LUCRETIUS AND THE ATOMIC THEORY 211 



able fluid of this kind has formed part of many theories, and 

 ether came to be very generally adopted as a favourite name for 

 the fluid, but caloric was also much thought of as a medium. 

 We even find half-a-dozen imponderable co-existent fluids re- 

 garded with favour one called heat, another electricity, another 

 phlogiston, another light, and what not, with little hard atoms 

 swimming about, each endowed with forces of repulsion and 

 attraction of all sorts, as was thought desirable. This idea of 

 the constitution of matter was perhaps the worst of all. These 

 imponderable fluids were mere names, and these forces were 

 suppositions, representing no observed facts. No attempt was 

 made to show how or why the forces acted, but gravitation being 

 taken as due to a mere ' force,' speculators thought themselves 

 at liberty to imagine any number of forces, attractive or repul- 

 sive, or alternating, varying as the distance, or the square, cube, 

 fifth power of the distance, &c. At last Boscovich got rid of 

 atoms altogether, by supposing them to be the mere centres of 

 forces exerted by a position or point only, where nothing existed 

 but the power of exerting a force. A medium composed of 

 molecules flying in all directions has been shown by Maxwell to 

 have certain properties in which it resembles a solid body rather 

 than a fluid. The less the molecules interfere with each other's 

 motion the more decided do these properties become, till in 

 the ultimate case in which they do not interfere at all, Maxwell 

 states that the elastic properties of the medium are precisely 

 those deduced by French mathematicians from the hypothesis 

 of centres of force at rest acting on one another at a distance. 

 Thus the most opposite hypotheses sometimes conduct to the 

 same result. Dalton, assuming that the idea of an atom with 

 an ambient ether was generally believed in, gave an immense 

 support to the atomic theory by his discovery of the simple 

 relations in which substances combine chemically. Since then 

 it has been heretical to doubt atoms, until Sir Benjamin Brodie 

 the other day broached ideas which seemed independent if not 

 subversive of the simple atomic faith. 



Eeviewing the various doctrines, we find that the problem 

 of the constitution of matter is yet unsolved ; but at least it can 

 now be fairly stated. We know with much accuracy the con- 

 ditions to be fulfilled by any hypothesis, and we possess a 



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