OHARLES A. YOUNG. 43 
usually, however, there is reason to suppose that they 
are constituted like clouds on the earth, that is, com- 
posed of particles, solid or liquid, surrounded and. inter- 
penetrated by gaseous matter, such as a cloud of water, 
rain, fog, snow, smoke or dust. The nebular hypothesis 
is aname given to an explanation of astronomical evolu- 
tion, according to which explanation the suns and 
planets, as we observe them, have come to their present 
condition by the gradual condensation and aggregation 
of nebulous matter. While much doubt remains as to 
‘the many details of the process, and although there 
seem to be some difficulties as yet insuperable in the 
way of all different and rival forms the history has 
taken, as expounded by different authorities, it seems 
almost certain that in its main features it is correct. 
Swedenborg, in 1734, seems to have been the first to ex- 
press the mainidea, though on finely speculate grounds 
and in a manner vague, and in many ways unsatisfactory 
and absurd. Kant, in 1755, independently and more 
fully developed similar ideas, and avowedly based his 
speculations on the Newtonian laws of motion and grav- 
itation, but his mathematics was hardly equal to the 
subject, and some of his ideas are simply absurd from the 
mechanical point of view. Thus he makes his original 
nebular absolutely homogeneous and quiescent, and 
makes it set itself to whirling by a mere process of con- 
traction. His speculations upon the inhabitants of the 
different planets are, of course, worthless. It is not 
certain whether Laplace (1796 to 1808) had or had not 
some previous knowledge of the speculations of Sweden- 
borg and Kant. However, in his work we do not find 
any mathematical or mechanical absurdities, but the 
theory of heat was not then understood and the more 
modern science of thermo-dynamics requires some very 
essential modifications of his views. More recent work- 
ers along the line are Roche of Montpelier; Faye, who 
