OTHER SENSES THAN OURS. 163 



which possesses a smell at all, exhibits that property 

 in virtue of its power of giving off fine particles of its 

 substance. It is similarly clear that the odour of any 

 body will be the more powerful according as its par- 

 ticles are freely given off. Take the case of musk, 

 for instance. We weigh a grain of musk in a chemical 

 balance which turns the scale with the merest fraction 

 of a grain. Thuswise, we secure exact weight, and 

 we place our grain weight of musk in a room. For 

 years, our grain of musk will appreciably scent that 

 apartment. During all this period it must, therefore, 

 have been giving off its particles to the air ; yet, mark 

 the astounding result if we weigh it years afterwards, 

 we shall find it show exactly the weight it originally 

 possessed. 



Plainly, then, the particles given off from the musk 

 in such numbers must have been of such microscopic 

 size as to leave practically unaffected the bulk of the 

 substance. Our minds fail to grasp any idea of the 

 size of such particles. Sir William Thomson's esti- 

 mate of the size of atoms may find a parallel in the 

 particles of our musk ; yet, small as these particles 

 are, you observe they excite the sense of smell, and 

 become appreciated by our brain as those of a well- 

 known perfume. It may interest us to know that 

 Sir William Thomson makes the ultimate atoms of 

 matter each measure, in diameter, the one-fifty-millionth 

 part of an inch. 



As far as sight is concerned, I believe Helmholtz 

 gives a particle which is the eighty-thousandth part 

 of an inch in diameter, as the smallest which can be 

 distinctly made out by the eyes in association with 

 other particles. Such estimates of what matter is, 

 microscopically regarded, may serve to teach us some- 



