STATIC GUIDING CONDITIONS. 439 



produce similar effects; but we know that in a great 

 variety and number of cases similar causes produce dis- 

 similar or even opposite effects, and therefore the sphere 

 of influence of transmuting conditions is a very great one. 

 A determining, deflecting, or guiding condition is one 

 which decides the kind or direction of effect which an 

 active force produces: for instance, railway points de- 

 termine the direction in which a train shall proceed, 

 telegraphic switches determine that in which a message 

 shall be sent ; reflectors, lenses, and prisms direct the 

 course of rays of light ; conducting wires determine the 

 course of an electric current, &c. &c. Carbon and oxygen 

 unite together chemically at a red heat, because the de- 

 termining or guiding condition (consisting probably of 

 suitable relative molecular states of the two substances) is 

 present; but carbon and chlorine will not chemically 

 unite at any temperature, because such a determining 

 condition is not present. When a force is active, it 

 always produces some effect, and the effect it produces is 

 usually more or less different in different cases : thus, heat 

 applied to ice, melts it ; to water, converts it into a 

 vapour ; to steam, enlarges its bulk ; to a thermo-electric 

 pile, produces an electric current ; to a magnet, destroys 

 its magnetism ; to oxide of silver, decomposes it ; to a 

 mixture of oxygen and hydrogen, causes them to com- 

 bine, and produce an explosion ; and so on. And, under 

 different determining conditions, even opposite effects are 

 sometimes due to the same cause : for instance, a ball of 

 iron sinks in water, but rises in mercury by the attractive 

 force of gravity. Not only does the same cause produce 

 different and even opposite effects under the guidance of 

 different conditions, but different causes sometimes pro- 

 duce the same effect : for instance, an electric current 

 may be produced by means of friction in an ordinary 



