Electricity of shavings o* wood*. 51 



I now thought I had found the caufe of the contradictory Other contra- 

 refults, but to be more certain about it, I began the following ^^ cx P en * 

 (et of experiments. I fliarpened a penknife to a very nice 

 edge, and ufed the fame pieces of wood that had been ufed 

 in the former trials. In 24 trials with cherry-tree, the chips 

 were always negative, and in four trials with elm, and in fix 

 with yew, the chips were always negative. I now drew the 

 edge of the knife lightly over a piece of iron to dull it, ex- 

 pecting to get pofitive chips, but on trial the chips were (till 

 negative. Suppofing the knife was not dull enough, I drew 

 the edge over the iron again, and made the edge very bad, 

 but the chips were flill negative. The edge of the knife was 

 next made rough by rubbing it on a grindftone, and this rough 

 edge gave negative chips. The knife was next ground and 

 fet on the hone very carefully, and this (harp edge gave nega- 

 tive chips. 



As nothing fatisfadtory was obtained from this fet of expe- Probability that 

 rirnents. I began to fufpeft that the degree of heat of the J^ e *. £ £« 

 wood was to be confidered (for the wood was cold in all the fluence the re- 

 tail experiments, and fometimes hot, and fometimes cold, but ult8# 

 mod frequently hot in the firft) or that perhaps the heat of the 

 wood, and the fharpnefs of the knife, were both to be taken 

 inLo the confideration. I therefore fet about the following fet 

 of experiments. 



I fplit the piece of cherry tree (that was ufed before) into Detail of e*pe- 



two pieces, one of which was made thoroughly hot at the "ments with hot 

 . . . r . ? r a °d co 'd wo °d i 



iire. This when chipped with the fame knife that was ufed and different 



in the Iaft experiments, without being- fliarpened, gave pofi- ftarpnefs of 

 tive chips every time in fix trials, and after this piece of wood 

 had cooled till it was fcarcely warm, gave pofitive chips every 

 time in four trials. I then took the other piece which had not 

 been near the fire for five or fix hours, this gave negative 

 chips every time in four trials. I now made this piece of 

 wood quite hot, thoroughly, and chipped it again with the 

 fame knife, and in feven trials the chips were pofitive every 

 time. Thefe two pieces of wood was now laid by for three 

 or four hours to get quite cool. In this Hate they gave nega- 

 tive chips every time in twelve trials. One of them when 

 made thoroughly hot again, gave pofitive chips every time in 

 fix trials. The other piece was now made warm (but only 

 externally fo) in eight trials it gave pofitive chips four times, 



E 2 and 



