of a Nac Hygrometer. 3£$ 



the quotient will express the ratio of the tension at the time 

 of the experiment, to the total tension regarded as unity. 



Some experiments appeal - to me to indicate, that the num- 

 bers which express the rise of the thermometer at different 

 temperatures, for different degrees of humidity, follow, with- 

 out any sensible error, the same ratio as the tensions of the 

 vapours at these same temperatures : For this comparison, I 

 employed the table constructed from the experiments of Dal- 

 ton. From this it followed, that the thermometer, taken out 

 of the acid, and exposed to the air, would indicate, by the 

 number of degrees it i*ose, two things at once ; 1st, The ratio 

 between the tension of the vapour in the air, and the total 

 tension at the same temperature ; and, 2d, The absolute 

 tension of the vapour in the atmosphere at the time of the 

 experiment. 



Along with the advantages which I have mentioned, the pre- 

 ceding process presents some inconveniences. In the first place, 

 it is not an instrument, but an apparatus which must be em- 

 ployed, as an experiment is necessary to obtain an indication. 

 In the next place, the employment of the process requires 

 some precautions. We must, as much as possible, make use 

 of a thermometer whose bulb is very small, both on account 

 of the great sensibility of the instrument, and because the 

 quantity of acid that adheres to the surface of the bulb re- 

 mains always the same. We must also employ an acid of the 

 same degree of concentration, though I have, however, not 

 observed that a small difference in the decree of con centra- 

 tion has a very great influence. We must, likewise, during 

 the experiment, endeavour to avoid every cause of heat fo- 

 reign to that which alone ought to act. On this account, it 

 it is proper to have, beside the thermometer in use, another 

 which will indicate the temperature of the air at every in- 

 stant. 



Before concluding this notice, I shall say a few words on 

 the fact, which constitutes the principle of it, viz. the re- 

 markable difference between the quantities of heat developed 

 by the condensation of vapours on sulphuric acid, according 

 to the degree of humidity of the atmosphere. 



It would seem at first sight, that however small be tin 



