Page:Harper's New Monthly Magazine - v109.djvu/421

Diagram showing the well-known deflection of cathode or Crookes rays in an ordinary high-vacuum tube by a steel magnet held near; their path being traced by a phosphorescent lateral screen, as arranged by Crookes. A measure of the deflection by a known magnetic field enables the mass of each particle to be determined, when its charge and speed are known. To measure the speed, another kind of deflection must be likewise measured, viz. the one caused by the presence of an electrified body acting upon the same charged particles. J. J. Thomson first showed how to produce this kind of deflection. The magnetic curvature was first measured by Schuster. To determine the charge, the particles are caught in a hollow vessel, and their aggregate charge measured,—as was first done by Perrin; but a counting of the particles is also necessary, and this is effected by a curious and most interesting method of mist-production applied by C. T. R. Wilson, of Cambridge.

they are stopped by any obstacle. They thus keep the vessel in which they are enclosed at a temperature a degree or two above surrounding bodies, at least in the case of the most active known substances, radium and its emanation. For radium converts its own intra-atomic energy into heat at so surprising a rate that it could, if all of the heat were economized and none allowed to escape, raise its own weight of water from ordinary temperature to the boiling-point every hour.

The number of atoms breaking up in any perceptible portion of radium salt must be reckoned in millions per second; nevertheless the proportion of atoms which are thus undergoing transformation at any one time is extremely small. If they could be seen individually most of them would appear quiescent and stable. Of every ten thousand atoms, if a single one breaks up and flings away a portion of itself once a year, that would be enough to account for all the activity observed, even in the case of so exceptionally active a substance as radium; hence the apparent stability of ordinary matter is not surprising.

The thus-projected atomic fragments were measured by Rutherford, who found them deflected by a magnet in the opposite direction to the electron projectiles, and were therefore proved to be positively charged; but they are deflected so slightly that they must be very massive bodies, 1600 times as massive as an electron, or twice the atomic weight of hydrogen. A substance with this atomic weight is known, viz. Helium; and surely enough the discoverer of Helium, Sir W. Ramsay, working with Mr. Soddy, a recent colleague of Rutherford, has witnessed the Helium spectrum gradually develop in a tube into which nothing but radium-emanation had been put.

Matter then appears to be composed of positive and negative electricity and nothing else. All its newly discovered, as well as all its long-known, properties can thus be explained:—even the long-standing puzzle of "cohesion" shows signs of giving way. The only outstanding still-intractable physical property is "gravitation," and no satisfactory theory of the nature of gravitation has been so far forthcoming. I doubt however if it is far away. It would seem to be a slight but quite uniform secondary or residual effect due to the immersion of a negative electron in a positive atmosphere. It is a mutual force between one atomic sys-

Apparatus by which Professor Rutherford, of Montreal, measured the atomic weight of the substance flung upwards by a layer of radium at the bottom of a vessel placed below a gold-leaf electroscope. It possessed a grid of thin plates, between which the projectiles had to make their way through a current of rarefied hydrogen gas. A strong magnetic field curved them into the plates, and diminished their action on the charged electroscope by a measured amount.