ELECTRICITY. 475 



press any opinion in regard to its substance. All that is 

 properly known of it, is its effects, and such of its char- 

 acteristics as may be fairly and logically inferred from its 

 effects. These last are however very few. The princi- 

 pal object therefore of the following tract, is to present 

 in a systematic form, the prominent facts, and classes 

 of facts, which it is common to refer to this cause. We 

 must first however say something of the theory. 



A theory in any science may be of great advantage, 

 even though there be not positive evidence of its truth. 

 Sometimes it is only a hypothesis, adopted simply for the 

 purpose of arranging and systematizing the facts observ- 

 ed. In such cases it is of great service in reducing to 

 order what would otherwise be a mass of inextricable 

 confusion. 



Some theory, for this latter purpose, is necessary for 

 the science of electricity. If we attempt to look at the 

 facts themselves individually, without any theory, we shall 

 have before us a confused and heterogeneous mass, which 

 it is almost impossible to remember, or to recall when 

 occasion requires. Consequently theories have been de- 

 vised for the purpose of grouping, and arranging the 

 facts, and assisting the recollection of them. 



There are two prominent theories which have been 

 employed for this purpose in electricity. The one which 

 is now almost universally received among philosophers, 

 we shall proceed to explain. We do not offer it as posi- 

 tively true or as proved by experiment. Subsequent dis- 

 coveries may entirely destroy it, but an acquaintance 

 with it is absolutely necessary to the general reader. For 

 in the first place, it is impossible for us to describe the 

 facts in this science without using the language of theo- 

 ry ; and in the second place, this theory is now so uni- 

 versally received, that no book on electricity would be 

 intelligible to a reader without a knowledge of its princi- 

 ples. 



The hypothesis is, that there is pervading all substances 

 a subtile and highly elastic fluid, which is however itself 

 entirely void of any sensible weight. This fluid is sup- 

 posed to be capable of moving over the surfaces of dif- 

 ferent bodies, in some cases with greater, and in 



