PRELIMINARY REMARKS. 343 



compressible, or nearly so ; the latter compressible and elastic. The 

 effects of light, heat, and electricity, are usually attributed to one or 

 more fluids, pervading all space ; but too subtile and diffusive to be 

 collected and weighed, and hence termed imponderable. Concern- 

 ing the constitution of matter, there are two opposite theories. The 

 dynamic theory, supposes that matter is continuous, without any 

 atomic organization, and without any pores or interstices. The 

 atomic theory, supposes all matter to consist of insensibly small, or 

 ultimate particles, called atoms ; each having a regular, and probably 

 a rounded shape; with interstices between them, like those in a pile 

 of cannon balls. This theory is now generally adopted ; and it is 

 strongly supported by the laws of crystallization, and of chemical 

 combination in definite proportions. 



There are two essential properties of matter, without which we 

 cannot conceive of its existence. They are extension, and resisti- 

 bility. Extension, is that property of matter by which it must 

 occupy a certain space ; and resistibility, less properly called im- 

 penetrability, is that property by which it will not permit other mat- 

 ter to occupy the same space at the same time. Four other properties 

 of matter may be termed general ; as they belong to all matter with 

 which we are acquainted; though not essential to its existence. They 

 are gravitation, inertia, porosity, and divisibility. Gravitation, is 

 that property of matter by which it tends to move towards other 

 matter, unless prevented by some force, to which, in that case, it 

 opposes a certain pressure or resistance : and inertia, is that property 

 of matter by which, being at rest, it tends to remain at rest, or being 

 in motion it ever continues in motion, unless affected by some extra- 

 neous force. Porosity, is that property of matter by which, accord- 

 ing to the atomic theory, it presents interstices or pores between its 

 particles : and divisibility, is that property by which matter may be 

 resolved into particles, at least as small as our vision will permit us to 

 discover. Of the remaining properties of matter, which may be 

 termed accidental, such as color, shape, hardness, elasticity, and the 

 like, we have no room here to speak. 



The progress of Acrophysics, was necessarily dependent, in a 

 great measure, on that of Mathematics ; and its study has been a 

 great stimulus to mathematical inquiries, by clothing them with new 

 interest and importance. How much knowledge in this department 

 was possessed by the ancient Chaldeans and Egyptians, we are 

 unable exactly to ascertain ; but it appears to have been considerable, 

 and derived from one common source. Their knowledge of these 

 sciences was carried into Greece, by Thales and Pythagoras ; in 

 whose doctrines truth was often mingled with error. Thus, Thales 

 taught that water was the sole element, of which all things were 

 composed ; and he believed amber, and the magnet, to be animate 

 beings. Pythagoras attached a mysterious importance to certain num- 

 bers, as exercising a control or influence over the material world. 

 Anaximenes and Diogenes regarded air as the primary element, of 

 which even water was composed; and Heraclitus maintained that 

 fire was the primary element, into which all others might be resolved. 

 Empedocles admitted the existence of four elements ; fire, air, water. 



