OF NATURAL PHILOSOPHY. 153 



and how such a heat can be maintained, we can 

 form no conception. Yet we are not, therefore, 

 entitled to deny the inference. 



(150.) 4th, That contrary or opposing facts are 

 equally instructive for the discovery of causes with 

 favourable ones. 



(151.) For instance: when air is confined with 

 moistened iron filings in a close vessel over water, 

 its bulk is diminished, by a certain portion of it 

 being abstracted and combining with the iron, pro- 

 ducing rust. And, if the remainder be examined, 

 it is found that it will not support flame or animal 

 life. This contrary fact shows that the cause of the 

 support of flame and animal life is to be looked for 

 in that part of the air which the iron abstracts, and 

 which rusts it. 



(152.) 5th, That causes will very frequently be- 

 come obvious, by a mere arrangement of our facts in 

 the order of intensity in which some peculiar quality 

 subsists ; though not of necessity, because counter- 

 acting or modifying causes may be at the same 

 time in action. 



(153.) For example : sound consists in impulses 

 communicated to our ears by the air. If a series of 

 impulses of equal force be communicated to it at 

 equal intervals of time, at first in slow succession, 

 and by degrees more and more rapidly, we hear at 

 first a rattling noise, then a low murmur, and then a 

 hum, which by degrees acquires the character of a 

 musical note, rising higher and higher in acuteness, till 

 its pitch becomes too high for the ear to follow. And 

 from this correspondence between the pitch of the 

 note and the rapidity of succession of the impulse, we 



