Page:Philosophical Review Volume 19.djvu/267

253 leads step by step to this generalization. The fertility of the argument, the wealth of illustration drawn from every order of phenomena, defy condensation. The terms are of the highest degree of abstractness, as indeed any such formula is bound to be, and, to those who have not given to Spencer's work very careful study, it cannot help but be difficult to understand. It will best serve the purpose of this exposition to give a single illustration, and from that I hope it may be possible for those who are not able to spare the time thoroughly to study the original to gain some slight idea of the meaning and range of the conception.

We are most of us acquainted with the nebular hypothesis, which indeed is now a necessary integral part of any rational present day astronomy. How a nebula originates need not now concern us; it is sufficient for our purpose that, as a matter of fact, there exist in various parts of the heavens many irregular shaped bodies of very low density, which, spectroscopic analysis informs us, are masses of glowing gas. This is the first stage in stellar evolution. This mass (note the Spencerian formula) is indefinite in outline, incoherent in substance (as a gaseous body must be), and comparatively homogeneous in content. The substance is gaseous, the lines of its spectrum are comparatively few, showing the presence of only a small number of chemical elements, while our knowledge of gaseous masses precludes the possibility of any considerable differentiation between its constituent parts. Thus the nebula is an excellent example of the first stage in the evolution of an aggregate.

Evolution of this nebula proceeds by "integration of matter and dissipation of motion." As the mass slowly cools the energy, whether of molar motion, position, or heat, radiates into space. The motion is dissipated. The matter integrates; the particles