THE CARBONIFEROUS PERIOD. 73 
cal fact to explain this statement. Limestone is 
carbonate of calcium. Calcium is a metal, fusi- 
ble as such, and, forming a part of the melted 
masses within the earth, it was thrown out with 
the eruptions of Plutonic rocks. Brought to the 
air, it would appropriate a certain amount of 
oxygen, and by that process would become oxide 
of calcium, in which condition it combines very 
readily with carbonic acid. Thus it becomes car- 
bonate of lime ; and all lime deposits played an 
important part in establishing the atmospheric 
proportions essential to the existence of the 
warm-blooded animals. 
Such facts remind us how far more compre- 
hensive the results of science will become when 
the different branches of scientific investigation 
are pursued in connection with each other. 
When chemists have brought their knowledge 
out of their special laboratories into the labora- 
tory of the world, where chemical combinations 
are and have been through all time going on in 
such vast proportions, — when physicists study 
the laws of moisture, of clouds and storms, in 
past periods as well as in the present, — when, in 
short, geologists and zoologists are chemists and 
physicists, and vice versd, — then we shall learn 
more of the changes the world has undergone 
than is possible now that they are separately 
studied. 
4 
