186 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
through gases, and led to great improvements in the instruments 
used in this type of research. It is not, however, to the power of 
probing dark places, important though this is, that the influence of 
Rontgen rays on the progress of science has mainly been due; it is 
rather because these rays make gases, and, indeed, solids and liquids, 
through which they pass conductors of electricity. It is true that be- 
fore the discovery of these rays other methods of making gases con- 
ductors were known, but none of these was so convenient for the 
purposes of accurate measurement. 
The study of gases exposed to Réntgen rays has revealed in such 
gases the presence of particles charged with electricity ; some of these 
particles are charged with positive, others with negative electricity. 
The properties of these particles have been investigated; we know 
the charge they carry, the speed with which they move under an 
electric force, the rate at which the oppositely charged ones recom- 
_bine, and these investigations have thrown a new light, not only on 
electricity, but also on the structure of matter. 
We know from these investigations that electricity, like matter, is 
molecular in structure, that just as a quantity of hydrogen is a col- 
lection of an immense number of small particles called molecules, so 
a charge of electricity is made up of a great number of small charges, 
each of a perfectly definite and known amount. 
Helmholtz said in 1880 that in his opinion the evidence in favor of 
the molecular constitution of electricity was even stronger than that 
in favor of the molecular constitution 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 
electricity is obtained. Nay, further, the molecular theory of matter 
is indebted to the molecular theory of electricity for the most accu- 
rate 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 
unelectrified 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 
estimates 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 vol- 
