June 20, 1902.] 



SCIENCE. 



mi 



millimeter of the air around us. Notwith- 

 standing tills apparently crowded assem- 

 blage, the individual molecules move about 

 in the liveliest manner, their average 

 speed being about five hundred meters per 

 second, and this in spite of the fact that 

 the average length of an unimpeded jour- 

 ney is barely visible by the aid of the best 

 microscopes. Each molecule must there- 

 fore collide with its neighbors astonish- 

 ingly often, the encounters occurring, in 

 fact, about five thousand million times per 

 second.* 



More surprising still than the proper- 

 ties of assemblages of molecules forming 

 gases are the properties of the individual 

 molecules, especially when they are made 

 up of tAvo or more atoms. Such miniature 

 systems, comparable, probably, in com- 

 plexity with the Martian and Jovian sub- 

 systems of the solar system, exhibit de- 

 grees of constancy which rival the invaria- 

 bleness of the fixed stars themselves. This 

 is particularly the case with their rates of 

 vibration as disclosed by the spectroscope. 

 These rates afford one of the most delicate 

 tests of the propex-ties of matter, whether 

 it is found on the earth or on the most 

 distant star; and yet the vibrations, which 

 recur with a regularity equal to, if not sur- 

 passing, the regularity of the rotation of 

 the earth, are executed at the rate of some 

 hundreds of millions of millions per 

 second, t Herein, perhaps, we may find a 



* See, for example, ' The Kinetic Theory of 

 Gases,' by Dr. Oskar Emil Meyer, translated by 

 Robert E. Baynes, Longmans, Green and Co., 

 New York, 1899. 



f The number of vibrations per second corre- 

 sponding to any given wave-length of light may 

 be easily computed. For the velocity of light 

 is about 300,000 kilometers, or 3 X 10" microns 

 per second, and this divided by the wave-length 

 in question gives the number of vibrations per 

 second. Thus the average wave-length of the 

 cadmium rays used by Professor Michelson (cited 

 above) is about half a micron. The material 



cosmic unit of time as well as a cosmic 

 unit of distance, though both appear to be 

 inconveniently small for terrestrial pur- 

 poses. 



But the smaller bodies of the universe 

 do not end with molecules and atoms of 

 gases. Recent investigations point to the 

 conclusion that there is another order of 

 bodies of much smaller dimensions and 

 possessing still more wonderful properties. 

 These have been called corpuscles.* Their 

 density is only about one thousandth as 

 great as that of the lightest gas, hydrogen ; 

 they are freely given off by several of the 

 so-called radio-active substances; and they 

 move about with speeds of the same order 

 as the velocity of light. It appears not im- 

 probable that they play a most important 

 role in cosmic as well as in terrestrial 

 physics, and the amount of attention being 

 given to them justifies the hope that their 

 study may illuminate many obscure cor- 

 ners in the realm of molecular science. 



Passing per saltum from the smallest 

 measureable and calculable quantities to 

 those with which we have an every-day 

 familiarity, I would direct your attention 

 to the great number of articles of commerce 



sources of these rays must vibrate, therefore, 

 about six hundred million million times per sec- 

 ond. 



* See a paper by Professor J. J. Thomson, ' On 

 Bodies Smaller than Atoms,' Popular Science 

 Monthly, August, 1901. 



See also a paper by Professor John Cox on 

 ' Comets' Tails, the Corona and the Autora 

 Borealis,' Popular Science Monthly, January, 

 1902. 



A fact of great interest in eonnectiron with the 

 'corpuscles ' considered in these two papers is 

 the repulsion of light impinging on bodies, the 

 amount of which has been actually measured re- 

 cently by several observers. This repulsion be- 

 tween the sun and the earth is very great, 

 amounting to about a hundred million million 

 dynes; but the gravitational attraction between 

 these bodies is about forty million million times 

 as great as that repulsion. 



