146 LECTUEES 



a silicate. It would seem to follow, then, that springs and volcanoes, 

 also, only return to the atroosphere what had been previously taken 

 from it. The only difference between these sources and the three first 

 is, that while decomposition, combustion, and respiration return to 

 the air what had been taken from it hut a little while before, springs 

 and volcanoes return to the air what had been taken from it during 

 some previous geological epoch. Thus the atmosphere becomes the 

 great original source of all the carbonic acid in the world. 



But whatever be the cause of the excess of carbonic acid in the at- 

 mosphere during the coal period, we cannot fail to see an evident and 

 beneficent design in its removal. Carbonic acid, as is well known, is 

 as poisonous to animals as it is nourishing to plants. Previous to the 

 coal period there lived none but aquatic animals of low order. These, 

 on account of low vitality, sluggish circulation, and little necessity 

 for rapid and constant oxygenation of the blood, have great endurance 

 of carbonic acid. But now the earth was prepared to receive air- 

 breathing animals, the atmosphere must be purified for the purpose. 

 This was accomplished by the astonishing vegetation of the coal period. 

 But observe, and never cease to admire and wonder, that the self-same 

 providential act which purified the atmosphere and rendered the earth 

 a fit habitation for reptiles and birds, had reference also to the coming 

 of man countless ages after, and laid up materials for his use. In the 

 carbon thus silently extracted from the atmosphere was buried a me- 

 chanical energy which, after a sleep of millions of years, was to rise 

 again as the great physical regenerator of the human race. 



ORIGIN OF COAL. 



It is now universally admitted among geologists that coal is entirely 

 of vegetable origin. There was a time, however, and that not many 

 years ago, when the vegetable or mineral origin of coal was a ques- 

 tion warmly contested by the best geologists ; but its vegetable char- 

 acter is now so firmly established and so universally admitted that 

 the history of the controversy has lost its interest. I will not, there- 

 fore, tire you with its details, but proceed to state the evidence upon 

 which the universal belief is founded. 



First, then, the enormous profusion of fossil plants, in the form of 

 impressions ot leaves, trunks and branches of trees, fruits, &c., found 

 in immediate connexion with a coal seam, afibrds strong presumption 

 in its favor. In the second place, this presumption is strengthened," 

 and becomes, in fact, almost certainty in the case of trunks of trees 

 retaining their external conformation, and under the microscope their 

 internal structure even to the minutest sculpturing upon their cell 

 walls, and yet turned to perfect coal. It might possibly be objected 

 that it may be a substitution of one substance for another, similar to 

 what takes place in petrification, where we find, also, the external 

 conformation and internal structure perfectly preserved, but the 

 organic matter all gone, that the ancient trunk having been buried 

 in bituminous matter and thoroughly impregnated therewith, as 

 particle by particle the woody matter was removed by decomposition 

 the bituminous matter took its place, and thus perfectly imitated its 



