144 MB. W. N. SHAW ON HYGIIOMETRIC METHODS. 



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removal of its moisture without altering its pressure furnishes an equally accurate 

 means of finding the original pressure of the vapour. Only one stage less direct is 

 the method of finding the increase of pressure produced by saturating a specimen of 

 air whose volume is kept constant and assuming the saturation pressure from 

 REGNAULT'S tables. Calling such determinations " absolute," an instrument for 

 making them is called an absolute hygrometer, and of these very many have been 

 suggested by various experimenters in recent years. Before describing them, I will 

 just mention that the simplicity of the method is entirely illusory. The differences of 

 pressure to be measured are small, never practically exceeding 30 mm. in our climate, 

 and being generally about 10 mm. Taking the latter amount, and supposing that an 

 accuracy of 1 per cent, is required, the pressure difference must be measured to O'l mm., 

 which is less than J-^QQ of the ordinary barometric pressure. Consequently the volume 

 or the pressure must be kept constant during the measurement to the same degree of 

 accuracy ; and, since a change of temperature of 1 C. alters the volume or pressure by 

 about '00366 of its amount, the temperature of the apparatus must be kept constant, 

 or its change compensated for, to within 0'02, which is a very difficult matter. 

 Further, any taps or connexions must hold sufficiently tight to guarantee that the 

 vessel will not lose or gain air to the extent of -shroff f i ts whole volume during the 

 course of an experiment. These are a few of the difficulties. 



The best known of these new instruments is that of SCHWACK.HOFER, described and 

 figured in JELINEK'S ' Zeitschrift' for 1879. In its modified form it is an apparatus 

 for finding the diminution of volume of air in consequence of the absorption of the 

 vapour by sulphuric acid. The air is first enclosed in a burette, with a graduated stem 

 communicating with a mercury cistern ; the burette is then, by a suitable arrangement 

 of taps, brought into communication with a second vessel, open at the bottom and 

 shaped like an elongated bell, containing a number of vertical glass tubes, and filled 

 up with sulphuric acid. The mercury is then forced by means of a plunger dipping 

 into its cistern into the burette, and the air passes over into the second vessel, dis- 

 placing the sulphuric acid, leaving, of course, a large surface of acid exposed, which 

 dries the air after four or five transfers backwards and forwards. The sulphuric acid 

 is then driven back to its original level, which is carefully marked ; the pressure of the 

 dried air is adjusted to be the same as before by means of an oil gauge, and the 

 diminution of volume is read off ort the graduated stem of the burette. The tempe- 

 rature is read by a thermometer sealed into the burette, and kept uniform by a jacket 

 of glycerine round the burette. 



I have omitted a large number of small precautions which are quite necessary in 

 taking an observation. The apparatus is very delicate, and is therefore available only 

 for laboratory use. 



PEENTER worked with an instrument of this kind. He says : " Bei fiinfzig Versuche 

 waren resultatlos, bis es mir gelang die Hiihne zu dichten, das Quecksilber luttfrei 

 und rein zu halten und die ganze Rohre vor aller Verunreinigung zu schiitzen. 



