MEASUREMENT OF TEMPERATURE 81 



Marking the boiling point. On account of the condensation of 

 vapour upon the thermometer, the method used in Expt. 30, iii. to 

 find the boiling point is not very accurate. More exact deter- 

 minations can be made by means of the apparatus shown in Fig. 65. 

 A can or flask F is fitted with a cork, through which a glass or brass 

 tujbe B passes. Surrounding this tube is a wider tube C, fitted upon 

 the inner tube by means of a piece of thick india-rubber tubing D. 

 At the top of the outer tube is a cork E having a hole in which a thermo- 

 meter can be fitted. When the water in the flask is boiled, steam 

 passes up the inner tube B, and down the wide tube C, and escapes 

 at the outlet G into the open air. To use the apparatus, the top of 

 the stem of the thermometer being graduated is pushed gently into the 

 cork which fits in the outer tube, and adjusted so that the point reached 

 by the mercury at the temperature of boiling water is just above the 

 cork. The cork is then fitted into its place, the water boiled, and when 

 steam has been coming off for about a quarter of an hour, an obser- 

 vation is made of the point reached by the top of the mercury in the 

 tube. The observation is repeated after a few minutes and when two 

 readings obtained at an interval of about ten minutes agree, the point 

 at which the top of the liquid stands is marked upon the stem. The 

 temperature observed is the boiling point of water under the particular 

 conditions existing at the time and place of the experiment. 



Precautions necessary in marking the fixed points. Since experi- 

 ments have shown that the mixture of common salt with ice or snow 

 lowers the temperature of the ice or snow, great care must be taken 

 that pure clean ice is used when the lower fixed point of a thermometer 

 is being marked. It must also be noted that the presence of substances 

 other than common salt similarly have an effect on the temperature. 



The temperature of a boiling solution of common salt in water is 

 higher, too, than that of the steam which is given off from it. More- 

 over, since it is the temperature of the steam from boiling water which 

 alone remains constant at sea-level (the nature of the containing 

 vessel and the presence of substances in solution affecting the tempera- 

 ture of the liquid), in marking the higher fixed point of a thermometer 

 the instrument should be surrounded by the steam and not placed in 

 the liquid. It will be seen more fully later that an increase of atmo- 

 spheric pressure, represented by an increased barometric height, raises 

 the temperature at which water boils, hence the height of the barometer 

 must be recorded when the higher fixed point is being marked. 



Thermometer scales. Some value must now be given to the two 

 fixed points which have been obtained as previously described, and 

 of course they could be called anything the maker of the thermometer 

 J.G.S. F 



