120 COHESION. [SECT. xiv. 



powerful, that they could not be separated without breaking. 

 Instances have occurred where two or three have been so 

 perfectly united, that they have been cut and their edges 

 polished as if they had been fused together, and so great 

 was the force required to make the surfaces slide that one 

 tore off a portion of the surface of the other. 



The size of the ultimate particles of matter must be small 

 in the extreme. Organised beings, possessing life and all its 

 functions, have been discovered so small that a million of 

 them would occupy less space than a grain of sand. The 

 malleability of gold, the perfume of musk, the odour of 

 flowers, and many other instances might be given of the 

 excessive minuteness of the atoms of matter; yet from a 

 variety of circumstances it may be inferred that matter is 

 not infinitely divisible. Dr. Wollaston has shown that in all 

 probability the atmospheres of the sun and planets as well as 

 of the earth consist of ultimate atoms no longer divisible ; 

 and, if so, that our atmosphere only extends to that point 

 where the terrestrial attraction is balanced by the elasticity 

 of the air. The definite proportions of chemical compounds 

 afford one of the best proofs that divisibility of matter has 

 a limit. The cohesive force,, which has been the subject of 

 the preceding considerations, only unites particles of the 

 same kind of matter ; whereas affinity, which is the cause 

 of chemical compounds, is the mutual attraction between 

 particles of different kinds of matter, and is merely a result 

 of the electrical state of the particles, chemical affinity and 

 electricity being only forms of the same powers. 



It is a permanent and universal law in all unorganised 

 bodies hitherto analysed, that the composition of substances 

 is definite and invariable, the same compound always con- 

 sisting of the same elements united together in the same 

 proportions. Two substances may indeed be mixed ; but 

 they will not combine to form a third substance different 

 from both, unless their component particles unite in definite 

 proportions, that is to say, one part by weight of one of the 



