52 
MR. J. J. WATERSTON ON THE PHYSICS OF MEDIA COMPOSED OF 
of the ethereal medium is not caused directly by the motion of material particles as 
with the grosser media of gases and liquids when affected by the motion of bodies 
passing through them. Something takes place in the vibratory motion of the 
elements of matter that does not take place in their collective motion. 
Molecular vibration consists in the struggle, as it were, between the vis insita of 
molecules and the forces that bind them together. It seems, therefore, as if the 
disturbance of the medium which answers to radiation of heat and light were 
derived from the disturbance of the molecular forces, and that when there was no 
such disturbance there was no resistance. 
But the difficulty yet remains of a body moving in a material medium without 
resistance. It is almost inconceivable in the present state of our knowledge of 
molecular physics. If it is at all possible (for there is almost an absurdity involved 
in asserting that it is possible) it can only be by means of a persistent or continuous 
relation of intense mutual organized activity between the ether and the perfectly 
elastic elements of matter, of which we can as yet have very little conception. 
The disturbance of molecular forces that allows the medium to absorb the vis insita 
of vibrating molecules may be likened to the ascent and descent of planetary bodies 
to and from the sun while revolving in elliptic orbits. There is a disturbance when 
the centripetal force on a body increases or diminishes, so that if a body revolved in a 
circle thei'e is no disturbance, no resistance, no absorption of its vis insita by the 
active medium, and if it revolves in an eccentric ellipse there is disturbance, resistance, 
and absorption. The number of luminous vibrations in a second is, by the undulatory 
theory, from 458 million million to 727 million million, and the vibrations that cause 
heat are probably not very different in the velocity of their succession. If a set of 
molecules thus vibrating in a perfectly cold region took one second to dissipate their 
molecular vis viva by radiation, this is sufficient time for so vast a multitude of 
revolutions in the molecular orbits that the loss of vis insita in our vibration is 
probably as infinitesimal as that of the planets during one revolution, and we might 
conjecture that this infinitesimal ratio was a function of the ratio of the elasticity of 
the ether to the molecular velocity of the vibrations, the elasticity of the ether 
being apparent only as an active centripetal force. Such a retardation would 
certainly follow if the velocity of the action of gravitation were not infinite, but there 
still remains to be suggested the physical condition of a medium that offers no 
resistance. Is such an entity possible ? 
Note H.—Formula for Measuring Heights by the Thermometer. 
The tension of the atmospheric medium varies, as we have seen, in the proportion 
of the sixth power of the depth below the summit, and the elasticity of steam varies 
as the sixth power of the ordinate to the line SF on the chart multiplied by the 
absolute temperature or square of the corresponding abscissa (see Note B). This 
