the Weights of Atoms. 183 



rt globes all of one size, influencing one another only through 

 " actual contact, we have for each molecule simply a zigzag 

 " path composed of rectilinear portions, with abrupt changes 

 " of direction .... But we cannot believe that the individual 

 " molecules of gases in general, or even of any one gas, are 

 " hard elastic globes. Any two of the moving particles or 

 " molecules must act upon one another somehow, so that 

 " when they pass very near one another they shall produce 

 " considerable deflexion of the path and change in the velocity 

 u of each. This mutual action (called force) is different at 

 "different distances, and must vary, according to variations 

 " of the distance, so as to fulfil some definite law. If the 

 M particles were hard elastic globes acting upon one another 

 " only by contact, the law of force would be . . . zero force 

 " when the distance from centre to centre exceeds the sum of 

 "the radii, and infinite repulsion for any distance less than 

 " the sum of the radii. This hypothesis, with its ' hard and 

 " ' fast ' demarcation between no force and infinite force, 

 " seems to require mitigation/' Boscovict/s theory supplies 

 clearly the needed mitigation. 



§ 30. To fix the ideas we shall still suppose the force 

 absolutely zero when the distance between centres exceeds a 

 definite limit, \ ; but when the distance is less than X, we 

 shall suppose the force to begin either attractive or repulsive, 

 and to come gradually to a repulsion of very great magnitude, 

 with diminution of distance towards zero. Particles thus 

 defined I call Boscovich atoms. We thus call ^\ the radius 

 of the atom, and \ its diameter. We shall say that two 

 atoms are in collision when the distance between their centres 

 is less than \. Thus il two molecules in collision will exercise 

 " a mutual repulsion in virtue of which the distance between 

 " their centres, after being diminished to a minimum, will 

 u begin to increase as the molecules leave one another. 

 " This minimum distance would be equal to the sum of the 

 '• radii, if the molecules were infinitely hard elastic spheres ; 

 " but in reality we must suppose it to be very different in 

 " different collisions." 



§ 31. The essential quality of a gas is that the straight line 

 of uniform motion of each molecule between collisions, called 

 the free path, is long in comparison with distances between 

 centres during collision. In an ideal perfect gas the free 

 path would be infinitely long in comparison with distances 

 between centres during collision, but infinitely short in 

 comparison with any length directly perceptible to our senses; 

 a condition which requires the number of molecules in any 



