398 



NATURE 



[February 21, 1901 



It is shown by experiment that it is sufficient to arrange 

 between the polar pieces of an electromagnet a hori- 

 :zontal photographic plate upon which is placed, in the 

 centre of a little leaden dish, the radiating source of very 

 small diameter. The rays emanating normally to the 

 plate and brought back orthogonally on it are the most 

 efficacious ; the impression, large and diffiise, is thrown 

 on one side of the field, and constitutes a sort of spectrum 

 which is sufficiently pure. If one works in the dark, 

 and places on the photographic plate bands of various 

 •substances, aluminium leaf, copper and platinum foil, 

 &c., it is seen that under the screens the impression of 

 the deviated rays is limited by elliptic arcs, distin- 

 guished from each other. Each screen arrests different 

 radiations, the most deviated being the most absorbable. 

 The dimensions of the elliptic arcs are in accordance with 

 theory. The absorption is the same in air and in an 

 absolute vacuum. These experiments constitute a sort of 

 magnetic spectrum analysis of the deviable radiation. 



The identification of this radiation with the kathode 

 rays requires two other verifications — the demonstration 

 of the existence of a transport of electric charges and 

 that of a deviation in an electrostatic field. M. and 

 Mine. Curie have made the first verification, and I have 

 realised the second. M. and Mme. Curie placed a 

 screen, isolated from all contact with the air, in con- 

 nection with an electrometer, and observed that the 

 radium radiation charged this screen negatively, whilst 

 the source itself, if it is properly isolated, is charged 

 positively. The current for each square centimeter of 

 radiating surface was about 4.10^^^ electromagnetic 

 'C.G.S. units. 



For my part, I showed that in an electric field the 

 radiation of radium undergoes a parabolic inflection in 

 the contrary sense to the field, as would be the case with 

 a flux of negatively charged particles. The comparison 

 of the electrostatic and magnetic deviations allows of 

 the determination, like that of Prof. J.J. Thomson for the 

 kathode rays, of the velocity of the particles. For the 

 particular radiations defined by H/i = 1600, the velocity 

 has been found equal to 1.6 x lo^*^ — about one-half of that 

 of light. The ratio of the material masses carried off 

 to the charges which they transport has been found 

 equal to io~^, a number identical with that correspond- 

 ing to the kathode rays. From these numbers, and that 

 which results from the experiment of M. and Mme. 

 Curie, it follows that for each square centimetre of sur- 

 face of the radiating substance studied there escapes a 

 flux of material which would amount to a loss of about 

 one milligramme in one thousand million years. If the 

 material emission, which appears to be of the same order 

 as the evaporation of certain scented substances, is 

 the first cause of the observed phenomena, there would 

 be no contradiction between the apparent absence of any 

 source of energy and the continuous emission of this 

 ■energy. 



The most deviable portions of the radium radiation 

 pass easily through all bodies when they are very near 

 the source, but are stopped when these bodies are some 

 centimetres distant. I have further recognised that, 

 after having traversed a screen, the radiation possesses 

 the same magnetic deviability. 



I can only give here the physical properties of radio- 

 activity. One of the most important applications has 

 been the discovery, by M. and Mme. Curie, of new 

 chemical elements. Radium has a high atomic weight, 

 and a characteristic spectrum observed by M. Demar^ay. 

 Polonium, according to recent researches, has a spectrum 

 in the ultra-violet. These bodies possess, then, the 

 characters of simple substances. 



Recent studies on induced radio-activity appear to 

 open still new horizons. It has been mentioned already 

 that a body becomes temporarily active when it receives 

 (the radiation of an active body. M. and Mme. Curie 



NO. 1634, VOL. 63] 



and M. Giesel have recognised that the induced activity 

 thus provoked was much greater if the body was mixed 

 in solution with an active salt and then afterwards separ- 

 ated by precipitation. In thus rendering barium active 

 with actinium, M. Debierne has recognised that the 

 active barium behaves as a different body from ordinary 

 barium, that it could be separated chemically and con- 

 centrated. Active barium thus resembles radium. It 

 differs from it by the absence of a peculiar emission 

 spectrum and by the fact that its activity weakens with 

 time. I would add that recently Sir W. Crookes, who 

 has made numerous experiments by the photographic 

 method, announced that he had prepared uranium that 

 was almost inactive. According to these experiments, 

 as well as those of M. Debierne, M. Giesel and myself, 

 it would appear to follow that the activity of uranium is 

 due in great part, if not altogether, to a small quantity of 

 actinium or of another radio-active body. 



These facts, although increasing the complexity of 

 radio-activity from the chemical point of view, do not, 

 however, remove from it its value ; it can be remarked 

 that if the existence of uranium as a simple body had 

 been unknown up to the present, its radiant properties 

 would have permitted of its isolation, even though they 

 are not inseparable from its existence. 



This short account shows that a new order of pheno- 

 mena has arisen from the study of a new property of 

 matter — radio-activity. Henri Becqup:rel. 



PROGRESS OF THE MAGNETIC SURVEY OF 

 THE UNITED STATES. 



n^'HE special division of the United States Coast and 

 •*- Geodetic Survey devoted to the magnetic survey of 

 the United States, and countries under its jurisdiction, 

 was created by the late superintendent. Dr. H. S. 

 Pritchetr, now president of the Massachusetts Institute 

 of Technology, on July i, 1899 ; and Dr. L. A. Bauer was 

 put in charge of the division. Since that date magnetic 

 observations — namely, declination, dip and intensity of 

 magnetic force, have been made up to December 31, 1900, 

 at about 500 stations distributed over the United States, 

 Alaska and the Hawaiian Islands. At most of the stations 

 permanent marks have been established for the use of the 

 surveyor. Special consideration has also been given to 

 the needs of the mariner,' especially in Alaskan waters, 

 where occur places of pronounced local attraction 

 affecting the compasses on board ship all the way from 

 I of a point to 4 points. 



Special stations, known as "repeat" or " secular varia- 

 tion " stations, have also been established in different 

 parts of the United States. At these, observations will 

 be repeated at stated intervals in order to determine the 

 amount of secular change in the magnetic elements. It 

 is the endeavour, whenever possible, to establish such 

 stations in the vicinities of colleges and universities, as 

 experience has shown that on college grounds there is 

 hope for a permanency of station for a fairly long 

 interval. 



Of special State surveys mention may be made first of 

 the completion of the magnetic survey of Maryland, 

 which was undertaken primarily by the Maryland 

 Geological Survey and assistance rendered by this 

 Bureau ; second, the completion of the magnetic survey 

 of North Carolina, conducted under the joint auspices of 

 this Bureau and the North Carolina Geological Survey ; 

 third, the completion of the magnetic survey of West 

 Virginia ; and fourth, the completion of the magnetic 

 survey of Iowa. 



Fair progress has also been made in the establishment 

 of the magnetic base stations, where the countless varia- 

 tions of the earth's magnetism will be recorded photo- 

 graphically. Thus, a temporary magnetic observatory 



