726 THE POPULAR SCIENCE MONTHLY. 



back three millions and forward one million of years. But we believe 

 that most geologists agree with the statement of Sir Charles Lyell that 

 " an attempt to assign chronological value to any except the latest 

 geological epochs must in the present state of science be hopeless." 



It may be stated, however, that during the period covered by the 

 computations, three epochs of greatest eccentricity of the earth's orbit 

 occurred, and in each of these it is found that the eccentricity is not 

 uniform, but rises and falls. Hence the coming on and departure of 

 each period may have been continuous, but by no means regular. 

 Sometimes the changes were at the comparatively short intervals of 

 10,000 or 12,000 years, and the close relation these changes may have 

 had to the life of the time, possibly to extinction of species, is plausibly 

 suggested by Mr. Croll. 



The period of great eccentricity to which the last glacial epoch is 

 referred began about 240,000 years ago, and extended over a period 

 of about 160,000 years. The conclusion, therefore, is that it closed 

 about 80,000 years ago. It is not easy to understand, however, what 

 value to attach to the words " close of the glacial epoch." Shall we 

 say that it closed w^hen the ice ceased to exist as a glacier along the 

 shores of New England, or when it extended no farther southward 

 than the Canadian highlands ? The gigantic fields of ice which now 

 cover both arctic and antarctic lands prove that, within areas more 

 limited indeed than in former time, the glacial epoch still exists in its 

 stern and sublime reality. 



Whether we consider the facts of geology or those of astronomical 

 computation, it seems evident that the growth and decline of the ice- 

 sheet, and of the causes which produced it, have been in no sense 

 cataclysmic or accidental, but secular ; only after ages have passed are 

 we enabled to realize, from the altered aspects of Nature, that a great 

 change is in progress. At present the eccentricity of the earth's orbit 

 is diminishing. In a little less than 24,000 years it will be "as nearly 

 circular as it can ever be ; " and if Mr. Croll's tables are to be relied on, 

 no cycles of extreme heat or cold will occur for the next 150,000 years. 

 We are entering a period of comparatively equable climate, arising 

 from a more uniform distribution of solar heat over the surface of the 

 globe. 







THE ARTIFICIAL PREPARATION OF ORGANIC BODIES. 



By IRA EEMSEN, 



PEOFESSOK OF CHEMISTRY IN WILLIAMS COLLEGE. 



THE "good, old" foundation upon which our fathers stood has 

 been sadly shaken. Its complete overthrow has at times seemed 

 inevitable. " Scientific men " have led the attacking army, and thus 

 gradually brought themselves into disgrace with a portion of the 



