6i6 



THE POPULAR SCIENCE MONTHLY. 



a pivot formed by a sewing-needle, and supported on a bent strip of 

 metal, as shown in the figure. By weighting the straw with a little 

 wire near iV, you so balance it that the plate N shall be just lifted 

 away from M. The wire w^ which may be 100 feet long, proceeds 

 from J/ to your glass tube, round which it is coiled. A single vigor- 

 ous stroke of the tube by the rubber sends electricity along w to 31 ; 



Fig. 6. 



iVis attracted downward, the other end of the long straw being lifted 

 through a considerable distance. In subsequent figures you will see 

 the complete straw-index, and its modes of application. 



A few experiments with either of these instruments will enable 

 you to classify bodies as conductors, semi-conductors, and insulators. 

 Here is a list of a few of each, which, however, difier much among 

 themselves : 



Conductors. 

 The common metals. Solutions of salts. 



Well-burned charcoal. Eain- water. 



Concentrated acids. Linen. 



Living vegetables and animals. 



Semi-conductors. 



Alcohol and ether. Marble. 



Dry wood. Paper. 



Straw. 



This is the place to demonstrate, in a manner never to be forgot- 

 ten, the influence of moisture. Assure yourself that your dry silk 

 string insulates. Wet it throughout, and squeeze it a little, so that 

 the water from it may not trickle over your glass tube. Coil it round 

 the tube as before, and excite the tube. The leaves of the electro- 

 scope immediately diverge. The water is here the conductor. The 

 influence of moisture was first demonstrated by Du Fay (1733 to 



