296 



PHENOMENA OF DEW. 



General bafes 

 for explaining 

 t)iern, 



Remarks. 



The laws ap- 

 plied to explain 

 is.ll the facts. 



BASES OF THE EXPLANATION. 



1. The lefs the temperature of glafs is elevated, the more humi- 

 dity it attracts from the air. 



2. Metals attract it very little. 



3. Glafs fenfibly exercifes its action on tlie humtdity of air, at a 

 diftance, and notzvithjianding the interprfition of different bodies, 

 fuch as plates of metal, &c. 



4. Metals give to vlafi, near ivhich they are placed, the property 

 of more fpeedily attracting caloric from hot air, and on the con- 

 trary, that of yielding it more fpeedily to cold air. 



N. B. When I fay that metals give glafs this property, I 

 mean that they a5i as if they gave it them; which is evident by 

 an examination of two thermometers, one of mercury the other 

 of alcohol, which are plunged at the fame time in air either 

 colder or warmer than that whofe temperature they indicate. 

 The metallic thermometer arrives much fooner than the other 

 to that of the new medium. Its glafs then, if colder, muft 

 take up more fpeedily from the medium the caloric which it 

 tranfmitsto the metal, or, if hotter, it muft more fpeedily give 

 out that of the metal. 



The firft balls has been long eftabliflied. 



The fecond and third are proved by the 17th and 18th fec- 

 tionsof the extract. 



The laft is a neceffary confequence of the conducting pro- 

 perty of metals. 



This being premifed, it is eafy to comprehend that, 



A. When the glafs is armed on its warm face (§ 27, lft, 

 A. B. and C.) it yields its caloric to the cold air more fpee- 

 dily than that which is not armed (bafis 4), and confequently 

 it attracts humidity more powerfully (bafis J), whether dire&ly 

 on the glafs, or through the- metal, or on the metal itfelf (bafis 

 3), if this be in contact ; but if it be at the diftance of fome 

 millimetres, the humidity not meeting the metal on its paftage, 

 accumulates on the oppofite glafs in a greater quantity than 

 el fe where. 



B. If the metal be applied on the cold fide (§ 27, II. A. 

 and B.) the giafs moil heated does not attract fo much humi- 

 dity (bafes 4 and 1 ), and it accumulates on the unarmed part of 

 the pane. 



C. If, in this cafe, the armour be covered with a plate of glafs* 

 the plate cools more fpeedily than if the metal were not pre- 



&ntj 



