264 



SCIENCE 



[N. S. Vol. XXX. No. 765 



tution of matter. How much stronger is 

 that evidence now, when we have measured 

 the charge on the unit and found it to be 

 the same from whatever source the electric- 

 ity is obtained. Nay, further, the molecu- 

 lar theory of matter is indebted to the mo- 

 lecular theory of electricity for the most 

 accurate determination of its fundamental 

 quantity, the number of molecules in any 

 given quantity of an elementary substance. 

 The great advantage of the electrical 

 methods for the study of the properties of 

 matter is due to the fact that whenever a 

 particle is electrified it is very easily 

 identified, whereas an uncharged molecule 

 is most elusive; and it is only when these 

 are present in immense numbers that we 

 are able to detect them. A very simple 

 calculation will illustrate the difference in 

 our power of detecting electrified and un- 

 eleetrified molecules. The smallest quantity 

 of unelectrified matter ever detected is 

 probably that of neon, one of the inert 

 gases of the atmosphere. Professor Strutt 

 has shown that the amount of neon in one 

 twentieth of a cubic centimeter of the air 

 at ordinary pressures can be detected by 

 the spectroscope ; Sir "William Ramsay esti- 

 mates that the neon in the air only amounts 

 to one part of neon in 100,000 parts of air, 

 so that the neon in one twentieth of a 

 cubic centimeter of air would only occupy 

 at atmospheric pressure a volume of half a 

 millionth of a cubic centimeter. When 

 stated in this form the quantity seems ex- 

 ceedingly small, but in this small volume 

 there are about ten million million 

 molecules. Now the population of the 

 earth is estimated at about fifteen hundred 

 millions, so that the smallest number of 

 molecules of neon we can identify is about 

 7,000 times the population of the earth. In 

 other words, if we had no better test for the 

 existence of a man than we have for that of 

 an unelectrified molecule we should come 



to the conclusion that the earth is unin- 

 habited. Contrast this with our power of 

 detecting electrified molecules. We can by 

 the electrical method, even better by the 

 cloud method of C. T. R. Wilson, detect the 

 presence of three or four charged particles 

 in a cubic centimeter. Rutherford has 

 shown that we can detect the presence of a 

 single a particle. Now the particle is a 

 charged atom of helium; if this atom had 

 been uncharged we should have required 

 more than a million million of them, instead 

 of one, before we should have been able to 

 detect them. 



We may, I think, conclude, since electri- 

 fied particles can be studied with so much 

 greater ease than unelectrified ones, that we 

 shall obtain a knowledge of the ultimate 

 structure of electricity before we arrive at 

 a eoiTCsponding degree of certainty with 

 regard to the structure of matter. 



We have already made considerable 

 progress in the task of discovering what 

 the structure of electricity is. We have 

 known for some time that of one kind of 

 electricity— the negative— and a very inter- 

 esting one it is. We know that negative 

 electricity is made up of units all of which 

 are of the same kind; that these units are 

 exceedingly small compared with even the 

 smallest atom, for the mass of the unit is 

 only %7oo P^rt of the mass of an atom of 

 hydrogen ; that its radius is only 10"" centi- 

 meter, and that these units, "corpuscles" 

 as they have been called, can be obtained 

 from aU substances. The size of these cor- 

 puscles is on an altogether different scale 

 from that of atoms; the volume of a cor- 

 puscle bears to that of the atom about the 

 same relation as that of a speck of dust to 

 the volume of this room. Under suitable 

 conditions they move at enormous speeds 

 which approach in some instances the veloc- 

 ity of light. 



The discovery of these corpuscles is an 



