have a speed one tenth of the velocity of light, and a molecular mass of atomic magnitude. They penetrate a few centimeters into air, pass through thin aluminum foil but are stopped by denser metals. As they are but slightly deviable in a magnetic field, their momentum is calculated to be enormous: until, however, better evidence of the total positive charge which they carry has been obtained, we can not consider the magnitude of the momentum as definitely established; especially since their speed does not appear to be uniform. From experiments wherein a particles are allowed to escape freely, and again restrained by a lead cylinder surrounding radium, much of the apparent heat of the latter body appears to be due to the impinging of the a rays upon the surrounding surfaces.
-Rays are similar to cathode rays; they are less absorbable than the variety, and proceed at various speed, many approaching the velocity of light; they are stopped by solids in proportion to their density.
-Rays are similar to X-rays, of great penetrating power, and they are thought by some to be secondary effects of and rays, just as the X-rays originate from the impact of cathode rays on the glass wall of the Crookes tube. Besides, we have a multitude of conflicting accounts of secondary tertiary rays, resulting from these three varieties.
The chief method of research is the study of ionization, with the interposition of screens and magnetic fields, to separate the different kinds of rays. On the other hand, the varieties of rays emitted, their relative strength, and their variations of intensity, are the characteristics upon which the identification of the various so-called transformation-products of radio-active material is based. I have, therefore, copied from Professor Rutherford's book tabulations of these properties.
With regard to these various transformations, we should realize that the majority of the names are titles of hypothetical substance, whose existence within certain mixtures is assumed upon the evidence of their momentary radio-activity. The only one really isolated is that emanation which has all the properties of a gas, including that of condensibility at low temperatures—with the exception that its liquid form shows no vapor pressure—but has in addition remarkable energy effects, and has, undoubtedly, undergone transformation in Ramsay's hands. Bearing in mind the infinitesimal quantities of emanation which Earnsay and his associates could obtain, we are alike astounded by their marvelous manipulative dexterity and by the nature of their observations. First we had the gradual appearance of helium, when the emanation was stored by itself; then came the appearance of neon, when the emanation came into contact with water, the latter being partially decomposed into oxygen and hydrogen; lastly the partial reduction of copper nitrate solution, with the simultaneous appearance of lithium,
- "Radio-activity," 1905.