New Outlines of Chemkul Phihsophj. 209 



The apparatus that 1 make use of for excitation is a barome- 

 ter tube oi.lv 3-lOths of an inch in diameter, and a silk hand- 

 kerchief; but these lo-,v electrical states require a verv perfect 

 mode of insulation. The material generally used for this jmr- 

 pose is glass ; but this is known to lie a conductor in some de- 

 gree and to remove this inconvenience, varnish and sealing- wax 

 have been used. But as these do not render glass a perfect non- 

 conductor, some more perfect mode of insulation is necessary when 

 low degrees of electricity are the objects of investigation. 



1 have fretjuently made use of thermometer tubes, because 

 they contain less surface than the glass rods generally used; but 

 these only lessen the inconvenience, without removing it. 



White flint glass contains much lead; whence I supposed that 

 green-bottle glass might be better for this purpose, because it 

 contains no metal, and a hint to this effect is given in these 

 Outlines*. How far my supposition was well founded will ap- 

 pear from the following experiments : , . , , r .i 

 ^ Exp 9 _1. Having fixed two slips of Dutch goid-ieat to the 

 end of a thermometer tube ten inches long, I suspended it in 

 the axis of an open-necked bell-glass, by means of sealing-wax ; 

 one-half of the tube being within the glass, and the other hall 

 above it —2. A solid stick of green-bottle glass, seven inches 

 long, and about the same thickness as the thermometer tube 

 mentioned above, was fitted up in tlie same manner. 



The excited barometer tui)e being held over the top of the 

 stick of bottle glass, at the distance of an inch, the leaves di- 

 verged to an angle of about 30 degrees ; but the effect was trans- 

 ient, the leaves soon collapsed. 



The barometer tube, without receiving any further excitement, 

 was then held over the top of the thermometer tube of flint glass, 

 at the same distance as before; the leaves gradually diverged to 

 an angle of more than 100 degrees, and continued electrified tor 



30 hours. , ,11 



From these experiments we may infer, that green-bottle glass 

 is greatlv preferable to flint glass for insulation, aud may be sub- 

 stituted for it in many parts of an electrical apparatus. 



A thermometer tube seven inches long, having its surface uni- 

 formly covered with black sealing-wax to the thickness of 1- 10th 

 of an'inch, is still a conductor : and when compared with a so- 

 lid stick of green-bottle glass of the same dimensions, their con- 

 ducting powers appear nearly equal,— but with this difference, 

 the green glass c(xiducts electricity much slower than the 

 other t • 



* Phil. Mag. vol. xlv, p. 4'25. , r .ii 



+ " The smallest wire uill lie a siilTiciciit coinliirtov, c(/vcrcd a foot thiik 



with sciiling-wax."— r.ncv. Rrit. .Sill), vol. ii. j). Gl.'). . * -i 



Vol.46. No. 209. SepM815. O A stick 



