86 ELEMENTS OF SCIENCE 



that which is the cause of all and every known kind of 

 motion. As there are the six kinds of active energy 

 above mentioned, so we may speak of their unknown cause 

 as so many " physical forces " in a quite elementary 

 work, such as the present one, questions as to the absolute 

 nature and distinctness of such forces cannot be touched 

 upon, though conceptions which serve science as working 

 hypotheses, will be referred to. 



We have already made acquaintance with the force 

 spoken of as gravity, but here we shall not further study 

 the energy due to that force. For it is a universal, con- 

 stant condition of all material bodies, constant, more- 

 over, not only in its existence and action, but also in the 

 precise amount of its action, which is ever in exact pro- 

 portion with the mass of which any body (its distance 

 from other bodies being unchanged) consists as has 

 been pointed out in the last chapter.* 



We have noted the various modes of motion and ten- 

 dencies to motion in solids, liquids, and aeriform bodies, 

 but we all know that one substance at least can exist in 

 all three conditions. We know that water can be both 

 frozen and changed into a vapour ; while steam and 

 every other vapour is an aeriform body. Steam is an 

 invisible vapour. What is popularly called "steam" is 

 a cloud of minute particles of water formed by the 

 resumption of its liquid condition. If we look at the 

 place where steam is issuing from a rapidly boiling kettle 

 or engine, we shall find that nothing is visible close to 

 the mouth of the spout or chimney, the cloud of what 

 is popularly known as " steam " only begins to appear at 

 a short distance from it. 



HEAT. It is notorious, as before said, that cold 



* See ante, p. 66. 



