EXPERIMENT IS A POTENT MEANS OF DISCOVERY. 307 



usually make suitable arrangements for that purpose. 

 Experiment is usually also a potent and immediate means 

 of testing a question. We might, in nearly all cases, wait 

 for ever and not meet with the conditions or results 

 obtainable by a single good test or arrangement ; for 

 instance, the alkali metals, potassium, sodium, &c., would 

 probably not have been seen to the present day, had not 

 special conditions been invented and arranged to set them 

 free, and to preserve them in that state. The progress of 

 discovery is generally more slow in subjects, in which we 

 cannot make many experiments, such as astronomy, be- 

 cause we have frequently to wait a long time for the ne- 

 cessary conditions or phenomena to occur, as in an eclipse 

 of Venus ; and when they do happen, we are generally 

 unable to obtain the most essential condition of a test ex- 

 periment, viz. changing only one condition at a time, and 

 are obliged to prepare everything beforehand, and make 

 the most of the brief opportunity. Phenomena which take 

 place in a living being during the course of a disease are 

 also similar in those respects. Discovery is especially 

 slow in those subjects where the phenomena not only occur 

 only at long intervals of time, and are very brief, but are 

 also in the highest degree uncertain as to time and place, 

 as in the study of the phenomena of falling stones, &c. ; 

 and the same remarks apply, though to a less extent, to 

 those of earthquakes. 



Every new question which has to be tested generally 

 requires a number of new experiments, or new modifica- 

 tions of old ones, and thus every experiment has to a greater 

 or less extent to be invented. During the course of each 

 experiment also new difficulties continually arise, and new 

 contrivances have at once to be devised in order to over- 

 come them, and thus ingenuity is constantly on the rack. ' 

 It is therefore not only desirable that the investigator 



x 2 



