^Q ON SPONTANEOUS EVAPORATION. 



piece of gut, but which I constructed witij particular at- 

 tention, as I may perhaps relate at large on some futute 

 oppoitunity. I shall only say, that the uniform degree of 

 moisture I chose for my experiments answered nearly to 50* 

 of de Saussure's hygrometer*. 

 Measurement. To measure the lowering of the surface of the water m 

 the vessels, 1 employed a scale of 1000 equal parts, accu- 

 rately divided by Canivet. Of these parts igo were pre- 

 cisely equal to a French inch. 1 took the measure on the 

 side of the glass vessels with a pair of spring compasses, 

 the points of which were extremely fine, and a lens. P'or 

 thobc of metal 1 employed a small commodious instru- 

 ment. It consists of a capillary tube of glass, firmly fast- 

 ened at right angles to a wooden ruler perfectly straight-; 

 and a scale similar to the preceding traced on a very tliiu 

 and narrow slip of brass, tixed to the tube. The inside of 

 the tube being wetted by a drop of water introduced into 

 it, the tube is immersed perpendicularly in the water of the 

 vessel, till the edge of the ruler rests on the brim of the 

 vessel. The water ascends in the tube by capillary atrac- 

 tion, and the point of the scale to which it rises at the 

 commencement of the experiment is noted down. The 

 same operation is repeated at the conclusion of the experi- 

 ment ; and the difference between the former point and that 

 to which it now rises measures the lowering of the surface 

 by evaporation. 

 Liw of cTapo- When I liad thus obtained five or six well defined eva- 

 ''^'"'•'* porations, corresponding to equal intervals expressed in 



degrees on the scale of the thermometer, or to equal differ- 

 ences of heat, I endeavoured to find the law these evapora- 

 tions followet). For this purpose I made many fruitless 

 trials, till an idea suggested itself, which from its simplicity 

 ought to have presented itself at first; that of introducing 

 between the two extreme evaporations as many geometrical 

 proportionals as there were evaporations observed between 



* In my experiments on the ratio of evaporation to the moisture of 

 the air I employed a method of d'.:termini"g this moisture more certain 

 than the most perfect hygrometers; ih.it of calculating directly the quan- 

 tity of water in vapour cjntained in a given bulk of air, by absorbing 

 this water by perfectly dry potash. 



them. 



