CONSTITUTION OF MATTEE. 27 



The phenomena of combustion, fermentation, putrefaction, 

 decomposition, etc., belong to the science of chemistry. In 

 short, all the phenomena in which the molecule remains un- 

 changed belong to the science of physics, while the phenom- 

 ena in which the molecule is changed or modified in its nat- 

 ure belong to the science of chemistry. 



23. Illustration. If you examine a piece of iron from a 

 purely physical point of view, you may describe it as black 

 or steel gray, opaque, hard, of a specific gravity of 7.8 i. e., 

 nearly eight times as heavy as water and a fair conductor 

 of heat, as you will find on heating one end red-hot. Be- 

 sides these qualities, it is a conductor of electricity, and may 

 be converted into a magnet possessing the power of attract- 

 ing and repelling other pieces of iron. It is also fusible, 

 malleable, ductile, elastic, capable of crystallization. In all 

 this, however, its nature is not changed it still remains iron, 

 its molecules are intact. Again, here you have some sul- 

 phur : it is, physically considered, yellow, brittle, electric, 

 easily fusible, readily crystallized, soluble in certain liquids, 

 etc., etc. Now take some iron filings having the physical 

 qualities named, and mix these with powdered sulphur 

 having its own properties ; mix and pulverize as fine as you 

 please ; each grain of the mixture will contain a particle of 

 iron and a particle of sulphur. The iron may be withdrawn 

 by a magnet, and the sulphur may be dissolved out in car- 

 bon disulphide. Examined under a powerful microscope, 

 each particle will be seen to consist of two distinct substan- 

 ces, iron and sulphur. Now apply heat to this mixture, and 

 thus set chemical force at work : the mass glows, a kind of 

 combustion takes place, and on cooling you have a dark sub- 

 stance which possesses physical and chemical properties of 

 its own. The iron has disappeared, the sulphur has gone, 

 each has united with the other, atom to atom. The magnet 

 will not now withdraw the iron, nor can the sulphur be dis- 



