ON SPONTANEOUS EVAPORATION. jg 



The experiments I made to determine the relation of Experiments 

 evaporation to lieat were very simple. On a table in the '» ascertain it. 

 •middle of a large room, the air of which was perfectly still 

 and heated to a given temperature, and the hnmidity of 

 which was also ascertained and constantly the same, I placed 

 ■ cylindrical or prismatic vessels of glass and metal, the dia- 

 meters of which were of no importance; but I made them 

 all above an inch, because evaporation goes on less freely in 

 vessels with srnall apertures. These vessels I filled with 

 spring water, heated precisely to the temperature of the 

 room, and noted the time when the experiment commenced. 

 Keeping the air of the room uniformly at the same temper- 

 ature, when I thought the quantity of water evaporated 

 might begin to render the air sensibly rnoister, and thus 

 diminish its solvent power, I measured how much the sur- 

 face of the water in the vessels was lowered. The time of 

 taking tliis measure, which was that of the conclusion of the 

 experiment, I noted down ; and by the rule of proportion [ 

 found what the evaporation would have been, if the experi- 

 ment had continued twentj'-four hours. 



These experiments, though very simple, are attended Difficulties. 

 with some difficulty, if well executed. It is not easy, to 

 keep the air of a large room at the same temperature for any 

 length of time, or to have it always at the same degree of 

 humidity. By care, however, I accomplished both these 

 points for a time sufficient to be perfectly sure of the results 

 of these experiments. 



To determine the degrees of heat I employed several ex- Instruments 

 cellent thermometers, constructed on the principle of Mr. ^"^^' "^^ ' 

 De Luc, and two of which were made by the late Mr. Paul 

 of Gieneva. I was not equally happy in my means of as- 

 certaining the degree of humidity; for whatever pains I 

 took, 1 could not procure one of Mr. Saussure's hair hy- 

 grometers. This instrument, considered as the most accu- 

 rate, or least defective, of the kind, is very difficult to be 

 met with since the death of this celebrated artist, who was 

 perhaps the only persod that succeeded in makinjj good 

 ones. But as all that was requisite in the present instance 

 was to find one constant degree of humidity, I endeavoured 

 to supply their want by means of hygrometers made of one 

 C 2 piece 



