Organic Substances and Formations. 303 



opaque powder when heated red hot. Silica, if pure, remains 

 unchanged in form, and keeps its transparency, or it becomes 

 white, if minute portions of other earths are present. In 

 sponge spicules an animal substance is present, and this 

 blackens with heat. 



Silica is contained abundantly in common minerals, agates 

 are chifly composed of it ; the opal has it for its chief consti- 

 tuent; sharp sand is mostly composed of particles of silex, ami 

 sandstone contains it in large quantity. 



The few hints given in this paper, and in the preceding one 

 on Organic Chemistry will, it is hoped, give young microsco- 

 pists some help, but they must make a special study of 

 chemistry if they wish to do more than approach the thresh- 

 hold of many very interesting questions in the growth and 

 decay of liviug beings on which the microscope is able to throw 

 light, but which without chemical knowledge must remain 

 more or less unintelligible. 



