556 Reviews — Dr. T. Sterry. Hunt's Chemico- Geological Essays. 



sphere greatly below its own temperature, and having a crust growing 

 out from and supported by the sides of the vessel, are widely different 

 from those of a liquid globe slowly cooling beneath a very dense and 

 intensely-heated atmosphere. 



The composition of the Earth's crust and atmosphere are the next 

 subjects discussed. The original crust, says Sterry Hunt, is now 

 everywhere buried beneath its own ruins, and he considers that it 

 must have resembled certain furnace slags or volcanic glasses, whilst 

 the primeval atmosphere would contain all the carbon, chlorine, and 

 sulphur in the form of acid gases with nitrogen, watery vapour, and 

 a probable excess of oxygen. Forbes took strong objection to these 

 speculations, especially as regards the primitive atmosphere. No 

 excess of oxygen, he said, could exist with so much sulphurous 

 acid ; moreover, hydrochloric and sulphurous acids at high tempera- 

 tures decompose each other with the formation of water, chlorine and 

 sulphur. From the affinity of sulphur for the metals, and from the 

 fact that sulphurous acid is decomposed by metals with the formation 

 of sulphides and oxides, he infers that this element (sulphur) in 

 reality united itself to the metals, and thus formed dense sulphides 

 which at once sank through the lighter fluid layer. So far as he 

 confined himself to criticisms, especially on the action of sulphur, 

 Forbes was pretty safe ; but in his reply he unfortunately ventured 

 on the production of a rival atmosphere in zones, where, as eagerly 

 pointed out by his opponent, he forgot the specific gravities of some 

 of his gases, and altogether ignored the law of their diffusion. Those 

 who care to pursue this subject further will find the several papers 

 in the Geological Magazine. (See 1867, Vol. IV., pp. 357, 433, 

 and 477, and 1868, Vol. V, pp. 49, 92, 93, 106, and 366.) 



The subject of the earlier condition of the atmosphere is again 

 discussed by Dr. Hunt in the preface to the second edition, and he 

 alludes especially to the probabilities that even a very moderate 

 extra amount of carbonic acid in the atmosphere would diminish loss 

 of heat from radiation. He recalls to the minds of his readers the 

 enormous amount of carbonic acid now locked up in the earth's 

 limestones, equal in weight to 200 of the present atmospheres. The 

 progressive diminution in the height of the atmospheric column has 

 been due to its elements having been condensed in the form of 

 liquid water, or fixed as hydrates, oxides, or carbonates ; hence a 

 gradual refrigeration of the earth's climate. The application of these 

 views is, he observes, contrary to the hypothesis of an alternation of 

 warm and glacial climates in past ages, but is more in accordance 

 with the facts of the geological story, where everything tends to show 

 that a warm climate prevailed everywhere at the sea-level. (Read in 

 conjunction with the highly suggestive and ingenious hypothesis 

 propounded by Mr. Ball at the Royal Geographical Society in June 

 last, which " really took Sir J. Hooker's breath away," we may 

 believe that we have now obtained some clue to the history of the 

 earth's climate in time, which has been so long obscured by ice on 

 the brain.) The amount of carbonic acid which the life of the 

 successive ages could endure presents however the chief difficulty, 

 and is a physiological question which must doubtless be answered. 



