comparison with which their own diameters are sometimes as insignificant as are the diameters of planets in comparison with their immense solar distances, and finally in motion, inconceivably rapid, and never ceasing. Such is the molecular theory of matter, in its most general form of expression. And in this form it seems destined to do for the science of physics what has been done for chemistry by the atomic theory, and for astronomy by the theory of gravitation. It seems competent to bring all branches into harmonious relation as constituents of a single science. It may do this even if the mathematical measurements of the magnitudes it describes should prove to be beyond the reach of human skill.
But, if molecules exist, what are their masses and their diameters? If they are not in contact, what is the measure of their separation; and, if in motion, with what velocity? These are legitimate subjects of research suggested by the theory itself, and no less important in the science of physics than the problems of astronomical magnitudes are in astronomy. It is, therefore, not strange that the best intellects among experimental and mathematical philosophers should be found bending their energies toward the solution of these problems. Already very wonderful progress has been made, and numerical values are assigned to these molecular magnitudes, in some cases with great confidence in their accuracy, and in other cases provisionally, awaiting better and more extended means of research. For example, Dr. Joule tells us that the hydrogen molecule is darting through the molecular spaces of this gas at the rate of 6,099 feet a second; and Clerk Maxwell, that the molecules of oxygen move at the more sluggish rate of about 1,525 feet a second. We are further informed that the distance from center to center of the molecules of a gas is probably about 1800000 of an inch. Different methods of investigation agree tolerably well in pointing to 130000000 f a millimetre as a fair approximation toward the diameter of a molecule—that is to say, about 760,000,000 of these bodies lying side by side would bridge the space of a single inch.
These magnitudes are of an order which only modern science has ever asked the intellect of man to contemplate. The human mind thus discovers its position between two infinities. It is able, through the agency of the senses, to acquaint itself directly with a very limited range of phenomena, but, planting itself upon this little fragment of solid ground, it reaches into space, and by observations and by computations made upon them becomes acquainted with the infinitely great; while in the other direction it pierces the recesses of minute bodies, and by observations and computations there it becomes acquainted with the infinitesimal. The results attained in both directions are alike incomprehensible. Who can, for example, accurately conceive the distance described as 1,000,000 miles? Even he who has made the circuit of the world can not rely on this extended experience to enable him to see the beginning, middle, and end of 1,000,000 miles in
