292 COSMIC J11IL0S0PHY. [ft. ii. 



here. What is called the arrest of motion by friction is now 

 known to be the change of molar motion into heat, when the 

 rubbing substances are alike in constitution, — into heat and 

 electricity, when they are unlike. In violent collisions, as 

 in the chipping of stones with a mason's chisel, the arrested 

 molar motion is partly changed into light. And when an 

 iron bar is suspended in the magnetic meridian and violently 

 struck or continually jarred, a portion of the arrested motion 

 reveals itself as magnetism. 



The transformation of heat into molar motion may be seen 

 in the rise and fall of the mercury in the thermometer, or 

 in the driving of a piston by the molecular dilatation of 

 aqueous vapour. When lime is introduced into an atmo- 

 sphere of burning hydrogen, we see the conversion of heat 

 into light. And when the heated ends of zinc and copper 

 wires are brought together, we see heat generating electric 

 currents. Conversely, electricity conducted down a light- 

 ning-rod is partly converted into heat; and in the bright 

 flashes which are followed by claps of thunder, we witness 

 electric energy partly consumed in originating light. 



The phenomenon commonly called light is but a species 

 of a mode of solar energy which may be called radiance or 

 actinism, and which, according to the manner in which it 

 affects our senses, is known as radiant heat, as light, or as 

 the energy which works changes in the daguerreotype-plate 

 and in the leaves of plants. The difference between the 

 higher rays of the solar spectrum, which manifest them- 

 selves chiefly in causing chemical changes, and the lower 

 rays, which are cognized as violet light, is generically the 

 same as the difference between these and the still lower 

 rays which are cognized as indigo, blue, green, yellow, 

 orange, or red light; and the same is true if we descend 

 to those still lower rays which are recognized only by their 

 thermal effects. If we call the energy manifested in the 

 solar beam by the general name of actinism, we may say 



