June i8, 1903] 



NATURE 



163 



at the focus of the mirror and in the optical axis of the 

 i-ame. 



The driving- motion of the clock is transmitted to the 

 telescope by two sectors, one of which is being run back 

 ready to be put into gear again whilst the other is being 

 used ; each sector allows of one hour's exposure being 

 made. The " following " is performed by means of an 

 auxiliary telescope rigidly attached to the plate holder 

 (Scientific American, May i6}. 



The Relationships between Arc and Spark Spectra. — 

 In No. 4, vol. xvii. of the Astrophysical Journal there 

 appears an advance translation, by the author, of a paper 

 on the above subject recently communicated to the 

 K. Akademie der Wiss. zu Berlin by Prof. J. Hartmann. 



In his experiments on the arc spectrum of magnesium, 

 using metallic poles, he found that the line at \ 4481, which 

 is generally regarded as essentially a " spark " line, 

 appears in the arc spectrum, and actually increases in in- 

 tensity as the current strength becomes less • this is plainly 

 shown in a table which accompanies the paper. From this 

 and similar results the author arrives at the conclusion 

 that the higher temperature of the spark, as compared with 

 that of the arc, is open to question. 



Further experiments showed that a high voltage was 

 not necessary for the production of " spark " lines in the 

 arc, for when a current of 20 volts and 4 amperes was 

 used the line 4481 was about thirty times more intense 

 than when 120 volts and 4 amperes were used. 



Prof. Hartmann arrives at the conclusion that the energy 

 of the electric discharge and of the chemical changes may 

 play a more important part in the production of " spark " 

 lines than temperature does, and in his experiments, in 

 which the arc was formed in an atmosphere of hydrogen, he 

 has shown that the dielectric is also an important factor in 

 determining the nature of the spectrum obtained. 



RADIO-ACTIVE PROCESSES.' 

 T^flKRE are three distinct types of radiation spon- 

 ■*• taneously emitted from radio-active bodies, which may 

 bo called the o, fi, and y rays. The o-rays are prominent 

 in causing the conductivity of a gas, they are easily absorbed 

 by metals, and are projected bodies, not waves. These 

 bodies are about the size of a hydrogen atom, they are 

 positively charged, and travel with about one-tenth of the 

 velocity of light. The )3-rays are similar in all respects to 

 the kathode rays produced in a vacuum-tube. The 7-rays 

 are probably like Rontgen rays, but of very great pene- 

 trating power. The o-rays are by far the most important. 

 In addition to these rays two of the radio-elements give off 

 radio-active "emanations," which are in all respects like 

 gases. The radiations from these emanations are not per- 

 manent, but fall off in a geometrical progression with the 

 time. The radiation of the thorium emanation falls to half 

 value in one minute, that from radium in four days. They 

 have all the properties of gaseous matter in infinitesimal 

 <juantity. Their coefficients of diffusion can be measured, 

 the order of their molecular weights is 100, they are occluded 

 by solid compounds producing them, and may be condensed 

 at low temperatures. The radium emanation condenses 

 sharply at —150° C, the thorium emanation between 



— 120° C. and 



The two emanations excite on 



objects with which they come in contact two kinds of 

 temporary radio-activity, that from the radium emanation 

 decaying much faster than that from the thorium eman- 

 ation. The latter decays in a G.P. with the time falling to 

 half value in eleven hours. These effects appear to be pro- 

 duced by solid matter in invisible and unweighable quantity, 

 which can be dissolved off in some acids but not in others. 

 On evaporating the solutions, the radio-activity is obtained 

 unchanged in the residue. The experiments of Crookes and 

 Becquerel in separating by chemical treatment the matter 

 responsible for the activity of uranium, called uranium X, 

 were referred to, together with the latter 's observation 

 that the separated activity had completely decayed after the 

 lapse of a year, by which time the uranium itself had com- 

 pletely recovered its activity. The work of Rutherford and 



1 Abstract of paper read before the Physical Societv on Tune ■;, bv 

 Prof. E. Rutherford, F.R.S. 



