146 BULLETIN OF THE 



as to produce an entirely different spectrum, is not s ° easily ex- 

 plained. The suggestion of a disruption or disassociation of the 

 atomic flight by centrifugal force is negatived by the fact of per- 

 fect restoration of the orbit under uniform conditions. Nor does 

 the hypothesis of a resolution Of the elementary molecules into 

 still more elementary types, (which seems to have gained some 

 favor,) render the physical conception of the phenomeua in any 

 respect more simple. In particular cases a precise equalization of 

 the energies of emission, and of absorption in surrounding heated 

 gas, might effect a neutralization and complete obliteration of one 

 or more of the lines. And it is conceivable that a certain increase 

 of amplitude in the aetherial wave may (as in the case of its length) 

 cease to be recognized by the optic nerves. 



The law of Atomic Orbits. — The conception being thus presented 

 to us— -of a particle moving in an elliptical or circular orbit of 

 constant period, irrespective of the length of the radius-vector, or 

 of the velocity, (a condition so wholly unlike the gravitative orbits 

 of planets, observing the laws of Kepler,) what is the dynamic in- 

 terpretation of such a system ? This problem has been anticipated 

 by the genius of Newton, who in his Mathematical Principles of 

 Natural Philosophy has demonstrated the imaginary case, — " if the 

 periodic times are equal, (and the velocities therefore as the radii,) the 

 centripetal forces will also be as the radii." * A law of force in- 

 creasing directly with the distance (as in the extension of an india- 

 rubber, or of a helical steel wire spring,) is undoubtedly a very re- 

 markable one : but whatever its range of action, it will manifestly 

 within that range, secure the atom from all possibility of detach- 

 ment. 



From the perfect uniformity both of chemical and of spectro- 

 scopic indications, whether in the smallest or the largest mass of 

 molecules, — from whatever source obtained, we are forced to con- 

 clude that the molecules of any simple gas are absolutely similar. 

 Whether we analyze a drop of petroleum or distill an insect or a 



* Newton's Principia. 1687: book I, sect. II, prop. 4, corol. 3. A very- 

 beautiful illustration of this orbit is presented by the conical pendulum, 

 when the length of the suspension is very great relatively to the ranges of 

 excursion of the ball, so that an ellipse or different circular orbits shall lie 

 sensibly in the same plane. Another similar example is furnished by the 

 orbits of the balls of a parabolic "governor." 



