PHENOMENA OF DEW. 293 



12. If inftead of this difc of metal a difc of glafs be fubfti- Glafs inftead of 

 tuted, the humidity is not accumulated oppofite this difc any ™^ al has no ef " 

 more than on the reft of the glafs. 



13. The inverfe phenomenon takes place on the outer fur- Outfide, there- 

 Face under like circum fiances. 



14. If the metallic difc (of fe&ion II.) be varnifhed on the Varnifh on the 

 fide which faces the interior of the chamber, the phenomenon ^om^he^laV 

 takes place in the fame manner. has no effeft 



15. If the fide next the window be varnifhed, or if both on the fide next 

 fides be varnifhed, the phenomenon does not take place,. * he S Ia [ s > ^£" 



r ■ '"'* i ill ir i ftr °y S thc eff eft. 



16. Thefe properties of the metals belong alio to other Otherplates as 

 conductors of electricity, but with fome modifications accord- wel1 as metals. 

 ing to their nature. 



17. If a plate of metal be fubftituted inftead of a fquare of Metal plate in- 

 glafs, humidity is fometimes produced, but always much lefs o "^ pane 

 than upon glafs. 



18. If this plate of metal be thin and flat, and a difc of glafs Pane of glafs oa 

 be fattened to it, the humidity is produced in preference, r a P lateof mctal » 

 more abundantly on the correfponding interior furface. 



J 9. All thefe phenomena take place in the fame manner, or The fame phe- 

 nearly fo, when panes of glafs fitted up with the metallic difes, nomena in ? e 



J „ r . ~ open air, where 



or apparatus before defcribed, are fupported upon fliort pegs the ground re- 

 in the middle of a field where the grafs has been mowed and Pj" efe n ts lne 

 clipped ; in thefe circumftances the upper furface of the glafs t h e upper fp ace 

 jrcprefents the outfide, and the lower the infide of the chamber. the outer air. 



20. The interior humidity may be excited provided the Artificial pro- 

 weather be rather cold, by means of a ftove, or by fprinkling duftl °nof dew 



, , i , i • , i • i ° in a chamber : 



water on a red hot body ; and in the country during dry wea- an d u 



ther, the fpace on which the glaffes are to be expofed may be doors. 



lightly watered after fun-fet, 



21. If different liquids fuch as water, mercury, alcohol, Other bodies 

 oils, acids, &c or even fmall leaden fliot be put into glafs di ^ nt hom 

 veffels equal and alike in all refpecls to the half, or two thirds 



of their height, and they be expofed to the dew out of a win- 

 dow, the humidity will not be depofited on the lower part of 

 the vefiel, but only above the level of the fubftances contained, 

 and at a diftance which varies according to the nature of thefe 

 fubftances, being greater for mercury than for water, greater 

 for water than for oil, &c. 



22. In all thefe phenomena when the humidity is too abun- The humidity 



dant, the refults become confufed and uncertain. muft be modc " 



: rate. 



2 23. Tiie 



