3Llme0tcme. 13 



bon atom is twelve times heavier than the hy- 

 drogen atom, and the oxygen atom is sixteen 

 times heavier. Hence it will be seen readily 

 how a ton of coal will form two and two-thirds 

 times its weight of carbonic dioxide. Lime, 

 having a strong affinity or attraction for this 

 gas, has absorbed it from the air and water, 

 forming what is known as carbonate of lime 

 which is the ordinary limestone. Chalk and 

 the various marbles are also carbonates of 

 lime. Limestone strata in the crust of the 

 earth are found in all the periods of the 

 earth's formation. All forms of sea shells 

 that were once the homes of animal life are 

 constructed of this compound ; and in the later 

 formations of limestone, in the Secondary and 

 Tertiary periods, we find this rock to be made 

 up almost entirely of marine shells, some of 

 them microscopic in size. The earlier or older 

 formations of limestone that are found deeper 

 down in the earth's crust are less mingled with 

 these marine shells. This comes from the fact 

 that the first deposition of limestone strata 

 occurred before the later forms of sea life had 

 developed. Whatever signs of life are found 

 in these lower stratifications are of the very 

 lowest order. It is not to be understood that 

 animal life is a necessary factor in the forma- 

 tion of limestone, but it has been an inciden- 

 tal feature which no doubt has been the 

 chief means of gathering up from the water 



