CHEMICAL COMPOSITION. 197 



ostrich, emu, and mimel thrush, is shown in the 

 adjoining cut. It is in this tissue that the chalky 

 deposit, which gives hardness to the shell, is placed ; 

 the calcareous matter, when 

 first deposited on the egg 

 previous to its expulsion, has 

 a crystalline aspect, being 

 composed of many sided 

 pieces. These however sub- 

 sequently unite together. 

 The engraving represents this 

 feature in the structure of the shell. 



It may appear trivial to enter into these minute 

 particulars upon the structure of the egg-shell; 

 in reality, however, it is not so. No fact is too 

 trifling to command our attention, if the know- 

 ledge of it will render our general information 

 about birds more definite and complete. The 

 knowledge, also, of the true structure of the egg- 

 shell, is the only means of attaining any precise 

 and satisfactory information as to the nature and 

 seat of colour in the egg-shell. 



The chemical composition of the shell has been 

 carefully inquired into. That it contains lime 

 was known to the older philosophers, who, with 



