Chemistry and Physics. 501 



CHE). 8vo, pp. 758 ; London, 1912 (Longmans, Green and Co.). 

 — Twenty-two years have elapsed since the first edition of this 

 well-known book of reference made its appearance as a companion 

 to Watts' Dictionary. During this period chemistry has advanced 

 to such an extent in its applications to the arts and manufactures 

 that a complete revision and a great enlargement of scope in 

 the present edition have been necessary. It will consist of five 

 volumes in the place of the original three volumes. 



An inspection of the new volume shows evidence of thorough 

 work on the part of the contributors, who have been selected not 

 only from the United Kingdom but also from America, Germany, 

 Switzerland, etc. The work appears to have been thoroughly 

 revised and modernized, particularly as far as the topics in which 

 great advances have been made in the last two decades are con- 

 cerned. No detailed review of a book of such magnitude and 

 complexity can be attempted here, but a few of the prominent and 

 interesting articles in the present volume may be mentioned, such 

 as " Acetylene as an Illuminant," "Analysis," "Aluminium," 

 "Brewing," "Carbohydrates," and "Cellulose." h. l. w. 



6. On the Properties of the Pays Producing Aurora Borealis. 

 —It has been generally assumed that aurora? are caused by elec- 

 tromagnetic disturbances in the earth's atmosphere due to radia- 

 tions from the sun, but the precise nature of these radiations has 

 not been established heretofore. An appreciable advance towards 

 the solution of the problem has been made by L. Vegard, who 

 starts with the hypothesis that the incident radiations are small 

 electrified particles or rays. The straight-lined streamers of the 

 aurorse would require a radiation which is but little scattered and 

 this condition is fulfilled by a-rays and not by /3-rays. The 

 abruptness with which the luminosity stops at the lower edges of 

 the streamers corresponds to the well-defined range of a-particles 

 in a gas. The ionization, due to a homogeneous pencil of a-rays, 

 is known to increase as the speed decreases, attaining a maximum 

 value near the point where the rays are stopped. This fact has 

 its counterpart in the increase in luminosity observed near the 

 lowest parts of auroral bands and streamers. By comparing the 

 altitudes at which the a-rays from various radio-active substances 

 would be stopped by the earth's atmosphere with the observed 

 heights of auroras, Vegard shows that the agreement is as good 

 as can be expected from the data at hand. The parallel, drapery 

 bands can be accounted for by the assumption of groups of homo- 

 geneous rays from the same source. A mathematical investiga- 

 tion of the paths which would be followed by charged particles 

 entering the earth's atmosphere from the sun, leads to the conclu- 

 sion that a positive charge is most consistent with the observed 

 positions of aurora?. Thus it is seen, that the majority of auroral 

 forms may be explained on the assumption that they are due to 

 a-rays emitted by radio-active substances of the sun. — Phil. Mag., 

 xxiii, p. 211, February, 1912. h. s. u. 



1. The Pressure of a Blow. — In a discourse delivered at the 

 Royal Institution on January 26, Prof. Bertram Hopkinson 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 197.— May, 1912. 

 33 



