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228, are still available for the same obiect; the condensation being cosmical not atomic. But this leads us to the wider question of the distribution of stars throughout space.

Stellar distribution may be compared to the distribution of human habitations on the face of the earth. These habitations, instead of being equally distributed, are grouped into towns and cities, with suburban mansions and villas. Again, a more general classification is afforded by their distribution into different kingdoms, often separated from one another by great natural boundaries.

A similar grouping is observable in the celestial bodies. The first step of aggregation is exemplified in the case of double stars. It was found that the number of twin stars was much greater than what could be accounted for by chance distribution; and it was therefore suspected that there was some physical bond of connexion. This surmise has been fully borne out by observation. Many have been found to revolve round the same centre, and several have completed their period since the first observations. Systems, however, are found in the heavens with every gradation, from twin stars up to clusters, whose constituent elements cannot be numbered. These clusters, too, are often evidently connected with a higher grade of grouping—bearing the same relation to the whole system, that a single square of a city bears to the city itself.