KINETIC THEORIES OF GRAVITATION. 281 



Still very far from establisliing an action by contact^ or from sustaining 

 the fond hypothesis that an atom " acts where it <*s/" Admitting a con- 

 necting rod for drawing the railway train of dynamic, as Professor Max- 

 well has remarked, " the action of the rod is explained by the existence 

 of internal forces in its substance; and the internal forces are explained 

 by means of forces assumed to act between the particles of which the 

 rod is composed, that is between bodies at distances which, though 

 small, must be finite. The observed action at a considerable distance is 

 therefore explained by means of a great number of forces acting between 

 bodies at very small distances, for which we are as little able to account 

 as for the action at any distance, however great."* 

 To the wondering inquiry what possible explanation can be given of 



* A Treatise on Electricity acd Magnetism : by James Clerk Maxwell. Oxford, 2 vols. 

 8vo, 1873. Parti, chap, t, sec. 105, vol. i, p. 123. 



If the attempts hitherto made by kinetic theorists to explain the tensile strength of 

 a rope or of a chain by the pressure of a vis a tergo have been exceedingly lame and 

 unsatisfactory, even the more direct examples of actual impact and propulsion are 

 really as little serviceable to the hypothesis of contact action. If we have good reason 

 to believe that the constituent molecules of a steel bar are actually separated by rela- 

 tively large spaces of intense repulsion, — a fortiori must the physical impact of the 

 most violent percussion be resolved into an action through a vacant space. Taking 

 the case of a steel ball struck suddenly by a steel bat, the interval of distance between 

 the first impression of the moving mall and its nearest approach to the ball, is suffi- 

 cient to permit the acceleration of motion in the missile through every gradation, from 

 zero to its full velocity. 



Perhaps no better " prerogative of instances" of a physical contact could be sug- 

 gested than that of a thick glass plate resting on the convex surface of a large glass 

 lens, since the perfectly-ground plane surface of the upper glass and the perfectly- 

 ground spherical surface of the lower one are best adapted to exclude a possible film 

 of air. Such an arrangement represents the well-known experiment by which Newton 

 determined from the measurable variation of distance between the glasses (when 

 closely pressed together) the wave-length of light for different colors. On this very 

 beautiful experiment Dr. Robison forcibly remarks in his excellent work on Natural 

 Philosophy : 



"The conclusion seems unquestionable that we have no proof from the black spot 

 between the glasses, that they are in mathematical contact in that place. We know 

 by the first experiment that a very considerable force is necessary for producing the 

 black spot. A greater pressure makes it broader, and in all probability this is partly 

 by the mutual yielding of the glasses. I found that before a spot, whose surface is a 

 square inch can be produced, a force exceeding one thousand pounds must be em- 

 ployed. When the experiment is made with thin glasses, they are often broken before 

 any black spot is produced There is therefore an essential difi:erence be- 

 tween mathematical and physical contact ; between the absolute annihilation of dis- 

 tance, and the actual pressure of adjoining bodies. We must grant that two pieces of 

 glass are not in mathematical contact till they are exerting a mutual pressure not less 

 than one thousand pounds per square inch. For we must not conclude that they are 

 in contact till the black spot appears ; and even then we dare not positively affirm it. 

 My own decided opinion is, that the glasses not only are not in mathematical contact 

 in the black spot, but that the distance between them is vastly greater than the 

 eighty-nine-thousandth part of an inch, the difference of the distances at two suc- 

 cessive rings." — A System of Mechanical Philosophy : by John Robison. 4 vols. 8 vo., 

 Edinburgh, 18'22, vol. i, sec. 241, 242, pp. 2.50, 251. 



