GEOLOGY. 371 



followed their emergence from pressure within the sun. The great 

 nuclei, on the other hand, might have been able to hold even the Ughtest 

 gases at all stages, except that a few particular molecules might 

 acquire very exceptional cumulative velocities and escape. Between 

 the very smaU and the very large bodies there thus arose a radical 

 distinction, but they graded into one another in the middle of the 

 planetary series. Numerical inspection showed that nuclei of the 

 smallest order of bodies, including all of the planetoids and the smaller 

 satellites, and possibly the nuclei of the terrestrial planets, would 

 probably not be able to hold under control even the molecules of the 

 stony and metallic substances at the temperatures at which they would 

 be volatile in appreciable quantities. The concentration of these 

 bodies must therefore probably liave followed some other than the 

 strictly gaseous line of condensation — some line in which the constit- 

 uents were more massive and less active. The probable nature of this 

 line revealed itself through the considerations under the next head. 



III. The Formation of Precipitates, Precipitate Aggregates, and Brownian 



Mixtures. 



Following the emergence of the gaseous masses from the solar 

 pressure, there must have been notable cooling by expansion as well as 

 by radiation. Hence the precipitation of such substances as were lia- 

 ble to it at the temperatures reached seems inevitable. This is the 

 more certain because it is highly probable that precipitation had already 

 begun in the sun ; at least the photosphere is commonly explained as a 

 zone of such precipitates. These precipitates would be scattered 

 through the gas and would have the effect of Brownian particles and 

 the subsequent evolution is to be treated as that of a Brownian 

 mixture. A^Tiile such a mixture is closely analogous to a ti"ue gas, it 

 has points of difference growing out of the replacement of molecules by 

 aggregates which are indefinitely more massive and are affected by 

 inelasticity^ In nuclei of sufficient mass, the Brownian gaseous 

 mixture would doubtless give rise to a central magma that would in 

 turn evolve as a liquid Brownian mixture, owing to the progressive 

 formation withm itself of crj^stalline or concretional aggregates. 

 These aggregates, it is beHeved, would gather into sohd planetary 

 cores. 



In the smaller and more open nuclei, however, such molecules as 

 remained free, while the stony and metallic substances were forming 

 precipitate aggregates, would escape with facility until the supply 

 was exhausted, and so only the precipitate aggregates would remain as 

 the residue of a quasi-evaporative process. The Brownian mixture 

 would thus give place to a cloud of precipitate aggregates. Though 

 minute, these aggregates might yet be veiy high multiples of the 

 heaviest molecules, and being in addition somewhat inelastic, they 

 would be much less active and much more amenable to mutual con- 



