3 6z THE POPULAR SCIENCE MONTHLY. 



than it now is. All conclusions, however, in regard to the sun's for- 

 mer temperature must be hypothetical ; but, if it be a gaseous body, 

 as suggested by Professor Young, it has been growing hotter all the 

 time it has been giving out heat.* To this, as to all other theories 

 heretofore advanced, there lies the serious objection that they ignore 

 the world-wide uniformity of light and actinic force, and no theory 

 that fails here can be satisfactory. 



A perpendicular axis alone does not account for the warmth in po- 

 lar regions. On the contrary, with such an axis, they would receive 

 during the year less heat than they now do, and hence Dr. Croll infers 

 that a perpendicular axis would make the polar climate less genial. 

 This is true, if temperature depends solely on the amount of heat 

 received. But, as every one knows, it depends far more upon the 

 amount retained. Green-houses and drying-houses are often uncom- 

 fortably warm when the mercury without indicates a temperature near 

 freezing. The solar rays readily enter through the glass, and are ab- 

 sorbed by the floors, walls, etc., while the heat radiated back is unable 

 to escape. # Many substances possess this property, and Professor 

 Tyndall has shown that among them are carbonic acid and aqueous 

 vapor. 



In the present state of our knowledge, it is impossible to form 

 even an approximate estimate as to the actual amount of carbon stored 

 in the earth's crust as graphite, coal, lignite, bitumen, petroleum, etc., 

 but it must be many times probably many hundred times greater 

 than that now remaining in the atmosphere. All these forms of car- 

 bon are directly or indirectly of vegetable origin, and hence it once 

 existed in the form of carbonic acid. It has been said, however, that 

 so much CO a was not found in the air at any one time, but that it was 

 given out by volcanoes just about fast enough to take the place of 

 that which was stored in the earth's crust. But, as Professor Dana 

 remarks, volcanoes do not originate this gas ; they give it out only as 

 their fires come in contact with limestone, and this occurs but rarely 

 now, and was still more uncommon in Palaeozoic times, f 



It appears, therefore, that at some remote period all the carbon 

 which has since been a portion of animal or vegetable forms, existed 

 as free carbonic acid, and formed a part of the then atmosphere. 

 "With the beginning of j)lant-life a process of elimination commenced. 

 It continued till about the close of the Tertiary, w T hen the amount 

 taken out by living forms and that restored to the atmosphere by de- 

 composition became equal a condition which still exists. 



The CO 2 is now diffused with great uniformity over the earth, and, 



* For proof of this curious paradox, discovered by Mr. J. H. Lane, see " American 

 Journal of Science," July, 1880 ; also, Newcomb's "Astronomy," p. 508. 



f " Manual of Geology," p. 363. Some think that there has been an accretion of 

 CO 2 from interstellar space. This appears to be too hypothetical to seriously affect the 

 argument. 



