4 C. E. FAWSITT. 



that she should be different from the scheme we had 

 arranged for her to fit in to; this phase, however, does not 

 last long, and we become aware that it is best to take nature 

 as she is, and build up some new theory to cover the new 

 observation. Our belief, then, in the uniformities of nature 

 goes beyond our present knowledge, but it is in the best 

 interests of scientific progress that we should hold to it. 

 By examination of new facts showing apparent irregulari- 

 ties or by an examination of cases of abrupt change, new 

 lines of investigation suggest themselves; and from a 

 practical point of view, examination of the exceptional 

 case is usually fruitful. 



The Number of Conditions in an Experiment. 



A novice at experimental work is usually taught, and 

 taught rightly, to believe that if he keeps to the same con- 

 ditions of work (as far as they are known) he should be 

 able to repeat any experiment with identical results. If 

 he caunot, then he is a bad experimenter. A little more 

 experience, however, will bring to an experimenter in 

 certain lines of work some uncertainty as to when all the 

 conditions for an experiment are the same. For example, 

 experience may suggest to me that provided the tempera- 

 ture and pressure are kept constant, or within narrow 

 limits at any rate, in a given experiment, I may expect the 

 results to be independent of other conditions, e.g., light- 

 radiation, but it is never possible to be absolutely certain 

 that one is correct in regarding other influences as negli- 

 gible. To quote again from Balfour: — (2) 



"Choose the most perfect experiment on record, idealise the 

 conditions to your heart's content; for greater security, suppose 

 it repeated to weariness, how will you be advanced? There are, 

 I suppose, millions of circumstances, for the most part utterly 

 unknown, which have coexisted with all the experiments already 

 tried, but will have vanished before the next experiment is under- 

 taken. Does this disturb you? . . . Not at all. . . . You trust 



