v*3P Remarks e?i HygrometnJ, 



true when confined to the phsenomena of vapour produced in 

 a vacuum ; but f am convinced that it fails in point of ac- 

 curacy when air is present, even if the experiment he made 

 in Mr. DaltonYown mauometer. 



. Every cue who alleys the justice of the preceding argu- 

 •ments must refuse h \o M. Berzelius, when he says 



that the hygrometer should discover to what column of 

 mercury the water gas of the air belongs ; because we are far 

 from being certain that the air contains any water gas at all. 

 The foregoing remarks relate to the hypothesis which 

 suggested the hygrometer iij question. As for the instrument 

 itself, it cannot possibly have any just claim to correctness, 

 before the following proposition is fairly established, in con- 

 junction with the other new doctrines already mentioned. 

 The substances of which these hygrometers have been or 

 shall be made, exercise no attractive force on the aqueous 

 •gas ; on the contrary, they only diminish the temperature of 

 this gas ; in consequence of which, part of it is condensed 

 upon the cooling surface by the pressure of the rest. Hence 

 it follows, that if this gas be equally cooled at the same in- 

 stant by two different substances, it will fall in the form of 

 water, and in equal quantities, upon them both. Various 

 experiments, which I have made at different times, enable 

 me to pronounce the proposition, to be incorrect; conse- 

 quently the instrument of M. Berzelius cannot be admitted 

 into the apparatus of a meteorologist under the name of a 

 hygrometer. To state this objection more clearly, let me be 

 understood to say, that if any one would try two instru- 

 ments of the kind at the same time, one of which consisted 

 of glass and the other of tin or silver, they would assign 

 different expansive forces to the water gas of the atmosphere, 

 by beginning to condense it at different degrees of tempera- 

 ture. Besides, if we advert to some experiments by Count 

 Rumford, it is evident that glass attracts water from the at- 

 mosphere when we suspect nothing of the kind, and that a 

 glass globe is frequently covered with a thin film of dew 

 when it is supposed to be dry. Moreover, the experiments of 

 M. De Luc inform us that the phenomena of the hygrome- 

 ter succeed in n. vacuum, where it would be absurd to -ima- 

 gine 



