168 



EVAPORATION. 



1 



The method adopted by Dalton for determining the tension of vapor sus- 

 pended in the atmosphere at any given time is, perhaps, in skilful hands, more 

 exact than any which has since been discovered, especially if the glass vessel 

 used be sufficiently thin. Dr. Thompson states that he has submitted to ex- 

 periment other instruments for the same purpose, and this simple one, and that 

 he is satisfied that the results obtained by the last are susceptible of the high- 

 est degree of accuracy. 



Other instruments, however, have been contrived for determining the quan- 

 tity of vapor suspended in the atmosphere, and are called hygrometers, or meas- 

 urers of the moistness of the air. Such instruments are generally constructed 

 from some substance which has a power of absorbing moisture, and which 

 gives some external indication of the quantity which it absorbs. 



The hygrometer of M. De Luc consists of an extremely thin piece of whale- 

 bone, which is stretched between two points and acts on the shorter arm of an 

 index or hand, which plays on a graduated scale, like the hand of a clock. The 

 effect of the whalebone absorbing moisture is to cause it to swell, and its length 

 increases ; and, on the contrary, when it dries, its length is contracted. The 

 index is moved in the one direction or the other by these effects, and the space 

 it moves over gives the change in the hygrometric state of the atmosphere. 



The hygrometer of M. Saussure consists of a human hair, previously pre- 

 pared by boiling it in a caustic ley. It then becomes a highly sensible absorb- 

 ent of moisture. One extremity is suspended from a hook, and the other ex- 

 tremity carries a small weight which keeps it stretched. It is turned once 

 round a grooved wheel, which moves an index playing on a graduated arch. 

 As the hair contracts and expands by the effect of absorbing moisture, the 

 wheel is turned in the one direction or the other, and the index shows the effect 

 by moving through a corresponding portion of the arch. 



That this instrument may indicate the absolute quantity of vapor suspended 

 in the air, it was necessary that some fixed points upon it should be determin- 

 ed, analogous to the boiling and freezing points of water on a common ther- 

 mometer. To effect this is, however, more difficult in the present case, inas- 

 much as the instrument is influenced at once by two causes, namely : by heat, 

 and by the quantity of vapor suspended in the air. M. Saussure first consid- 

 ered the application of the instrument when exposed to an invariable tempera- 

 ture. He placed it in a vessel which contained perfectly dry air at the pro- 

 posed temperature. He thus obtained the point of extreme dryness. He then 

 successively introduced into the receiver several small known quantities of wa- 

 ter. This he accomplished by depositing the liquid on small pieces of linen, 

 which he weighed exactly, and determined the quantity of liquid thus intro- 

 duced. When each successive portion of the liquid was vaporized, he observed 

 and marked the indication of the hygrometer. He then withdrew them and 

 weighed them again, thus determining exactly the quantity of liquid evaporated 

 on each occasion. 



Having repeated very often the experiment at the same temperature, he found 

 that whatever variation the hygrometer had previously undergone, it always 

 returned to the same point when the quantities of water vaporized in the re- 

 ceiver were equal. He found the same result at various temperatures, the in- 

 dications at the same temperature being always the same ; but the absolute 

 quantity of water necessary to be vaporized in the space, in order to move the 

 hygrometer through the same number of degrees, was different at different tem- 

 peratures. To obtain, therefore, the actual quantity of water suspended in the 

 form of vapor, it is necessary at the same time to observe the indications of the 

 thermometer and hygrometer. These two indications are always sufficient for 

 the exact solution of the question. 



