THE MOLECULAR THEORY. 469 



comparison with which their own diameters are sometimes as insignifi- 

 cant 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 the- 

 ory, and for astronomy by the theory of gravitation. It seems com- 

 petent to bring all branches into harmonious relation as constituents 

 of a single science. It may do this even if the mathematical measure- 

 ments 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 isthe 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 8 \ of an inch. 

 Different methods of investigation agree tolerably well in pointing to 

 3 0*0 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 compu- 

 tations 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 ena- 

 ble him to see the beginning, middle, and end of 1,000,000 miles in 



