July 12, 1901.] 



SCIENCE. 



55 



FLUORESCENCE. 



Most of the substances that show fluor- 

 escence under the action of ultra-violet 

 light or the X-rays also fluoresce under the 

 action of the radium radiations, while 

 those that fluoresce under ordinary light 

 do not fluoresce under the radium radia- 

 tions ; but there are numerous exceptions 

 to these rules. 



It is a very interesting fact that when 

 freshly prepared the radium salts are 

 faintly self-luminous, and this property 

 seems to be retained as long as the salt 

 does not absorb moisture. It is this prop- 

 erty that has excited much of the popular 

 interest in the radium salts, for it is a case 

 of the longed-for light without heat. In 

 fact, it is light with no apparent source of 

 energy whatever. If radium chloride ever 

 becomes cheap we may be given an op- 

 portunity to court fortune by investing in 

 preferred stock of some ' International 

 Eadium Illuminating Company.' Just at 

 present gold is dirt cheap in comparison 

 with these radium salts. The self- lumines- 

 cence is due to the fact that under the 

 action of its own rays either the salt itself, 

 or some of the unavoidable impurities, 

 fluoresces. 



Some of the tissues of the eye fluoresce 

 under the radium rays, so a sensation of light 

 is felt when some of the salt is brought before 

 the closed eyelids or placed on the temple. 



IONIZATION OF GASES. 



Any gas traversed by the Becquerel rays, 

 as Mme. Curie has named the new radia- 

 tions, is made capable of conducting elec- 

 tricity. This conductivity is of the same 

 nature as that produced in gases by the 

 kathode and X-rays. According to the ac- 

 cepted hypothesis, the positive and negative 

 particles or ions of the gas are knocked 

 apart by the radiations, and the motion of 

 these free charged ions when directed by an 

 electric field constitutes the electric current. 



If an ionized gas is left to itself the positive 

 and negative particles soon reunite, and, in 

 fact, the reuniting process goes on all the 

 time in proportion to the number of free 

 ions in the gas, so that under any given in- 

 tensity of ionizing radiation a condition of 

 equilibrium is soon reached in which the 

 reuniting goes ou as fast as the ionizing. 

 Since the amount of ionization may be 

 measured by measuring the electrical con- 

 ductivity of the gas, this affords a conven- 

 ient means of comparing the relative 

 strengths of radio-active substances, and 

 one which is much more rapid and accurate 

 than the photographic one. It is by no 

 means certain, however, that the radiations 

 most efiective for ionization will therefoi^e 

 produce most efiect on a photographic plate. 

 Ionization is proportional to the absorption 

 of the radiations by the gas, so that if a bit 

 of radio-active substance be placed between 

 two metal plates a greater current may be 

 sent between them when they are two centi- 

 meters apart than when they are only one, 

 the greater thickness of the air in the first 

 case absorbing more of the radiations. 



An ionized gas is in many respects similar 

 to an ordinary liquid electrolyte, and Lord 

 Kelvin has shown that when a plate of 

 copper and a plate of zinc are connected by 

 a wire and the air between the plates ex- 

 posed to radiations from uranium com- 

 pounds, a current flows through the con- 

 necting wire just as if the plates were im- 

 mersed in a liquid electrolyte. It has 

 recently been shown that the Becquerel 

 rays decrease the resistance of selenium, 

 just as light and the X-rays do. 



PENETRATING POWER. 



The radiations from the various sub- 

 stances are not at all homogeneous, some 

 being verj' penetrative, others being easily 

 stopped by any substance. Polonium radi- 

 ations, while intense, are of the non-pene- 

 trating kind, being stopped by even the 



