Scientific Lectures. 131 



mercury, and then, withdrawing it a very little, we have a beautiful 

 boquet of fire-sparks, of burning steel and mercury, flying outward in 

 dense clouds, all ignited by the intense vibrations produced by the 

 electric force in traversing that little space of resisting air. 



Yet again, substituting other terminals, I have here a thin rod of 

 carbon connecting the polls, and, as you see, the vibrations caused by 

 the flow of the force through its resisting substance causes it to glow 

 with a dazzling whiteness.* 



But these are not the only methods in which electricity resisted 

 may become a cause of light-vibrations. Thus, for example, we have 

 quite a different condition of the electric forces which we recognize 

 as static, or " frictional electricity." In this case the force has the 

 power of passing through considerable thicknesses of resisting mate- 

 rial. Thus I have here an instrument for the development of electric 

 force in this form. It is called an induction coil, and was con- 

 structed for me by Mr. E. S. .Ritchie, of Boston. It throws, as you 

 see," sparks in the air twenty-one inches in, length, or in other words 

 the electric force will pass through the air for that distance, produc- 

 ing vibrations in the matter along its path, which are recognized by 

 your eyes as light. The same spark will also pass through three 

 inches of solid glass, leaving a delicate line of fracture to mark its 

 path, or by the use of this sheet of metallic paper, whose surface has 

 been broken by crumpling, it will give us a lightning-like discharge 

 of six or seven feet in length. f 



By concentrating this flash, on the other hand, into a brief time 

 and short space, we obtain, as you see, a more dazzling light, accom- 

 panied by a very sharp report, for by a secondary action, i. <s., that of 

 heating and expanding the air, sound-vibrations, as well as those of 

 light and heat, are in this case produced. \ 



When the resisting medium in the case of air or other gas or vapor 

 is rarefied, the electric force, in its passage through, no longer confines 



* These batteries of large surface and low resistance are of course unfit to produce an " electric 

 light," in the ordinary sense. But the method described above serves as a very fair substitute for 

 purposes of illustration ; where, as in this case, the large batteries were needed for other experiments. 



+ This experiment is arranged as follows : One or more sheets of metallic paper, such as is 

 used to wrap up coffee in packages by some dealers, are well crumpled so as to break up the 

 metallic surface without tearing the paper. These are then stitched on a piece of cloth, such as a 

 strip of " turkey red," so as to make a hanging banner, which is further completed by a cross-piece 

 of wood suspended from a pole. One terminal of the coil is connected with the top of the metallic 

 paper by a wire and the other with a long wire attached to a glass rod, by which it may be held and 

 passed over various parts of the paper. My attention was first directed to the use of this paper for 

 such a purpose by Mr. C. T. Chester, of New York. 



% For these effects the Leyden jar and also the " secondary condenser," shown in the accom- 

 panying cut (see fig. 9), and fully described in the journal of the Franklin Institute, vol. 53, p. 256, 

 with which dense white sparks fourteen inches long were shown,, the condenser containing twenty 

 coated panes. 



