in Electrical Instdation. ■ 167 



non-conducting coat. When a piece which insulates well is 

 cut, the surface exposed has a resinous lustre and a compact 

 character that is very distinctive ; whilst that which conducts 

 has not the same degree of lustre, appears less translucent, 

 and has more the aspect of a turbid solution solidified. I 

 believe both moist steam-heat, and water-baths are used in 

 its preparation for commerce ; and the difference of specimens 

 depends probably upon the manner in which these are applied, 

 and followed by the after process of rolling between hot cy- 

 linders. However, if a portion of that which conducts be 

 warmed in a current of hot air, as over the glass of a low gas 

 flame, and be stretched, doubled up, and kneaded for some 

 time between the fingers, as if with the intention of dissipating 

 the moisture within, it becomes as good an insulator as the 

 best. 



I have soaked a good piece in water for an hour ; and on 

 taking it out, wiping it, and exposing it to the air for a minute 

 or two, found it insulate as well as ever. Another piece was 

 soaked for four days and then wiped and tried : at first it was 

 found lowered in insulating power ; but after twelve hours' ex- 

 posure to air under common circumstances it was as good as 

 ever. I have not found that a week's exposure in a warm air 

 cupboard of a piece that did not insulate made it much better: 

 a film on the outside became non-conducting ; but if two fresh 

 surfaces were exposed by cutting, and these were brought into 

 contact with the electrometer and the finger, the inside por- 

 tion was still found to conduct. 



If the gutta percha in either the good or the bad condition 

 (as to electrical service) be submitted to a gradually increasing 

 temperature, at about 350° or 380°, it gives off a considerable 

 proportion of water ; being then cooled, the substance which 

 remains has the general properties of gutta percha, and insu- 

 lates well. The original gum is probably complicated, being 

 a mixture of several things ; and whether the water has existed 

 in the substance as a hydrate, or is the result of a deeper 

 change of one part or another of the gum, I am not prepared 

 to say. All I tiesire in this note is to make known its use in 

 the arrangement of extemporary or permanent electrical ap-« 

 paratus for the advantage of working philosophers, both juve- 

 nile and adult. 



I am, dear Phillips, 



Yours, 



M. FAKAnAY. 



