34 



SCIENTIFIC AMUSEMENTS. 



cipated considering the price ; when anything like close measure- 

 ments of temperature are requisite it is always essential to " verify " 

 the thermometer indications, that is to check them and see if they 

 are fairly exact. The most important check is to see that the 

 ice-point is correctly recorded ; for this purpose a few ounces of 

 fresh snow or pounded pure ice are placed in a tumbler, and the 

 thermometer plunged into the mass and used to stir it about for 

 a few minutes, or preferably lumps of ice are placed in a per- 

 forated vessel (fig. 18), and the 

 thermometer plunged in the ice ; at 

 the end of this time the quicksilver 

 will have sunk to the lowest possible 

 level under the circumstances, and 

 by carefully looking at it it will be 

 at once seen whether the mercury 

 stands exactly at the zero-point or 

 not. If not, a note should be made 

 of the point at which it does stand ; 

 suppose this to be 2, then it follows 

 that the thermometer reads 2 too 

 high at that part of the scale. If, 

 on the other hand, the reading were 

 1, the thermometer would obvi- 

 ously be erroneous in the other 

 ^ direction, reading 1 too low. 



The boiling-point may be similarly 

 checked by immersing the ther- 

 mometer in the steam given off by 

 boiling distilled water when the 

 barometer is at the average height, about 30 inches. The ther- 

 mometer should be suspended inside a sufficiently wide glass 

 tube placed vertically over the vessel of boiling water (fig. 19), 

 so that the bulb is not immersed in the water, whilst the whole 

 stem is thoroughly heated by the steam' which must issue freely 

 at the top. In this way the error of the reading at the boiling- 

 point is determined. For most ordinary purposes an error at 

 either part of the scale not exceeding one or two degrees is of 

 little consequence ; but for many kinds of scientific observations 

 a much greater degree of accuracy than this is indispensable. 



Expt. 30. Circumstances modifying the Freezing-Point of 

 Water. If on a cold winter's day some pure distilled water be 

 placed in the open air (or in a sufficiently cold room) and kept 

 stirred up by means of a thermometer placed therein, it will be 

 noticed that ice begins to form when the thermometer marks 



Fig. 18. Verification of Ice- 

 Point. 



