128 COEKELATION OF PHYSICAL FORCES. 



With regard, however, to liquid and gaseous bodies, there 

 are very great difficulties in viewing them as consisting of 

 separate and distant molecules. If, for instance, we assume 

 with Young that the particles in water are at least as distant 

 from each other comparatively as 100 men would be if dis- 

 persed at equal distances over the surface of England, the dis- 

 tance of these particles, when the water is expanded into 

 steam, would be increased more than forty times, so that the 

 100 men would be reduced to two, and by further increasing 

 the temperature this distance may be indefinitely increased ; 

 adding to the effects of temperature rarefaction by the air- 

 pump, we may again increase the. distance, so that, if we as- 

 sume any original distance, we ought, by expansion, to in- 

 crease it to a point at which the distance between molecule 

 and molecule should become measurable. But no extent of 

 rarefaction, whether by heat or the air-pump, or both, makes 

 the slightest change in the apparent continuity of matter ; 

 and gases, I find, retain their peculiar character, as far as a 

 judgment of it can be formed from its effect on the electric 

 spark, throughout any extent of rarefaction which can exper- 

 imentally be applied to them : thus the electric spark in prot- 

 oxide of nitrogen, however attenuated, presents a crimson 

 tint, that in carbonic oxide a greenish tint. 



Without, however, entering on the metaphysical enquiry 

 as to the constitution of matter (or whether the atomic phil- 

 osophers or the followers of Boscovich are right), a question 

 which probably human appliances will never answer : and 

 even admitting that an ethereal medium, not absolutely im- 

 ponderable as asserted by many, but of extreme tenuity, per- 

 vades matter, still ordinary or non-ethereal matter itself must 

 exercise a most important action upon the transmission of 

 light ; and Dr. Young, who opposed the theory of Euler, that 

 light was transmitted by undulations of gross matter itself, 

 just as sound is, was afterwards obliged to call to his assis- 

 tance the vibrations of the ponderable matter of the refract- 



