414 Scientific Intelligence. 



served to enumerate the tine drops. The latter, under a micro- 

 scope, appear brilliant, upon a dark suface. The following are 

 some of the results obtained by this method : 



Source of the air. Number per c. c. 



Outside air, raining 32,000 



Outside air, fair 130,000 



Room 1,860,000 



Room, near ceiling 5,420,000 



Bunsen Flame 30,000,000 



—Nature, March 1, 1888, p. 428. j. t. 



10. 3fagnesium and Zinc. — Hirn has investigated the electro- 

 positive character of magnesium with the view of replacing zinc 

 in certain batteries. In the Daniell cell its E. M. F. is 2 volts ; 

 in a Grove 2*9 volts; in a Leclanche, 2*2 volts, and in a bichro-' 

 mate-cell, it gives as much as 3 volts. The local action, however, 

 is considerable and its constancy uncei'tain. — Nature, March 22, 

 1888, p. 497. j. t. • 



11. Gravity. — In a discussion upon gravitation at a meeting of 

 the Physical Society of Berlin, Helmholtz explained his concep- 

 tion of the action of gravitation. He considers gravitation as 

 being the law of nature, established by experience, that every 

 body, when, in the neighborhood of another body is subject to 

 an acceleration which is proportional to its mass and diminishes 

 in the ratio of the inverse square of the distance between them. 

 Such a law of nature as this, established as it is on the basis of 

 experience, is on the whole not unsatisfactory. — Nature, March 8, 

 1888, p. 455. j. t. 



II. Geology and Mineralogy. 



1. Geology: Chemical, Physical and Stratigraphical / by 

 Joseph Prestwich. In two vols. Vol. II, pp. 606. 8vo, Strati- 

 graphical and Physical. Oxford, 1888. (Clarendon Press.) — 

 The first volume of Prof. Prestwich's valuable work was issued 

 in 1886. This second volume commences with the oldest forma- 

 tions, and closes with the Quaternary. Its first chapter gives a 

 very convenient table of the geological series of England with 

 their equivalents in the different countries of Europe, and follows 

 this with a corresponding table of the rocks of India, with an 

 enumeration of their prominent genera of fossils; and the same 

 for North America, Australia, New Zealand and South Africa. 

 The author makes the Cambrian end with the Tremadoc slates, 

 and divides the Silurian (or the remainder of it) into Upper and 

 Lower. The volume is full in its accounts of the several forma- 

 tions and their distribution in Great Britain and other countries, 

 and in illustrating figures ; and its plates of fossils are particu- 

 larly fine. A handsome folded plate represents " the probable 

 extent of land covered by ice and snow during the Glacial period, 

 their extent now and the j>resent boundaries of floating ice," and 

 its importance is doubled by being also a good tfathymetric map 

 of the oceans from recent data. An excellent colored geological 



