130 



distinction, called galvanism, and it is electricity in this form 

 that is employed in blasting. If we connect a copper and a 

 zinc plate together, by a metallic wire, and immerse them in 

 dilute acid, a violent effervescence will be seen; this is caused 

 by the action of the acid upon the zinc, which action generates 

 a current of electricity flowing from the copper plate through 

 the wire to the zinc, and it is with this current we work, 

 whether it be to decompose water, gild metals, or what 

 is now more to our purpose, to generate such an intense 

 heat in the conducting wire as will ignite gunpowder. Such 

 are the elements of an apparatus for blasting, and even this 

 simple battery would be sufficient could we place it within a 

 few inches of the rock to be rent ; but as safety requires 

 that it should be some distance off, and as a contrivance must 

 be used to enable the workman to throw the apparatus into 

 action, and also to arrest the current of electricity at the 

 required moment, some complication in its construction will 

 be necessary : thus, to allow of the requisite distance between 

 the machine and the rock, we must give the current of electric 

 fluid a sufficient power to overcome the resistance a long wire 

 would ofi'er to its progress. 



Electricity may be said to exist in two conditions, viz., 

 intensity and quantity. Intensity I have defined as that 

 quality which tends to overcome obstacles to its progress ; 

 quantity ^ its power to act upon those obstacles ; for example, 

 if we wish to pass a current through a wire of considerable 

 length, there must be sufficient intensity in that current to 

 overcome the resistance off"ered to its transit through the 

 wire; for although metals are excellent conductors of elec- 

 tricity, yet the best of them oppose its transmission to a 

 considerable degree, and, in order to heat any part of this 

 wire, there must be a quantity of electricity passing propor- 

 tionate to the thickness of the wire. In a galvanic battery 

 we gain quantity of electricity by increasing the size of the 



