-1859] WRITINGS OF JOSEPH HENRY. 89 



effort of mind which is always necessary in order to secure 

 an important truth and make it fully our own. 



Constitution of Matter. 

 Laws of force and motion. — All the objects which are pre- 

 sented to us in the material universe, and all the changes 

 which we observe taking place continually among them, 

 whether those which immediately surround us or those 

 which we perceive at a distance, either by the naked eye or 

 by means of a telescope, are referable to two principles — mat- 

 ter and force. By matter, we understand the substratum 

 of that which affects our senses; and by force, that which 

 produces the changes which we constantly observe in the 

 former. The idea of force was probably first suggested to 

 us by our muscular exertions: and indeed the original 

 meaning of the term is a muscle or tendon ; the Latin vis 

 (force) being probably derived from the Greek 'is, or fis. But 

 we cannot imagine a force without some bodily substance by 

 which, or against which it is exerted; the two ideas there- 

 fore of matter and force are co-existent in the mind, and 

 on a clear and definite conception of them depends that 

 precise relation of the phenomena of nature denominated 

 science. Though the essence of force and matter may never 

 be known to us, we can study the laws by which they are 

 governed, and adopt such a conception of the constitu- 

 tion of matter as will enable us to generalize a vast num- 

 ber of facts ; to connect these with each other, or with a 

 central thought ; to perceive their dependencies, and thus 

 in some cases to control phenomena; to relieve the memory, 

 and call into play the reasoning powers; and finally, to pre- 

 dict new facts, the existence of which had never yet been 

 proved by actual experience. But such a generalization 

 must be based on the well-established principles of the laws 

 of force and motion, and be in strict accordance with accu- 

 rately ascertained facts in the various branches of physical 

 inquiry, in order that it may be an exact expression of the 

 apparent cause of the phenomena, and that the prediction 

 from it may be ti-ue in measure as well as in mode. 



