202 SCIENTIFIC RECREATIONS. 



it with a little suet. It is then capable of floating on the surface of water 

 running in the direction of the north pole. We might go on multiplying 

 indefinitely examples of physical experiments without apparatus, but we 

 have probably already given a sufficient number to aid our readers in 

 imagining others. 



We have now in a simple manner shown how we can easily produce 

 electricity. We may understand that electrical phenomena are produced 

 (i) friction between different bodies ; (2) by placing bodies which differ in 

 contact ; (3) by the transition of bodies from one condition to another ; (4) 

 by chemical changes ; (5) by animals. The two first, and the fourth, are 

 the most usual causes. 



We know that certain substances when rubbed with silk or wool 

 acquire the property of attracting other substances. But in the case of a 

 rod of glass or stick of wax, the attraction will only be perceived when the 

 rubbing has been applied. But metal will behave differently. Any part of 

 the metal rod will continue to attract. So metals are CONDUCTORS of 

 electricity ; while glass, wax, silk, amber, sulphur, etc., are bad, or NON- 

 CONDUCTORS. Metals are the best conductors we have, but trees, plants, 

 liquids, and the bodies of animals, including men, are all good conductors of 

 electricity. Dry air is a bad conductor. 



There are two kinds of electricity, known as positive 

 and negative (plus or minus), vitreous or resinous. We saw 

 in fig. 200 that we can attract a small ball of pith or cork 

 by a piece of sealing-wax rubbed with flannel. If we then 

 c present a glass rod rubbed with silk to it, it will be equally 

 attracted, but will be at once repelled ; and after being so 

 repelled, if we put the wax to it, it will be attracted to the 



Fig. 204. Attraction sealing- wax again. So wax at first attracts then repels the 

 and repulsion. ball, and so does glass, but either will attract the ball if pre- 

 sented alternately (fig. 204). The reason for this is as follows : 



When we have rubbed the glass with silk, we charge it with positive 

 electricity, and when the rod touches the ball, the latter imbibes that 

 electricity, and flies away from the glass rod. The sealing-wax imparts 

 negative electricity in the same way. 



The law is, that bodies charged with the same kind of electricity repel 

 each other, and those containing the opposite kinds attract each other. 

 Positive repels positive ; negative repels negative. But positive attracts 

 negative, and negative attracts positive. Opposite electricities unite, and so 

 neutralize each other that no effect is perceived ; but it must be borne in 

 mind that all bodies possess both electricities in some quantity, greater 

 or less. By rubbing we separate these electricities, the rubber becoming 

 negative, the rubfe positive. The friction of glass supplies positive 

 electricity, and sealing-wax supplies negative electricity, or we can obtain 

 the same effect by rubbing either with certain material. 



The manner in which a body is electrified depends upon its nature 



