52 SCIENTIFIC AMUSEMENTS. 



ing experiment, filling the decanter with a freezing mixture of 

 snow and salt, or sulphate of sodium and hydrochloric acid (Expt. 

 21 and 24), instead of ice-cold water; the moisture deposited on 

 the cold glass will be frozen to hoarfrost as fast as it forms, so 

 that after a few minutes a perceptible quantity can be scraped off 

 the glass with a knife. 



Expt. 45. To Determine the Dew-Point. The dew-point is 

 that temperature to which moist air must be cooled in order that 

 it will just begin to deposit dew ; the greater the degree of cooling 

 requisite to produce this, the further removed is the air from being 

 saturated with moisture. Obtain a test-tube about J inch in 

 diameter and 6 inches long with a cork fitting tightly into it. By 

 means of a " rat-tail file " bore three smooth holes side by side 

 through the cork. Through one pass a piece of quill glass tubing 

 about 7 or 8 inches long, so that it may reach almost to the bottom 

 of the test-tube ; through the second pass a shorter similar piece 

 about 2 inches long, and through the third the stem of a thermo- 

 meter, preferably one with the scale engraved on the glass, and 

 with a cylindrical bulb no wider than the stem, which 

 is J inch thick or so (fig. 26). (Thermometers are 

 specially made of this form for chemical purposes, 

 so that they can be passed through corks, &c.) Instead 

 of a cork an india-rubber stopper ready perforated with 

 the three holes can be purchased. The thermometer 

 must descend so as nearly to reach the bottom of the 

 test-tube. Attach a long piece of thin india-rubber 

 tubing to the projecting short piece of glass tube, pour 

 ether into the test-tube so as to fill it about an inch deep, 

 Fig. 26. fix it in a holder obtainable at the instrument dealers, 

 Dew-point and insert the cork with the longer tube and thermo- 

 Hygro- meter bulb dipping into the ether. 



er ' Now stand a short distance off and suck the air out of 

 the test-tube by means of the india-rubber tube ; if the cork fits 

 sufficiently tightly air will bubble through the ether passing in 

 through the longer tube from the outside air. This will make the 

 ether evaporate quickly, the vapour being carried off by the current 

 of air ; in consequence the ether will be cooled down (as in Expt. 

 25) on account of heat being rendered latent during evaporation 

 (Chapter XXI.), and the thermometer will gradually fall. By 

 and by you will notice that the lower part of the tube becomes 

 dim from the formation of dew ; the temperature indicated by the 

 thermometer when this just begins to form is the dew-point. To 

 be very accurate, stop sucking the air, when the dew will begin to 

 clear off again, the thermometer slightly rising. The average of 



