August 28, 1919] 



NATURE 



5»5 



that the force exerted on them appears to diminish 

 with time. Now the electrical conductivity of grey, 

 crystalline selenium, cooled suddenly from above 

 o^ C, rises to a maximum and then steadily 

 creases with time. In the condition of maximum 

 nductivity the selenium has pronounced metallic 

 properties, and this suggests that the strongly light- 

 positive selenium is of this kind, and becomes 

 gradually transformed into a more stable and less 

 light-positive modification as time proceeds. Tel- 

 lurium appears to behave similarly. 



In continuation of Arrhenius's work on comets' tails, 

 Schwarzschild applied the theory of light pressure to 

 objects of the order of magnitude of the wave-lengfth 

 of light, and showed that a scattering of the incident 

 energy occurs on such particles. For any one kind of 

 matter there is, as a consequence, a definite size of 

 particle for which the ratio of the impressed force to the 

 incident energy is a maximum. It is interesting to note 

 that Ehrenhaft found a maximum velocity for particles 

 of a substance of a particular size, the critical radius 

 for silver (light-positive) being in accord with the 

 demands of theory, viz. 98 . 10- * cm. But a maximum 

 velocity exists also for light-negative particles, the 

 critical radius being 26 . 10-* cm. for sulphur and 

 15. 10-' cm. for light-negative selenium particles. In 

 the interpretation of light-negative photophoresis, for 

 which no theory at present exists, it must not be over- 

 looked, however, that spherical particles, say, of sul- 

 phur or selenium are apparently attracted by the 

 light, even when their dimensions correspond with 

 several wave-lengths of light. 



An interesting astronomical application of the pheno- 

 menon of photophoresis has been suggested by F. 

 Zerner (Phys. Zeit., xx., 93, 1919) to explain those ano- 

 malous comets' tails, which are directed towards the 

 sun. He refers to the observations by I. Schmidt 

 (Athens) of the 1882 comet, and suggests that whereas 

 normal comets' tails may be composed of light- 

 positive matter, it seems equally probable that ano- 

 malous comets' tails are made up of light-negative 

 material. Ehrenhaft's laboratory separation of ele- 

 ments by photophoresis would thus seem to have an 

 analogon in astronomy, and doubtless this point will 

 form the subject of much interesting research in the 

 future. Robert W. Lawson. 



STANDARDS OF MASS. 



A CIRCULAR recently issued by the United States 

 Bureau of Standards ^ furnishes information 

 concerning the verification of standards of mass and 

 the most suitable forms of such standards for different 

 purposes. An account is first given of the funda- 

 ipental and national standards of mass of the United 

 States. The standard is the kilogram, from which 

 the pound is derived by the relation i lb. avoir- 

 dupois =0-4535924277 kilogram, a relation which 

 shows that the avoirdupois pound of the United States 

 is the same as the British pound. The distinction 

 between mass and weight is then considered, and it 

 is explained that weight is measured in units of force, 

 9nd that, as it is not feasible for the purposes of 

 metrology to base the unit of force on some concrete 

 standard force, the unit is derived from the estab- 

 lished units of mass and acceleration. 



The next section of the Circular is a convenient 

 classification of weights, describing the forms recom- 

 mended for particular classes of work. Weights 

 intended to be of high precision, such as the primary 

 standards of the various States of the Union or 



J " Design and Test of Standards of Mass." Circular of the Bureau of 

 Standards. No. 3, 3rd edition, pp. 89. (Washington, 1918.) 



NO. 2600, VOL. 103] 



reference standards used by first-class manufacturers, 

 are only accepted for verification at the Bureau if 

 they comply with a certain specification as to material, 

 form, and structure. Unless they are made of 

 platinum or a metal which resists atmospheric corro- 

 sion, they must be protected by a plating of gold or 

 platinum. Nickel-plating is not allowed. The 

 material and plating must be such that no discolora- 

 tion appears on the surface of the weights when they 

 are placed in boiling water or when dried at a tem- 

 perature of 1 10° C, as is done in preparing them for 

 test. Manufacturers are advised that in machining 

 such weights the knob, top, and sides should be 

 finished first, next the outer rim of the bottom, and 

 then the central portion of the bottom hollowed out 

 by an amount approximately equal to the volume of 

 the knob. The preliminary adjustment should be com- 

 pleted in the last operation. 



As regards the adjustment of commercial test- 

 weights, it is of great practical importance that the 

 means of closing the adjusting hole shall be such 

 that the weights can be readily readjusted, but that 

 the operation shall necessarily involve the defacement 

 of the stamp. Various forms of adjusting plugs suit- 

 able for such weights are illustrated and described. 



The second half of the Circular is devoted to the 

 verification of weights and the reduction of observa- 

 tions. The different methods of weighing are described 

 and the particular purposes are indicated for which 

 each method is most appropriately applicable. Illus- 

 trations are given of the various weighing forms in 

 use at the Bureau, and examples of the methods of 

 comparison, as well as of the computations, are set 

 out in a very explicit manner. The important question 

 of the correction for the buoyancy of the air is very 

 fully treated. As regards the determination of 

 hurnidity, it is pointed out that the hair hygrometer 

 is almost the only form of instrument that can be 

 used inside a closed balance-case. Such hygrometers 

 should not be verified by placing them in saturated 

 vapour, as this leaves them almost worthless for some 

 time. Brief tables for use in the reduction of ob- 

 servations are appended, and the work is concluded by 

 two very convenient tables giving the equivalents of 

 avoirdupois pounds in kilograms, and vice versa, from 

 I to 999 in each case. 



In its present extended form this Circular is a 

 typical example of the useful publications issued by 

 the Bureau, the aim of which is not only to aid 

 scientific investigation, but also to encourage and 

 facilitate the employment of scientific methods in the 

 commercial world. 



THE FOLK-SONGS OF THE TETON 

 SIOUX.^ 



THE tribe of .American Indians selected by Dr. 

 Densmore for the researches now published is 

 the Teton division of the Dakota Sioux tribe, to which 

 the United States Government in 1868 assigned the 

 portion of territory known as the Standing Rock 

 Reservation, comprising some twenty million acres 

 of the provinces of North and South Dakota. Strictly 

 speaking, " Dakota " is the name applicable to the 

 natives rather than to the region, and the largest 

 division of the tribe or nation was known as 

 Ti'totjwar;, whence the contraction Teton. 



The author, who had previously published two 

 volumes on Chippewa music, has now transcribed, 

 with the help of the phonograph, more than six 



1 "Teton Sioux Musir." By Frances Densmore. Pp. xxviii +561-1-8* 

 plates. Smithsonian Institution. Bureau of American Ethnology, Butletin 

 61. (Washington : Government Prinlin? Office, iqi8.) 



