292 PHENOMENA OF DEW. 



If the whole ex- 9. If inftead of this lafl mentioned fmall difc, a glafs of the 



cov n e«dwith be fame flZe asthe cxternal metallic difc be applied fo as cxaftly 

 glafs, its effect to cover this laft, the humidity is difcovered over the whole 

 ceafes. f t h e interior rectangle in the fame manner as if the glafs were 



bare on the outfide. 



Metal upon this a. If upon the rectangle of glafs there be applied another 

 laft glafs plate . Ir ' . , .,., . . , r \ ,, 



revives the ef- meta *» c rectangle, no humidity is again produced on the coi- 

 fed, refponding interior furface, though it continues to be formed 



on the fame fide on the reft of the metal and glafs. 

 Another glafs b. But the humidity is again condenfed, if on the laft ex- 

 ternal metallic rectangle there be applied a rectangle of glafs. 

 Another metal c . And this again difappears, or ceafes to be produced, by 

 1 ' ' the application of a third metallic leaf. 



d. A third rectangle of glafs caufes it again to appear. 



e, fi g, &c. &c. &c. 



This alternation Neverthelefs there is a term at which the phenomenon be- 

 is limited. comes irregular ; and this is when the whole thicknefs of the 



metallic bales and plates of glafs amounts to 16 or 20 millime- 

 tres (three quarters of an inch). 



10. Though the above phenomena clearly prove that the 

 metal does not act as a defence or -cover in the production of 

 thefe effects, the author proves it ftill more ftrongly in the fol- 

 lowing manner. 

 Gilt paper ope- He applies to a pane of glafs gilt papers of the fame fize and 

 rates like a me- fjg Ure> f ome D y tne ir metallic faces, and others by the furfaces 

 be outfide; which are uncovered, all the other circumftances being the 

 otherwife not. fame. Thofe papers only of which the metallic furfaces were 

 expofed to the air produced the effects of metals, and the 

 others produced no effect. This experiment repeated with 

 pieces of glafs coated with tin foil inftead of gilt paper, pro- 

 duced the fame refults. 

 Infide metal at a 1 1. If in the infide of a chamber, and parallel to one of the 

 diftance from the p anes fgl a f s ' m a window, there be placed a difc of metal 

 denfation on the nx or feven centimetres in diameter (two inches and a quarter) 

 glafs, and not f that it fliall be fupported only by the middle, and the face 

 fliall be only a few millimetres (or twenty-fifths of an inch 

 diftant from the glafs, when the humidity comes on withii 

 there is none depofited on the difc, unlefs it be extremelj 

 near, but on the oppofite glafs it is depofited in much greatt 

 quantity than any where elfe. 



12, II 



