OBSERVATION AND EXPERIMENT. 421 



causes are connected with what effects, we may begin this 

 search at either end of the road which leads from the one 

 point to the other : we may either inquire into the effects of a 

 given cause, or into the causes of a given effect. The fact that 

 light blackens chloride of silver might have been discovered 

 either by experiments on light, trying what effect it would 

 produce on various substances, or by observing that portions 

 of the chloride had repeatedly become black, and inquiring 

 into the circumstances. The effect of the urali poison might 

 have become known either by administering it to animals, or 

 by examining how it happened that the wounds which the 

 Indians of Guiana inflict with their arrows prove so uniformly 

 mortal. Now it is manifest from the mere statement of 

 the examples, without any theoretical discussion, that arti- 

 ficial experimentation is applicable only to the former of these 

 modes of investigation. We can take a cause, and try what it 

 will produce : but we cannot take an effect, and try what it 

 will be produced by. We can only watch till we see it pro- 

 duced, or are enabled to produce it by accident. 



This would be of little importance, if it always depended 

 on our choice from which of the two ends of the sequence we 

 would undertake our inquiries. But we have seldom any 

 option. As we can only travel from the known to the un- 

 known, we are obliged to commence at whichever end we are 

 best acquainted with. If the agent is more familiar to us than 

 its effects, we watch for, or contrive, instances of the agent, 

 under such varieties of circumstances as are open to us, and 

 observe the result. If, on the contrary, the conditions on 

 which a phenomenon depends are obscure, but the phenomenon 

 itself familiar, we must commence our inquiry from the effect. 

 If we are struck with the fact that chloride of silver has been 

 blackened, and have no suspicion of the cause, we have no 

 resource but to compare instances in which the fact has 

 chanced to occur, until by that comparison we discover that in 

 all those instances the substances had been exposed to light. 

 If we knew nothing of the Indian arrows but their fatal effect, 

 accident alone could turn our attention to experiments on the 

 urali ; in the regular course of investigation, we could only 



