What force then shall we say it is that causes the motion of the stars? As far as we know none but gravitation. It appears therefore as if the gaseous primeval substance of the stars were not governed by this force. It might prove hazardous, however, to make this assumption as gases also possess weight and even the most rarefied strata of the Earth’s atmosphere exert barometric pressure by virtue of their attraction to the mass of the earth. Rather the immobility of the nebulæ is due to the frequent collisions between the molecules in any quantity of gas even if it be attenuated to such a high degree as in the nebulæ. Thus, the molecules strike a balance, as it were, against each other so that the different parts of the gas accumulations shortly are brought to rest relative to each other. The irregular gas mists around the Milky Way form therefore a continuous whole. A different condition obtains in regard to condensed stellar bodies such as the stars. They may in the densest throng move during billions of years before they collide; but they might on the other hand enter nebulous masses and thereby suffer gradual retardation. We now refer to stars moving outside of the vapour clouds. They are therefore unrestricted and the longer they have obeyed gravitation without impeding encounters with nebulous matter, in other words the longer the time elapsed since they emerged from the gas accumulations which gave them birth, the swifter is their motion. Their (average) velocity can of course not exceed a certain limit which in our parts of the universe appears to be about 18 km. (11.2 miles) per second. Campbell’s measurements show that for the youngest stars (all except the red) the velocity is greatest in the plane of the Milky Way, a natural enough condition as the attracting matter here is most abundant. The planetary nebulæ possess a greater velocity although they, as consisting of mist vapours, are in the first stage of evolution. Faster yet do the spiral nebulæ move according to measurements by Wolf of Heidelberg. This shows that they are of a different nature from the irregular nebulæ, which form the matrix of the Milky Way. A closer examination of the few--thirteen in all--planetary nebulæ, determined by the American astronomer Keeler, convinced me that they approach the Galaxy from its poles with a moderate speed, and subsequently under the influence of its attraction curve their orbit, rapidly gain in velocity, and finally rush into the nearest part of the Milky Way with a very high speed.