Everyone has heard of radium, and knows that it is in a state which can best be described as a long-drawn-out explosion. It is always shooting off tiny particles. Night and day, year in and year out, it is firing off these exceedingly minute projectiles, of which there are two kinds, one of which appears to be atoms of helium. Some years ago, when radium was being much talked about and the names of M. and Madame Curie were in everyone's mouth, little toys were sold, the invention, I believe, of Sir William Crookes, called spinthariscopes. Each of these consisted of a short brass tube with a small lens in one end. Looking through the lens in a dark room, one could see little splashes of light on the walls of the tube. Those splashes were caused by a tiny speck of radium in the middle of the tube, the helium atoms from which, by bombarding the inner surface of the tube, produced the sparks. Now if we can count those splashes we can tell how many atoms of helium are being given off per minute. And if then we reckon how many minutes it takes to accumulate a cubic centimetre of helium we can easily reckon how many atoms go to the cubic centimetre. But the difficulty is to count them. So the learned Professor called in the aid of the electrometer. He could not count all the atoms shot off, so he put the piece of radium at one end of a tube and an electrometer at the other. Every now and then an atom shot right through the tube and out at the farther end. And since each of these atoms from radium is charged with electricity, each as it emerged operated the electrometer. By simply watching the twitching of the instrument, therefore, it was possible to count how many atoms shot through the tube--one atom one twitch. And from the size and position of the tube it was possible to reckon what proportion of the whole number shot off would pass that way. The result of the experiment showed that there are in a cubic centimetre of helium a number of atoms represented by 256 followed by seventeen noughts. And as helium is one of the few substances in which the molecule is formed of but one atom, that is also the number of molecules. And now consider this, please. A cubic centimetre is about the size of a boy's marble. That contains the vast number of molecules just mentioned. And the electrometer was able to detect the presence of those _one at a time_. Need one add another word as to the inconceivable delicacy of the instrument.