SCIENCE.] INTRODUCTORY. 17 



describe the commonest occurrence with even an ap- 

 proach to accuracy. That is to say, either he will 

 omit something which did occur, and which is of im- 

 portance ; or he will imply or suggest the occurrence 

 of something which he did not actually observe, but 

 which he unconsciously infers must have happened. 

 When two truthful witnesses contradict one another 

 in a court of justice, it usually turns out that one or 

 other, or sometimes both, are confounding their in- 

 ferences from what they saw with that which they 

 actually saw. A swears that B picked his pocket. 

 It turns out that all that A really knows is that he 

 felt a hand in his pocket when B was close to him ; 

 and that B was not the thief, but C, whom A did not 

 observe. Untrained observers mix up together their 

 inferences from what they see with that which they 

 actually see in the most wonderful way ; and even 

 experienced and careful observers are in constant 

 danger of falling into the same error. 



Scientific observation is such as is at once full, 

 precise, and free from unconscious inference. 



Experiment is the observation of that which hap- 

 pens when we intentionally bring natural objects 

 together, or separate them, or in any way change 

 the conditions under which they are placed. Scien- 

 tific experiment, therefore, is scientific observation per- 

 formed under accurately known artificial conditions. 



It is a matter of common observation that water 

 sometimes freezes. The observation becomes scien- 

 tific when we ascertain under what exact conditions 

 the change of water into ice takes place. The com- 

 monest experiments tell us that wood floats in water, 



