Modes of Physical Force. 9 7 



air. With iron the experiment is equally instructive. 

 Iron is volatilized by the voltaic arc in nitrogen or in an 

 exhausted receiver : and when a scarcely perceptible film 

 has lined the receiver, this is washed with an acid, which 

 then gives, with ferro-cyanide of potassium, the prussian- 

 blue precipitate. In this case we readily distil iron, a 

 metal by ordinary means fusible only at a very high tem- 

 perature." A very good vacuum, like that which remains 

 in a receiver after carbonic acid has been absorbed by 

 caustic potash, will, until it becomes partly filled by the 

 volatilization of the material of the electrodes entirely 

 stop the discharges of a Rhumkorf coil : and a conductor 

 of insufficient dimensions is ignited or actually fused by 

 such discharge or by the current equally. Matter is 

 needed for the transmission of the electricity in both 

 these cases. The matter, too, must be in a particular 

 state. One body is favourable to the transmission of 

 electricity, another not. The same body may be favour- 

 able at one time -and unfavourable at another: thus 

 carbon is a good conductor in the state of graphite and 

 a very bad conductor in the state of diamond. The 

 transmission of electricity, that is to say, is dependent 

 upon the presence of certain molecular conditions. In 

 a certain part of the voltaic circuit these take that form 

 of chemical disturbance which is known as electrolysis. 

 Whether an analogous condition is present in other 

 parts of the circuit is another question, brat this must be 

 conceded that there is some material change in these 

 other parts. Thus : a wire is fused if it be too small to 

 allow the free passage of the current or discharge, and 

 even in the case of a thick large wire, stretched as in the 

 ordinary telegraph, where little or no heat may be de- 

 veloped by the electricity, the sonorous vibrations which 

 attend the passage of the electricity, and the manifest 



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