54 



SCIENCE. 



[N. S. Vol. XIV. No. 341. 



The spectrum of radium has been care- 

 fully examined by M. Demarcay, who as- 

 signs to it several new lines, the strongest 

 having wave-lengths 4,683 and 3,814.7, and 

 two bands. No characteristic spectrum has 

 yet been found for polonium or actinium. 

 On account of the small quantities of these 

 substances available, no accurate atomic 

 weight determinations have yet been made, 

 but from what has been done it appears 

 that radium has a higher atomic weight 

 than barium. 



At first it was supposed that the uranium 

 itself was the source of the radio-activity of 

 the uranium salts, since often the activity 

 of the compounds seemed to depend on the 

 amount of uranium present, but after the 

 discovery of radium and polonium the ra- 

 dio-active power of uranium began to come 

 under suspicion. Professor Crookes found 

 that often different specimens of the same 

 uranium salt would have very different 

 radio-active strength, which difference in 

 strength could be very little changed by 

 changes in chemical or phj^sical conditions, 

 the strongly active salt remaining so and 

 the less active salt never gaining strength. 

 Suspecting that the radio-activity was due 

 to something other than the uranium, Pro- 

 fessor Crookes set about separating this 

 irrepressible element. Such was his suc- 

 cess that, starting with active uranium ni- 

 trate, he was able to obtain from it ura- 

 nium nitrate which had no effect on a 

 photographic plate even with an exposure 

 of seven days. At the other end of the 

 separation he had a substance many times 

 as active as the original salt. The best 

 and simplest of several methods of separa- 

 tion makes use of ether as a solvent. The 

 ether dissolves the uranium nitrate and 

 leaves undissolved most of the radio- 

 active substance. This substance Professor 

 Crookes calls UrX, the X testifying to 

 our present ignorance of its real nature. 

 Professor Crookes has also tried to separate 



the active material from the thorium com- 

 pounds, but has so far met with only par- 

 tial success. It seems not unlikely, though, 

 that it may yet be done as completely as in 

 the case of uranium. Meanwhile it is con-, 

 venient still to speak of uranium and tho- 

 rium radiations. 



PHOTOGRAPHIC AND CHEMICAL EFFECTS OF 

 THE RADIATIONS. 



The photographic efi"ect was the one 

 first discovered and it remains the most 

 delicate test for radio-activity, for the 

 effect is cumulative, and the exposure may 

 be made as long as desired. Any one can 

 obtain very good radiographs from any of 

 the ordinary uranium salts by using rapid 

 dry-plates and an exposure of two days, 

 while visible effects can be obtained with 

 a much shorter exposure. Polonium and 

 radium affect a photographic plate in a 

 few minutes if sufficiently close to it. Even 

 with a distance of a meter between the 

 radium and the plate, radiographs have 

 been obtained after a few days' exposure. 

 These are very sharp, showing thus the 

 rectilinear propagation of the radiations. 

 Polonium radiations are so rapidly ab- 

 sorbed by the air that no effect is pro- 

 duced with a greater distance than a few 

 centimeters. 



Under the action of the radium ra- 

 diations glass takes a permanent brown 

 or violet tint. The haloid salts of the 

 alkali metals become colored just as 

 under the action of the X-rays. Paper is 

 sometimes discolored and under certain 

 conditions ozone may be formed in the 

 neighborhood of the very active « sub- 

 stances. Barium platino-cyanide is colored 

 brown. 



The action of the radium rays on the 

 skin is the same as that of the X-rays. 

 At first there is a slight reddening of the 

 skin, but after three or four weeks' exposure 

 severe inflammation sets in. 



