LIGHT AND HEAT. 12 i 



which it is desirable that they should know who 

 are anxious to observe nature with pleasure and to 

 profit. But they are all either less understood, or 

 less open to the common observer, than the great 

 principle of gravitation ; and so they may be more 

 advantageously noticed along with the substances 

 or the places in which their operations are dis- 

 played. Those that perhaps demand the first at- 

 tention are they which, without any other apparatus 

 than the substances in which their effects are seen, 

 counteract or suspend the general influence of gravi- 

 tation. 



SECTION V. 



Observation of Light and Heal. 



THE class of agents or agencies (for we have no 

 means of ascertaining whether they are the one or 

 the other whether they bo 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 phenomena 

 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 resistance 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 know- 

 Li 





