360 RECENT PROGRESS IN PHYSICS. 



On the other hand, such substances as ordinarily become negative 

 by friction could not, by the employment of similar means, be so 

 changed as to become positive. 



In relation to the rubbing substance, it is shown by the experiments 

 that for cloth, may be substituted leather, sealing wax or silk, but 

 not Kienmaier's amalgam; on the other hand, a glass rod prepared in 

 the flame of a spirit lamj) and rubbed with tin foil shows negative 

 electrcity; the same effect is produced by the other metals ; even on 

 dipping a prepared glass rod but once into mercury, it is drawn out 

 with negative electricity ; by repeated dippings,, however, it is ren- 

 dered positive. 



To say " that the glass rod, held in the flame of any combustible 

 substance, or dipped into concentrated acids undergoes a change upon 

 its surface, which cannot be discovered immediately by the senses, but 

 which can be recognized by means of the electroscope," can by no 

 means be called an explanation, it is simply a modified statement of 

 the fact, 



§ 5. On the conducting power of certain substances. ^i?te-9s has ex- 

 amined many substances with reference to their conducting power, 

 and their capability of becoming electrified by friction. — (Pogg. Ann., 

 LXIV, 51.) 



A small rod oi selenium, three lines thick, will discharge a gold leaf 

 electrometer almost instantaneously, and by means of it sparks may 

 be drawn from the conductor of an electrical machine ; insulated and 

 rubbed in one spot by flannel, it becomes negatively electrified in every 

 part. In its ordinary condition, consequently, the surface of selenium 

 conducts. If in one spot a new surface is made by fusion, it does not 

 conduct electricity as well as before, and a thread of selenium drawn 

 out in a flame insulates as well as shellac. Rubbed with flannel, leather, 

 linen, or even drawn between the dry fingers, such a thread becomes 

 strongly negative. 



Selenium, therefore, is a non-conductor, and becomes electric by 

 friction, if its surface be perfectly clean. 



Iodine is an imperfect conductor of electricity. A rod of this sub- 

 stance, 6|- lines thick and 20| lines long, discharged an electroscope 

 in one second ; without insulation this cylinder could not be electrified ; 

 when insulated and rubbed against flannel it became feebly negative. 



Iielinas2:)haHuin insulates, provided that pieces with a clear vitreous 

 surface are used. Leather — brown pieces with a rough ragged surface, 

 on the other hand, conduct, as is also the case with bits of amber hav- 

 ing rough surfaces. 



Aluminum and glucinum in the form of powder, when properly dried, 

 are non-conductors. 



§ 6. Production of electricity by steam escaping through narrow 

 PASSAGES. — Mr. Armstrong, of Newcastle-upon-Tyne, towards the close 

 of 1840, received information that, at Saghill, near Newcastle, a very 

 extraordinary })henomenon had been observed on the escape of steam 

 from a boiler. (Pogg. Ann. LII, 328, Phil. Mag. vol. XVII, p. 370 

 and 4:o2_, vol. XVIII, p. 50.) — Steam was escaping from a leaky joint 

 near the safety valve, and the engine tender, having one hand acci- 



