102 ANSWERS TO PRACTICAL QUESTIONS 



8. Why *V s/ate found fetwcm- seams of coal ? 



{See-Geotogfr page 150.) 



The* 'cbai' represents a period of vegetation, and the 

 slate, one of convulsion. During the former, a deposit of 

 the leaves, branches, trunks of trees, etc., was made ; 

 during the latter, one of gravel, sand, etc., accumu- 

 lated. 



9. Why was the coal hidden in the earth ? 



It is natural to think that one object was to protect it 

 from accidental combustion. 



10. Where was the C, now contained in the coal, before 

 the Carboniferous age ? 



(See Geology, pages 150-1.) 



In the atmosphere, which was then so full of CO 2 that, 

 according to certain authorities, it contained 7 to 8 parts 

 in 100. 



1 1. Must the air have then contained more plant food ? 



(See Chemistry, page 71, and Geology, page 150.) 



12. What is the principle of the aquarium ? 



The inter-dependence of animals and plants, whereby 

 each supplies the wants of the other. The aquarium is a 

 microcosm a world in miniature. 



* I have read somewhere a beautiful Persian fable in which a nightingale 

 and a rose are represented as being confined in a cage together, and being 

 dependent upon each other for life. The fable is truth symbolized. The idea 

 has now become more practical, but not less beautiful. In the modern 

 aquarium , or drawing-room fish-pond, we see the world in miniature. It is a 

 self-regulating, self-subsisting establishment, and is constructed on the most 

 perfect principles of chemical economy. 



" Before this truth of compensation between animals and plants was dis- 

 covered, many attempts were made to keep fish in small glass globes. As 



