210 SCIENTIFIC RECREATIONS. 



Recapitulation of foregoing Chapter. So far, we have seen there is 

 electricity in everything, although some bodies are termed conductors and 

 others non-conductors ; though, as in applying the terms heat and cold, we 

 must remember that no body is entirely devoid of electricity, and no body 

 is therefore an absolute ;z0;z-conductor any more than any object is absolutely 

 devoid of heat. Faraday, indeed, was of opinion that " conduction and 

 insulation are only extreme degrees of one common condition " ; they are 

 identical both in principle and action, except that in conduction an effect 

 common to both is raised to the highest degree, and in the case of insulation 

 it occurs in an almost insensible quantity. 



We have also read of positive and negative electricities, and we must not 

 fancy there is any particular reason for this distinction. It was Du Fay, 

 whom we have mentioned, who gave the names " vitreous " and " resinous " 

 to the two kinds, as one was developed by rubbing glass, and the other by 

 rubbing resin. But, as shown by our experiments, either kind of electricity 

 can be excited in glass or sealing-wax, and both kinds are produced at once. 

 You cannot get " positive " without negative electricity. " Positive " is the term 

 applied to the kind produced by rubbing glass with silk or wool ; " negative " 

 is the term applied to the kind developed by rubbing sealing-wax, but the 

 kind developed by friction depends on the rubbing substance and certain 

 conditions. 



Bodies charged with the same electricity repel; if charged with different 

 kinds they attract each other. The more readily displaced particles when 

 bodies are rubbed become negatively electrified as a rule. 



Similar electricities repel each other with a force inversely proportional 

 to the squares of the distance between their centres, as established by 

 Coulomb. So if the space between any two similarly electrified bodies be 

 reduced by say one-Jialf^ the force of the repulsion will be increased four 

 times. The rule for attraction is similar so when two bodies are charged 

 with opposite electricities, and the distance between them is increased, the 

 attractive force is diminished in proportion as the square of the distance 

 between them. Many confirmations of this theory were made by the 

 late friend of our boyhood, Sir W. Snow Harris, and published in the PJiilo- 

 sopJiical Transactions. 



The following full list of conductors and non-conductors (copied from 

 Professor Noad's Text-book of Electricity, and compared with De La Rive's 

 Treatise) may be useful : 



CONDUCTING BODIES IN ORDER OF INSULATORS IN THE INVERSE ORDER OF 

 CONDUCTING POWER. INSULATING POWER. 



All the metals. Dry metallic oxides. 



Well-burnt charcoal. Oils (heavier the better). 



Plumbago. Vegetable ashes. 



Concentrated acids. Transparent dry crystals. 



Powdered charcoal. Ice below 13 Fahr. 



Dilute acids. Phosphorus. 



Saline solutions. Lime. 



