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SECTION V. 



Observation of Light and Heat. 



THE class of agents or agencies (for we have no 

 means of ascertaining whether they are the one or 

 the other whether they be real things, or mere 

 phenomena of other things) to which we shall very 

 briefly allude in this section, are light, heat, elec- 

 tricity, and some others, which are sometimes (not 

 very sensibly) called " imponderable" substances. 

 Being "ponderable," that is having weight, is 

 the only real test that our observation can have of 

 what we are accustomed to call material substances, 

 that is, can be the objects in which those pheno- 

 mena, which we are in the habit of calling the 

 effects of the " laws of nature" can be exhibited 

 or revealed to us through the medium of the senses. 

 And even weight, though we can feel it, in resist- 

 ance to our muscles and in the muscles themselves, 

 in more minute portions than we can see with the 

 eye, is yet never felt alone, so as that we can have 

 any knowledge of it. In order to that, there must 

 be something which we can call substance, and that 

 substance must be of some extension, or measure, 

 or bulk; that is, it must occupy space, and space in 

 which there can be no other substance at the 

 same time. That space must be of some shape or 

 figure, too ; and the shape of the space must be 

 exactly the same as that of the body which fills 

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