Chap. 5.] [ 481 ] 



CHAP. V. 



OF RAIN. 



Recapitulation of Doflrines relative to fpontaneous E-Tjaporation,^f r a- 

 four ly fame fnppofed to conftji of hollow FeJicles.-Rain. Different 

 Theories of Rain. Sno-iv. Hail. Rain ivbicb froze in coming in 

 contact ivit/j the Earth. Large Hail-ftones>-~-Fogs.Dc-iv. Hoaf 

 Froft,Injlances of partial freezing when the general Temperature 

 is above the freezing Point. 



IN a preceding volume, when treating of the effects 

 of heat or fire, and particularly of vapour, it was 

 neceffary to introduce a few obfervations relative to 

 fpontaneous evaporation. It is proper, however, to 

 repeat, in this place, that philofophers are by no means 

 agreed with rcfpedt to the caufe of this phenomenon. 

 By fome it has been attributed to a folution of water 

 in air fimilar to that of faline fubftances in water j by 

 others, to the adion of the eletric fluid. The firlt 

 of thefe opinions had till lately obtained almoft uni- 

 verfal afTent, but is now relinquished, fince it is found, 

 that evaporation proceeds, as well in the exhaufted 

 receiver of die air pump, as when air is prefent. Elec- 

 tricity has been found to promote evaporation, and 

 clouds are almoft univerfally electrical ; but evapora- 

 tion is carried on continually where there is no reafon 

 to fuppofe the prefence of the eledric fluid, at Jeaft 

 where its.operation cannot be difcovered by any known 

 teft. It is at prefent commonly fuppofed, that redun- 

 dant heat is the general caufe of the evaporation of 

 water, and that it always proceeds in proportion to its 

 VOL. II, I i tempera- 



