5IO 



KNOWLEDGE & SCIENTIFIC NEWS. 



[August, 1906. 



oxide is rendered doubtful owing to the very small amount 

 of radium employed. With the quantity actually used the 

 efficiency was less, but this might not be the case if more 

 radium were available. 



Decay of Radium A, B. and C. 



Dr. H. L. Bronson is continuing his researches on the 

 decomposition products of radium, and claims to have 

 proved — 



(i) That within the limits of experimental error the 

 experimental decay curves agree with the theoretical 

 decay curves, calculated on the assumptions that the 

 three products, A, B, and C are successive, and that 

 their periods are 3, 26, and 19 minutes. 



(2) That radium B emits Beta-rays of less penetrating 

 power than those from radium C ; and that on this 

 account the Beta-ray decay curves are unsatisfactory 

 for the purposes of analysis. 



(3) That these Beta-rays from radium B completely 

 explain the divergence which Rutherford found be- 

 tween the experimental and theoretical Beta-ray curves. 



Image irv a Concave Mirror. 



If a lighted candle be placed in front of a concave mirror 

 between the focus and the centre of curvature, a real in- 

 verted image of it upon a screen can be obtained in a definit ^ 

 place beyond the centre of curvature. If the candle is 

 placed so that this image is fairly distant, anyone looking 

 into the mirror will see a second image of the candle ap- 

 parently behind- the mirror. Now since the rays from any 

 point of the candle must be converging after reflection to a 

 point on the real distant image, it follows that the image 

 seen in the mirror must be seen by means of converging 

 cones of rays, .^ny eye which sees this image with anv- 

 thing like perfect definition must be able to adjust itself 

 to more than infinitely distant objects. Short-sighted 

 people will not see the image except as a blurr of light. 

 But it is astonishing how well-defined it can appear. An 1 

 it is still more astonishing that, as far as I am aware, no 

 text-book contains any account of this phenomenon 

 Instead of a candle, the obser\'er may examine the image 

 of his own face in this way, and it is interesting to observe 

 the changes as the observer draws close to the mirror, when, 

 of cour.se, the usual, erect vertical image is obtained. Those 

 interested in this question are recommended to draw the 

 rays, in various cases, by which the images are seen. 



The Magnetisation of a Magnet. 



A curious misconception in ver)- elementary text- 

 books is as to the portion of an ordinary bar magnet which 

 is the more highly magnetised. Examinees tell one ad 

 ■naxiseam that if is the ends which are so. Of course, it 

 is near the ends that most external evidence is obtained of 

 the bar being magnetised at all ; that is, it is there that the 

 so-called free magnetism is in evidence. But though it is 

 perhaps not quite so obvious how to obtain proof of the 

 statement, it is nevertheless true that it is the middle of 

 the magnet that has the greater magnetisation. Thos:" 

 who are familiar with the molecular theory of magnetisa- 

 tion, which attributes magnetisation to the alignment of 

 already existing molecular magnets, should have no diffi- 

 culty in realising this fact, for it is clear that the reversing 

 action of the free poles at the ends will tend to make the 

 alignment less perfect near them. The fact that lines of 

 force leave the magnet in the neighbourhood of the poles 

 also points to the same conclusion ; for it indicates that there 

 must be more lines passing through the centre than near 

 the poles, the rest having escaped. The simplest experi- 

 mental proof is from the phenomena of induced currents. 

 A coil of wire connected to a galvanometer is traversed 

 by an induced current not only while the coil is being 

 brought up to the pole, but while it is moved by stages over 

 the magnet from pole to centre. The induced flow is due 

 to the change in the number of magnetic lines threading 

 the coil, and the direction of the flow is that corresponding 

 to an increase in this number, as the centre is approached. 

 The confu.sion, of course, is between magnetisation and 

 free magnetism. Near the centre the magnetisation is 

 great, but practically no free magnetism shows itself, be- 



cause north and south kinds are present together in nearly 

 equal quantity. Near the ends the magnetisation is less ; 

 but free magnetism, either north or else south, is 

 conspicuous because there is very much more of one kind 

 than of the other. 



ZOOLOGICAL. 



By R. Lydekker. 



The Sea-Serpent. 



Ix'TEREST, for the moment, is concentrated on the account 

 given at the meeting of the Zoological Society on the igth 

 of June by Messrs. Kicoll and Waldo, of their experiences 

 of llie sea-serpent, when cruising off Para on the morning 

 of December 7 of last year. That these gentlemen, both 

 experienced observers, .saw some huge marine animal can- 

 not be doubted ; while their description of such portions of 

 it as came under their notice does not enable us to refer it to 

 any known group of living vertebrates. An especiallv note- 

 woithy feature is that the description tallies remarkably well 

 with that of the monster seen many years ago by the officers 

 of H.M.S. Dcrdahis. Two great difficulties enter into the 

 question of the existence of a sea-serpent, namelv, the 

 absence in Tertiary formations of the fossil remains of any 

 such animal, and the infrequency of its appearances. 

 Being presumably an air-breather (as otherwise there would 

 be no occasion for its ever coming to the surface), it should 

 make its appearance at coraparativelv frequent inter\'als. 

 and, therefore, ought to be seen not uncomraonlv. 



The Indian Ocean. 



At a recent meeting of the Royal Geographical Society, an 

 important communication was made by ^Ir. J. .S. Gardiner 

 concerning the natural history and mode of formation of 

 the Indian Ocean, based on an exploring exoedition under- 

 taken in H.M.S. SeaJark. One of the problems specially 

 considered was the manner in which the land-bridge 

 foimerly connecting Madagascar with Ceylon and India 

 was removed. In the author's opinion, this was eflected 

 entirely by currents ; but in the discussion it was pointed 

 out that these could not have formed the basin of the 

 Indian Ocean, which can .scarcely be due to anything but 

 subsidence, and consequently that the later changes were 

 piobably also due to the same agency. 



The Fins of Fishes. 



.\ controveesv has been long in existence with regard to 

 the origin of the paired fins of fishes, and thus the limbs of 

 vertebrates generally. According to one theor)', these fin^ 

 are derived from gill-structures ; the arches, or supports, of 

 the gills having become modified into the shoulder-girdle 

 (scapula and coracoid) and pelvis, while from the gill-flaps, 

 with their supporting gill-rays, the fins themselves have 

 been e\olved. According to the alternative view, the fins 

 are modified portions of a longitudinal fold of skin running 

 along each side of the body. It is to be hoped that the 

 controversy may be finally closed as the result of a paper 

 communicated by Mr. Goodrich, of Oxford, to the June 

 issue of the Quarterly Journal of Microscopical Science. 

 .\ttention is there directed to the fact that the paired fins 

 nnd the median fins are structurally similar; and that while 

 this is in perfect acrord with the lateral-fold theory, it can- 

 not possibly be explained by the gill-theory. 



South America and Africa. 



In the course of a paper on the freshwater fishes of South 

 and Central America, published in the June issue of the 

 Fopuhir Science Monthly, Professor Eigenmann expresses 

 his opinion with regard to another much-debated question, 

 namely, a foimer land-connection between South America 

 and .Africa. 



" In tiie earliest Tertiai-y," he writes, " tropical America 

 cojisistcd of two land-areas, Archiguiana and Archamazonia, 

 separated by the lower valley of the Amazon, which was 

 siill submerged. There was a land-mass, Hellcnis, between 

 Africa and South .America, possibly in contact with Guiana 

 .-ind some point in tropical .Africa. This land-mass, which 