NO. T755, VOL. 68] 



Soddy on thorium was then discussed in detail. Thorium 

 precipitated in solution by ammonia retains only 25 per 

 cent, of its activity. If the solution is evaporated and 

 ignited the remaining 75 per cent, is found in the extremely 

 small residue left, which by reason of its separation is 

 ditlerent chemically from thorium, and was called 

 thorium X. Left to themselves, the thorium gradually re- 

 covers its activity, and the ThX loses it. The activity of 

 the latter falls in a G.P. with the time, the half value being 

 reached after four days. At any time the sum total of the 

 two activities is a constant. This would occur if the ThX 

 were being continually produced by the thorium, and this 

 was shown to be the case by precipitating thorium at 

 definite intervals after its separation from ThX. The ThX, 

 and not thorium, produces the thorium emanation. The 

 production of ThX by thorium, of the emanation by ThX, 

 and of the matter causing the excited activity by the eman- 

 ation, are all changes of the same type, although the rates 

 of change are distinct in each case. The change of uranium 

 into uranium X is also similar, being the slowest of all. 

 Twenty-two days elapse before uranium freed from ThX 

 recovers one-half of its activity. In radium the radium 

 emanation is the first product produced, and since this in 

 a solid is almost completely occluded, the activity of a 

 radium salt after it has been obtained from its solution 

 rises after precipitation to several times its original value, 

 due to the occlusion of the emanation. In all three radio- 

 elements a part of the radio-activity is non-separable, and 

 this part consists only of o-rays. The j8-rays only result 

 at the last stages of the process that can be experimentally 

 traced. In all cases the radiation, from any type of active 

 matter, is a measure of the amount of the next type pro- 

 duced. Thus the radio-activity of ThX at any period 

 throughout its life is always a measure of the amount of 

 emanation it produces. These results find their explanation 

 if it is supposed that the o-particles projected form integral 

 portions of the atom of the radio-active element. Thus 

 ThX is thorium minus one or more projected o-particles. 

 The emanation similarly is ThX less a further o-particle, 

 and so on. The non-separable activity is due to the atoms 

 of the original radio-element disintegrating at a constant 

 rate. The whole of the processes take place unaltered in 

 velocity, apparently under all conditions of temperature, 

 state of aggregation, and chemical combination. This is 

 to be expected of a subatomic change in which one system 

 only is involved at each change. On this view the spon- 

 taneous heat-emission of solid radium salts, discovered by 

 Curie, is explained by the internal bombardment by the 

 o-particles shot off and absorbed in the mass of the sub- 

 stance. The amount of energy given out in these sub- 

 atomic changes is enormous, and from Curie's experiments 

 it can be deduced that each gram of radium gives out 10" 

 gram-calories during its life, which is sufficient to raise 

 500 tons a mile high. It seems probable that the internal 

 energy of atoms in general is of a similar high order of 

 magnitude. 



SOME UNSOLVED PROBLEMS IN 

 ENGINEERING.' 

 'T'HE present lecture is devoted to the indication of some 

 -*■ of the directions in which the further aid of the 

 physicist is more immediately required by the engineer, 

 while it is hoped that in future lectures each branch of 

 inquiry thus pointed out will be dealt with in detail by 

 someone who has made that particular subject his special 

 study. 



In v=sw of the great interests — monetary and otherwise 

 —involved, it appears to me that the whole question of 

 steam-jacketing, and particularly the application of such 

 jackets to compound or multiple-expansion engines of 

 modern types and of large power, using steam at high 

 pressures, deserves a much more thorough and systematic 

 investigation than it has hitherto received. 



The action of steam-jackets is, however, only one of 

 several important problems relating to steam-engine 

 economy at present remaining unsolved. Another is the 



1 Abridged from the eleventh "James Forrest" lecture delivered by 

 Mr. W. H. Maw to an EngineeringjConference on June 16, at the Institution 

 of Civil Engineers. 



