IV PREFACE. 
so arranged as to reply to the questions, " What 
is?" "What are?'? or How do you know? " 
For instance : " What is flint ?" (See vol. i. p. 53.) 
The answer will be found thus : " Flint is a pecu- 
liarly hard and compact kind of stone, generally 
of smoke-grey colour, passing into greyish white, 
reddish, or brown. It is nearly thrice as heavy as 
water, and, when broken, will split in every direc- 
tion, into pieces which have a smooth surface." 
The author is aware that, in many instances, the 
definitions are defective : but this has, in general, 
arisen from a necessity of rendering them short, 
and at the same time of using such terms as would 
be likely to convey information to the minds of 
persons who have had no previous knowledge of 
the systems of natural history. 
After the definition, a further illustration some- 
times follows; and in the large characters will be 
found a brief detail of the history and uses of the 
object described. The articles are numbered, for 
the greater convenience both of reference and 
explanation, but particularly the latter. Thus, 
under the explanation of CARBON, it is stated that 
"in combination with oxygen (21) it forms car- 
bonic acid (26), and that it is the chief component 
part of pit-coal (217), petroleum (213), and other 
bituminous substances." By a reference to the 
