nitrogen, have been identified in nebulae, while' a third has been 

 discovered which has not yet been identified with any known element. 



Every modification of the nebular hypothesis yet put forth has 

 been compelled to assume that the original nebulous mass must be in 

 an incandescent state. Certain it is that all visible nebulae are self- 

 luminous. But this is a condition of their visibility. It can not be 

 known how many may exist which have not yet reached this state, 

 and are, therefore, invisible. It does not seem necessary to suppose 

 that the contraction of a nebulous mass is either due to, or requires, a 

 high temperature. No reason exists why cold particles may not be- 

 come collected into a diffused mass. The inherent motions of these 

 particles are not increased or diminished. But, these motions remain- 

 ing the same, their circuits of motion are reduced, the frequency of 

 contact is increased, and heat and light are evolved from the friction. 

 The tendency of all matter under the law of gravitation, considered 



