November 21, 1907] 



NATURE 



surveyors and astronomers. The examples are mostly 

 taken from actual observations, and the necessary cal- 

 culations are g'iven in considerable detail. In the 

 earlier chapters a knowledge of determinants is not 

 assumed, and the explanations given ought to make 

 the method intelligible to readers of quite moderate 

 mathematical ability. For the more difficult and con- 

 troversial points of the theory, reference is made to 

 the treatise of Czuber ; at the same time, a very good 

 example of the unavoidably empirical nature of the 

 whole subject is given by working out the same 

 elementary problem according to each of three dif- 

 ferent laws of error. Now that the measurements of 

 physics and chemistry are approaching, not to say sur- 

 passing, in exactness those of astronomy and geodesy, 

 a practical work of this kind is likely to assist a 

 larger and larger body of experimenters. 



Die Purpurbakterien. Eine mikrobiologische Studie. 



By Prof. Hans Molisch. Pp. 92. (Jena : Gustav 



Fischer, 1907.) Price 3 marks. 

 Tuts memoir deals with an interesting group of 

 chromogenic microorganisms, viz. those producing 

 brilliant pigments ranging in tint from pink, through 

 rose and deep red to reddish-purple. They are prob- 

 ably more nearly allied to the coloured alga 

 (PhycochromaceEe) than to the bacteria proper, and 

 one of the earliest descriptions of a member of the 

 group was given by Sir Rav Lankester in 187-; 

 under the title of a " peach-coloured bacterium." The 

 author first discusses the occurrence in nature of these 

 organisms. Sometimes they are met with in great 

 abundance on the sea-coast, in river estuaries, and 

 in hot_ and sulphur springs. Directions are given for 

 obtaining growths in various organic mixtures, for 

 the preparation of suitable culture media, and for ob- 

 taining pure cultures in the latter. The biological 

 and physiological properties are next considered ; 

 while light has an inhibitory, or a germicidal, action 

 on most bacteria, the "purple " bacteria develop best 

 in its presence. They are sensitive to all light rays, 

 but in particular to the ultra-violet ones ; thev do 

 not. however, evolve oxygen in the presence of light, 

 and their need for oxvgen varies much, some species 

 being almost anaerobic. 



The colouring matter produced bv the " purple " 

 bacteria is a mixture of two pigments — a green, 

 " bacteriochlorin," and a red, " bacteriopurpurin." 

 The chemical and other properties of these are de- 

 scribed fully. 



As regards classification, the organisms are grouped 

 in a special order, the Rhodobacteria. This is divided 

 into two families, distinguished by the presence or 

 absence of sulphur granules, and several new species 

 isolated by the author are described. .Altogether the 

 book forms a very useful summary of our knowledge 

 of an interesting and peculiar group of micro- 

 organisms. R. T. Hewlett. 



The Case of Existence. By Norman Alliston. 

 Pp. xiii + 262. (London : Kegan Paul, Trench, 

 Trijbner and Co., Ltd., 1907.) Price 55. net. 

 " Of the enigmas of life," says Mr. Alliston in his 

 introduction, "all speak; but nobody acts as if there 

 were an enigma." It is his object to remove this 

 inconsistency bv exposing the confusions of thought 

 of those who " want life speculatively to be a mys- 

 tery." The book has three parts. The first con- 

 tains a good account of the nature of Explanation 

 and some not ineffective criticisms of Knowledge, 

 Nature, &c., mingled, it must be admitted, with much 

 rather ill-informed dogmatism. The second, in the 

 course of a review of man's " obstinate questionings " 

 about existence, develops the author's peculiar egoistic 

 optimism. The third, in which he draws his ethical 



NO, 1986. VOL. 77] 



corollaries, unfortunately contains some chapters 

 which many readers will find offensive both in matter 

 and in tone. The book contains little to engage the 

 attention of the practised student of philosophy, but, 

 being written with obvious conviction and enthusiasm, 

 may here and there attract a useful recruit to the 

 study of first principles. At a later stage the student 

 may not unprofitably return to these pages to detect 

 and analyse the crudities and ambiguities which 

 abound under a surface of apparent lucidity. 



Science German Course. By C. W. P. Moffatt. Pp. 



xii + 22S. (London : W. B. Clive.) Price 35. 6d. 

 The portion of this book devoted to grammatical 

 construction and word formation occupies about eighty 

 pages, and is followed by extracts for translation 

 from the German. These selections deal with various 

 scientific subjects, and can be commenced after the 

 student has made himself familiar with the first few 

 pages of the grammar that precedes them. Short 

 vocabularies are given of technical terms in mathe- 

 matics, physics, chemistry, geology, botany, and 

 zoology. The book thus provides a convenient means 

 of obtaining sufficient acquaintance with the _ German 

 language to read simple scientific descriptions in it with 

 intelligence. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinion; 

 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 Wehnelt Kathode in a High Vacuum. 



That a good vacuum can be made into a good con- 

 ductor by the use of an incandescent kathode is known 

 since the discovery of the Edison effect, and has been 

 investigated with great thoroughness by O. W. Richard- 

 son (Trans. Roy. Soc, 1903, 201 A, 497). Wehnelt has 

 shown [Ann. d. Physik, 1904, iv., 14, 425) that if the 

 incandescent kathode is coated with one of the alkaline 

 earths, surprising results can be obtained. From a 

 platinum foil kathode at 1300° C. to 1400" C. coated 

 with lime, two to three amperes per sq. cm. of surface 

 can be passed through a good vacuum, the kathode fall 

 being practically negligible, and the total voltage across 

 the vacuum tube being below 30 volts. This result is so 

 very remarkable that I have repeated it in the following 

 way to test whether, as is commonly supposed, the pheno- 

 menon is really independent of the perfection of the 

 vacuum. 



In a tube provided with a Wehnelt kathode of about 

 a sq. cm. area was mounted an anode of the metal calcium. 

 I have recently shown (Proc. Roy. Soc, 1907, 78 .A, 429) 

 that calcium at its volatilising temperature (700° C. to 

 Soo° C.) absorbs practically instantaneously and very_ per- 

 fectly all known gases and vapours except the chemically 

 inert gases of the argon family, and have described a form 

 of vacuum furnace suitable for this operation. The tube 

 was prepared in the usual way by preliminary exhaustion 

 and washing out with oxygen to remove argon, and then 

 subjected to the action of calcium heated in a furnace 

 attached to the apparatus. When a good vacuum had 

 been obtained, current from the 250-volt supply was passed 

 through the tube between the heated Wehnelt kathode and 

 the calcium anode in order to heat the latter. 



The gases evolved from the anode and tube under this 

 treatment were absorbed by the calcium in the furnace. 

 The current was regulated by a resistance to about 

 1-2 amperes, and was interrupted at intervals to give the 

 evolved gases time to flow out of the apparatus. When 

 the gases had been for the most part removed the current 

 was passed continuously, heating the calcium anode up to 

 its volatilising point. Quite suddenly and completely the 

 current through the tub? stopped, and at the same moment 

 a copious mirror of calcium was volatilised from the anode. 

 In a little while a very feeble glow started and passed 



