xix.] EXPERIMENT. 437 



the 88ooth part of a degree centigrade has been detected 

 by Dr. Joule. The spectroscope has revealed the presence 

 of the io,ooo,oooth part of a gram. It is said that the 

 eye can observe the colour produced in a drop of water by 

 the 50,ooo,oooth part of a gram of fuschine, and about the 

 same quantity of cyanine. By the sense of smell we can 

 probably feel still smaller quantities of odorous matter. 1 

 We must nevertheless remember that quantitative effects 

 of far less amount than these must exist, and we Should 

 state our negative results \vitli corresponding caution. We 

 can only disprove the existence of a quantitative phenome- 

 non by showing deductively from the laws of nature, that 

 if present it would amount to a perceptible quantity. As 

 in the case of other negative arguments (p. 414), we must 

 demonstrate that the effect would appear, where it is by 

 experiment found not to appear. 



Limits of Experiment. 



It will be obvious that there are many operations of 

 nature which we are quite incapable of imitating in our 

 experiments. Our object is to study the conditions under 

 which a certain effect is produced ; but one of those con- 

 ditions may involve a great length of time. There are 

 instances on record of experiments extending over five or 

 ten years, and even over a large part of a lifetime ; but 

 such intervals of time are almost nothing to the time 

 during which nature may have been at work. The con- 

 tents of a mineral vein in Cornwall may have been under- 

 going gradual change for a hundred million years. All 

 metamorphic rocks have doubtless endured high tempera- 

 ture and enormous pressure for inconceivable periods of 

 time, so that chemical geology is generally beyond the 

 scope of experiment. 



Arguments have been brought against Darwin's theory, 

 founded upon the absence of any clear instance of the 

 production of a new species. During an historical interval 

 of perhaps four thousand years, no animal, it is said, has 

 been so much domesticated as to become different in 



1 Keill's Introduction to Naiwral Philosophy, yd ed., London, 

 1733, PP- 48-54. 



