1 84 Dielectric effects on various substances 



371] Plates of other substances*. 



372] The coatings of all these plates were circular. 



In computing the charge of the glass plates, the diameter of the coating 

 was corrected on account of the spreading of the electricity as in the 

 fourth column, the electricity being supposed to spread -07 of an inch if 

 the thickness is -21 and -09 if the thickness is -08, and so on in proportion 

 in other thicknesses. But no correction is made in computing the charges 

 of the other plates, as I was uncertain how much to allow. 



373] The method I used in making all the plates of the second table 

 was this. I first cast a round plate of the substance, three or four times 

 as thick as I intended it should be, and rather thinner near the edges than 

 in the middle, taking care to cast it as free from air bubbles as I could. 



I then heated it between two thick flat plates of brass, till it was 

 become soft, and then pressed it out to the proper thickness by squeezing 

 the plates together with screws f. In order to prevent its sticking to the 

 brass plates, I put a piece of thin tinfoil between it and each plate, and 

 I found the tinfoil did not stick to it so fast but what I could get it off 

 without any danger of damaging them. 



374] The heat necessary to melt shell lac is so great as to make it 

 froth and boil; which makes it impossible to cast a plate of it free from 

 air bubbles. The plate mentioned in the preceding table was as free from 

 them as I could make it. It contained, however, a great quantity of 

 minute bubbles, but no large ones. 



375] Bees wax melts with a heat of about 145. If it is then heated 

 to a degree rather greater than that of boiling water, it froths very much, 

 and seems to lose a good deal of watery matter, and if it is kept at this heat 

 till it has ceased frothing, it will then bear being heated to a much higher 

 degree without frothing or boiling. Bees wax thus prepared I call 

 dephlegmated. 



In order that the plates of dephlegmated bees wax should all be equally 

 so, I dephlegmated some bees wax with a pretty considerable heat, and 

 * [See Art. 674.] f [Art. 514.] 



