February 4, 19 15] 



NATURE 



61 



each, which occupies the greater part of the book, 

 follows the ordinary, old-fashioned lines, even to 

 the use of the Millerian indices, and the author 

 has wisely refrained from complicating the discus- 

 sion by devising a brand new set of names for the 

 several classes. The sections on the Goldschmidt 

 method are, on the other hand, scattered through- 

 out the book; it would have been wiser to 

 collect them together and to have expanded 

 them. The argument is incomplete. For instance, 

 the fundamental property of the gnomonic pro- 

 jection, viz., that all zones are represented by 

 straight lines, is very indefinitely stated, and not 

 proved at all. Examples of working out crystals 

 belonging to the six systems should have been 

 included in the several chapters, instead of re- 

 printing more or less fully at the end of the book 

 a few original papers, in which the actual work- 

 ing is subservient to the interest of the particular 

 research. For a full understanding of the Gold- 

 schmidt method the student must still refer to the 

 original source. 



We are informed that the Hill Publishing Co., 

 Ltd. , are the London publishers, but their name is 

 not given on the title-page. 



First Booh of Physiology and Hygiene. By Ger- 

 trude D. Cathcart. Pp. vi-f-158. (London: 

 Macmillan and Co., Ltd., 1914.) Price 15. 6d. 

 It is apparently the notion in certain educational 

 circles that hygiene can be taught without a 

 preliminary knowledge of the science physiology 

 on which it is founded. If such an idea still 

 lingers anywhere it will be immediately dispelled 

 by a perusal of this attractive little book. The 

 author shows quite clearly that the laws of health 

 are direct deductions from physiological prin- 

 ciples. These are explained in clear, simple 

 language, so free as possible from technical 

 terms, and we can highly recommend the book 

 as suitable for readers commencing the study of 

 the subject, or for those who do not wish to 

 take it up from the professional and medical 

 point of view. Where so much is excellent, it 

 seems ungracious to point out a serious mistake, 

 the only one so far as can be ascertained ; this 

 is the erroneous statement that the red blood 

 corpuscles are the carriers of carbon dioxide. 



W. D. H. 



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 Nature. No notice is 

 taken of anonymous communications.^ 



The Density of Lead from Ceylon Thorite. 



Last May, in conjunction with Mr. H. Hyman, I 

 published the result of an examination of the relative 

 atomic weight of a small specimen of lead, less than 

 one gram, separated from a kilogram of Ceylon 

 thorite, which showed a value rather more than a 

 unit in excess of that found for a specimen of ordinary 

 lead. I have since been engaged in extracting the 

 lead from 33 kilograms of Ceylon thorite, which was 

 first carefully sorted by hand, piece by piece, into 



NO. 2362^ VOL. Q4.1 



various grades. From the finest grade, consisting of 

 20 kilograms, about 80 grams of lead were obtained, 

 in agreement with the percentage obtained by 

 analysis. 



This specimen, and a similar weight of ordinary 

 assay lead, have been carefully purified by identical 

 processes, and finally obtained as metal, by fusing the 

 oxide with cyanide, and repeating the fusion with the 

 metal. The very porous castings so obtained were 

 melted in a mercury pump vacuum in glass tubes with 

 drawn-out jets, and cast into cylindrical graphite 

 moulds in the vacuum, hydrogen to atmospheric pres- 

 sure being then admitted, and the lead allowed to 

 freeze from the bottom. 



It was thought that a determination of the specific 

 gravity would }ield results of interest. It is to be 

 expected that the atomic volumes of isotopic elements 

 should be identical, so that, on this view, the densities 

 should be in proportion to the atomic weights. The 

 density of lead distilled in a vacuum was found, by 

 Kahlbaum, Roth, and Siedler, to be 11-3415 (D^O* 

 and after pressing to 10,000 atmospheres, 11-3470. 

 For the ordinary lead, prepared as above detailed, 

 D^=' was found to be 11-3465, as the mean of thret 

 determinations agfreeing within 8 units in the last 

 place, in good accord with that found for distilled 

 lead. On the other hand, the value found for the 

 sp>ecimen of thorite lead was 11-376, which is 0-26 per 

 cent, greater, and higher than has been found pre- 

 viously in any trustworthy determination. The atomic 

 weight calculated from the density, taking 207-10 as 

 the figure for ordinary lead, is 20764. It remains to 

 be seen whether the constants of the lead will be 

 altered by further purification, but one would expect 

 that the effect of any possible impurity would be to 

 decrease, 'rather than increase the density. 



During the purification of the lead, bismuth was 

 specially looked for, but, if any at all was present, its 

 quantity was certainly less than one part in ten millions 

 of the mineral. This seems to dispose of the specula- 

 tion that bismuth is one of the end products of the 

 thorium disintegration. On the other hand, I was 

 surprised to find a perceptible quantity of iodine in the 

 mineral, and separated between one and two grams, 

 which Mr. J. A. Cranston is now examining. So far 

 as the tests have yet gone, there seems to be a 

 distinct trace, also, of thallium present. 



Frederick Soddy. 



Chemical Department, .Aberdeen University, 

 January 30. 



The Cause of Streaks upon Lath and Plaster Walls. 

 In reply to Mr. Thomas D. Cope's letter in N.\tl'RE 

 of January 21, it may be stated that he is correct 

 in supposing that the best explanation of the streaks 

 on the plaster he refers to is that they are due to 

 the hot-air molecules driving the dust particles into 

 contact with the plaster, and the colder the plaster 

 the weaker is the f)ower of the cold-air molecules 

 next it to resist the deposition. This tendency of 

 hot air to deposit its dust on cold surfaces can be 

 seen in a very marked way in any house heated with 

 hot water or steam. Wherever a hot pipe comes 

 j through a wall there will always be found a dirty- 

 vertical streak on the wall just above the hot pipe, 

 caused by the stream of hot air rising from the 

 pipe depositing its dust on the cold surface. 



This action of hot air on cold surfaces accounts for 

 the difference in cleanness of surfaces in rooms heated 

 by open fires, and those warmed bv hot air or bv 

 the so-called radiators, which do most of their heat- 

 ing by warming the air by contact. In a fire-heated 

 room the furniture is principally heated bv radia- 

 tion, and, being warmer than the air, it repels the 



