214 Intelligence and Miscellaneous Articles. 



The peculiarities of island attraction (gravity) are succinctly ex- 

 plained in Chapter XVIII. The density of the crust in general is 

 2-68, and that of the fluid substratum 2-96; and the pressure of 

 the crust upon this liquid substratum is about 10,000 tons on the 

 square foot. 



In judging the cause and amount of compression in the Earth's 

 crust (Chapter XIX.), the Author thinks that a mean compression 

 of 4 per cent, (of the linear dimensions) may have elevated the 

 continents from the sea-level to their present height (p. 260). 

 Cooling of the crust and contraction of the interior may have been 

 causes of the compression, — also, to a slight degree, extravasation, 

 from beneath the crust, of the water now comjosing the ocean. It 

 is also pointed out by the Author that the expansive magma (with 

 its included water-gas) forced up from below w T ould widen cracks, 

 thus causing some compression ; and that such rocks as whinstone 

 and granite when solidifying would swell, as water does in 

 freezing. 



V. Chapters XX. to XXIV., appropriately following the dis- 

 cussion of hypotheses and phenomena in the earlier chapters, are 

 rich with facts and inferences concerning Physical Greology. Dis- 

 turbance of rocks ; folding, crumpling, shearing, faulting; fissures, 

 volcanic dykes, mineral veins; geological movements generally; 

 volcanos, volcanic action, and distribution of volcanos, are subjects 

 of great interest and are here well handled by a master. Chapter 

 XXV. gives an interesting speculation as to the origin of ocean- 

 basins, — comprising Prof. Darwin's hypothesis. 



VI. At pages 342-381 we have a concise summary of the objects 

 and results of each Chapter in the book, forming a most valuable 

 resume, which both physicist and geologist will fully appreciate. 

 An excellent Index completes this elaborate work. 



The careful references to all observers who have treated of the 

 physics of the Earth's crust, and straight-forward expositions of their 

 views, enhance the value of the book; and we may remark that all 

 the different authors and their opinions have been treated in a 

 characteristically considerate and courteous manner. 



XXIII. Intelligence and Miscellaneous Articles. 



ON THE RESISTANCE OF HYDROGEN AND OTHER GASES TO THE 

 CURRENT AND TO ELECTRICAL DISCHARGES, AND ON THE HEAT 

 DEVELOPED IN THE SPARK. BY E. V1LLARI. 



A X arc-light between carbon points of 1 centim. diameter is 

 -^*- well known to be shorter in a horizontal than in a vertical 

 direction ; and the arc is somewhat longer in a vertical position with 

 an ascending current than with a descending one, doubtless owing to 

 the greater heat of the anode, which must become more strongly 

 heated when it is uppermost. 



Arc-lights were formed in glass bulbs which were filled with dry 

 gases. The carbon electrodes were superimposed in a vertical 

 position, and were then drawn apart from each other until the arc- 



