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by means of each of our different senses and perceptive 

 powers respectively, we perceive its different sensible 

 qualities ; its duration of existence by our perception of 

 time ; its form, magnitude, motion, velocity, colour, &c., 

 by means of our sight and perceptions of space and colour ; 

 its kind of structure, weight, &c., by our sense of touch 

 and perception of resistance ; its sound, by means of our 

 sense of hearing ; its odour and volatility, by smelling, &c. 

 Also by comparing each of its sensible qualities together, 

 or with those of other phenomena, we discover similarities, 

 differences, and general truths ; and by comparing these 

 facts and truths with others, and reasoning upon them, we 

 discover the more hidden phenomena, truths, and prin- 

 ciples of nature. Upon these two c laws ' are based all 

 the rules of the art of scientific research. 



Although we have very strong reasons for believing 

 that, whenever we subject a force or substance to new 

 conditions new effects are produced, unless some circum- 

 stance exists to prevent them, in the great majority of 

 really new experiments (especially those which, if success- 

 ful, would yield important results) no apparent effects 

 occur. There are several reasons for this. First, in a 

 great number of cases we require new methods of de- 

 tection, the effects produced being not perceived because 

 neither our senses nor any known methods are suitable for 

 detecting them. Second, in another large number of cases 

 the effects produced are so small, feeble, or distant, that 

 we are unable to detect them even by the aid of the most 

 powerful and delicate instruments and appliances we at 

 present possess : for example, nearly all substances are 

 probably altered in temperature by exposure to light, but 

 in most cases the effects are so small that we cannot at 

 present detect them. Many, however, might be detected 

 if the requisite researches were made : for instance, many 



