Apparent influence of thickness of the plate 1 8 5 



suffered it to cool and harden, and out of this lump I made all three plates, 

 taking care in casting them not to heat them more than necessary. 



I used the same precautions also in casting the plates of a mixture of 

 rosin and bees wax, the proportion of the rosin to the bees wax was forgot 

 to be set down. 



What are called in the table the 4th and 5th plate of rosin and bees 

 wax are in reality the same plate as the 3rd, only with a smaller coating. 



376] It appears from these experiments, first, that there is a very 

 sensible difference in the charge of plates of the same dimensions according 

 to the different sort of glass they consist of, the charge of the plates 

 and Q, which consisted of the greenish foreign plate glass mentioned in 

 [Art. 301], being the greatest in proportion to their computed charge of 

 any, next to them the crown glass, and the flint glass being the least of all. 



Secondly. The charge of the Lac plate is much less in proportion to 

 its computed charge than that of any glass plate, and that of a plate of 

 bees wax, or of the mixture of rosin and bees wax, still less. 



But it must be observed that there is a very considerable difference 

 between the three different plates of dephlegmated bees wax in that 

 respect. The same thing, too, obtains in the mixture of rosin and bees 

 wax*. 



377] As the proportion of the real charge to the computed is greater 

 in the thick plates than the thin ones, one might be inclined to think that 

 this was owing to the electricity being not spread uniformly. But as the 

 difference seems to be greater than could well proceed from that cause, 

 I am inclined to think that it must have been partly owing to some 

 difference in the nature of the plates. Perhaps it may have been owing to 

 some of the plates having been less heated, and consequently having 

 suffered a greater degree of compression in pressing out than the others. 



378] The piece of ground crown glass mentioned in the first of the 

 foregoing tables was made out of a piece of crown glass about Jf of an 

 inch thick, and ground down to the thickness mentioned in the table, care 

 being taken by the workman to take away as much from one side as the 

 other, so that the plate consisted only of the middle part of the glass. 



My reason for making it was that as there appears to be a considerable 

 difference in the charge of different sorts of glass, it was suspected that 

 there might possibly be a difference between the inside of the piece and 

 the outside, and if there had, it would have affected the justness of the 

 experiments with the ten pieces of glass ground out of the same piece. 



* 'Note 27, p. 418.] 



+ There are pieces of that thickness sometimes blown for the use of the Opticians. 



