248 



NA TURE 



[January 12, 1905 



of a discussion on the zoological sj'stem as commonly 

 taught, Prof. H. E. Ziegler emphasises the view that 

 the rhizopod and flagellate animalcules, together with 

 the Sporozoa, form an allied assemblage, while the 

 ciliated animalcules, both as regards the nature of 

 the nucleus and the mode of reproduction, are 

 altogether different. In a fourth important com- 

 munication Dr. Bresslau amplifies and illustrates his 

 discovery that the marsupium of the marsupials, in 

 place of being a simple organ, is really formed by the 

 amalgamation of a number of small pouches. These 

 pouchlets, which at first form solid ring-like growths 

 of the epidermis, soon begin to degenerate, and are 

 merged in the wall of the marsupium. R. L. 



The Optical Dictionary. Edited bv Charles Hyatt- 

 Woolf, F.R.P.S. Pp. x+77. (London: The Guten- 

 berg Press.) Price 45. net. 

 This is an optical and ophthalmological glossary of 

 English terms, symbols, and abbreviations, together 

 with the English equivalents of some French and 

 German terms arranged alphabetically. The mean- 

 ings are, as a rule, very clearly given, and the book 

 should prove of use to students (especially medical 

 students) who suddenly come upon an unfamiliar term 

 in the course of their general reading. Of course, it 

 must be understood that it is practically impossible 

 to explain properly any scientific term in a line or two, 

 and this is all that is attempted ; the meanings given 

 must therefore in most cases be somewhat unsatis- 

 factory. But the book will doubtless succeed in its 

 aim, especially in the translation of foreign terms. 

 As regards accuracy — the sine qua non of a dictionary 

 — we only notice a very few actual errors, e.g. 

 dioptricaily does not mean by reflection, and in the 

 definition of numerical aperture the words refractive 

 index of the medium in which the object is immersed 

 scarcely indicate that the medium must extend into 

 contact with the objective. Underlant is apparently 

 a misprint for undulant, and one-third of p. 70 has 

 got into its wrong place. 



But these are not very important blemishes, and we 

 cordially recommend the book to those whom it may 

 concern. 



Practical Professional Photography. Vols. i. and ii. 

 By C. H. Hewitt. Pp. 126 and 114. (London: 

 IlifTe and Sons, Ltd., 1904.) Price is. net each. 

 These two volumes form a very useful addition to the 

 Photography bookshelf series, of which they form 

 Nos. 17 and 18. Although the author does not'profess 

 to go into any great detail, he gives an excellent 

 account of the necessary requirements of the pro- 

 fessional photographer, from the choice of business 

 premises, the handling of customers, book-keeping, 

 &c., down to the packing up of the finished pictures 

 and their dispatch. The chapters on portraiture, com- 

 position, and lighting are especially satisfactory, and 

 many a valuable hint is contained therein. 



A great number of illustrations accompany the text, 

 and serve the useful purpose of illustrating the author's 

 remarks on many lines of work. 



Solutions of the Exercises in Godfrey and Siddons's 

 Elementary Geometry. Bv E. A. Price. Pp. 172. 

 (Cambridge : The Universitv Press, 1904.) Price 

 5^. net. 

 This book will be found very useful to all, both pupils 

 and teachers, who use the well known work of Messrs. 

 Godfrey and Siddons. The solutions, 1836 in number, 

 contain not only the deductive, but the drawing 

 exercises, the figures being all such as the pupil is 

 required to construct. We cannot refrain from plead- 

 ing for a better figure of a hyperbola than that given 

 on p. 143, which a trained eye rejects at once, although 

 it is not essential to the pupil's work. 



NO. 1837, VOL. 71] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of N'.^TURE. 

 No notice is taken of anonymotis communications.] 



Average Number of Kinsfolk in Each Degree. 



May I ask you to insert yet another brief communication 

 on the above subject, because private correspondence shows 

 that parado.\ical opinions are not yet wholly dispelled? 

 The clearest way of expressing statistical problems is the 

 familiar method of black and white balls, which I will now 

 adopt. 



Plunge both hands into a dark bag partly filled with black 

 and white balls, equal in number, and well mixed. Grasp 

 a handful in the right hand, to represent a family of boys 

 and girls. Out of this unseen handful extract one ball, still 

 unseen, with the left hand. There will be on the average 

 of many similar experiments, as many white as black balls, 

 both in the original and in the residual handful, because 

 the extracted ball will be as often white as black. Using 

 my previous notation, let the number of balls in the original 

 handful be 2d. Consequently the number in the residual 

 handful will be 2d— i, and the average number in it either 

 of white or of black balls will be half as many, or d — J. 

 It makes no difference to the average result whether the 

 hitherto unseen ball in the left hand proves to be white or 

 black. In other words, it makes no difference in the 

 estimate of the average number of sisters or of brothers 

 whether the individual from whom they are reckoned be a 

 boy or a girl ; it is in both cases d — J. The reckoning may 

 proceed from one member of each family taken at random, 

 or from all its members taken in turn ; the resultant average 

 comes out the same. 



This, briefly, is my problem. Francis Galton. 



On the State in which Helium Exists in Minerals. 



In 1898 I published in the Proceedings of the Royal 

 Society the results of some experiments on the evolution of 

 gases from minerals on heating them. I succeeded in 

 proving that the hydrogen and carbon monoxide in the gases 

 could be accounted for quantitatively by the reduction of 

 water vapour and carbon dioxide by ferrous oxide, or by 

 similar substances, and that, except in cases in which 

 cavities could be proved to exist, the evolution of a gas 

 from a mineral implied chemical change at the moment of 

 heating. In the cases in which helium was evolved on heat- 

 ing a mineral, I pointed out that by the action of heat it is 

 possible to obtain only half the helium, though the evolution 

 of this gas never really ceases, but only becomes very slow. 

 This I took to be evidence of the existence of a chemical 

 compound of helium with some constituent of the mineral. 



Recently {Trans. Roy. Dublin Soc, 1904) Mr. Moss has 

 shown that by grinding pitchblende in vacuo helium is 

 evolved, and considers this result as certain evidence of the 

 existence of the gas in the free state in cavities. Since, 

 however, helium is evolved, though slowly, from the crushed 

 mineral at a temperature not above 300° C, the liberation 

 of the gas in Mr. Moss's experiment may be attributed to 

 local heating set up in the process of grinding. 



In view of recent discoveries it appears to me that both 

 of us have been on the wrong track in looking for an 

 explanation of the phenomenon. .\s Sir William Ramsay 

 and .Mr. Soddy have shown, the presence of helium in the 

 minerals may have resulted from the decomposition of radio- 

 active matter, formerly present in them. Recently Dr. 

 Jaquerod, of Geneva {Comptcs rcndus, 1904, No. 20. p. 789), 

 has found that when helium is heated in a quartz bulb to 

 a temperature above 500° C. the gas passes out through the 

 quartz with a velocity which increases with the temperature. 

 .4t 1100°, in a comparatively short time the pressure in 

 the bulb fell considerably below that of the atmosphere. 

 Hydrogen appeared to behave similarly. 



This experiment shows that quartz, and probably sub- 

 stances of the nature of the minerals we are considering, 

 though impermeable to helium at low temperatures, become 

 permeable at moderately high temperatures, and furnishes 

 us with a solution of the second part of our problem. 



