4- HEAT. 



downwards, and the bulb is heated till the mercury boils, the mercury 

 vapour rising and displacing the remaining air. On again inverting the 

 tube and placing the open end under mercury, as the temperature falls, 

 the whole space will, as a rule, be filled with mercury. Should the air 

 not be entirely expelled, however, the operation must be repeated. The 

 tube is then drawn out near the end till the bore is nearly closed, the 

 bulb is again heated until the mercury flows past the narrow part, and the 

 latter is rapidly sealed up by means of a blowpipe flame. It is found by 

 experience that the bulb of a thermometer undergoes contraction. The 

 contraction is very considerable during the first few weeks or even 

 months after it has been blown, but after two or three years it takes 

 place very slowly and is hardly appreciable over short intervals of time. 

 It is advisable, therefore, to wait till this stage is reached before the 

 graduation of the instrument is proceeded with. 



Fixed Points. If we only had to use one thermometer, to indicate a 

 rise in temperature in one particular case, it would be sufficient to mark 

 on the stem divisions of some chosen length, say 1 mm. each, to number 

 these from the bottom to the top of the tube, and to call each step of one 

 " division " a degree. If, for example, the mercury stood 50 mm. above 

 the lowest mark, we might call the temperature 50. But we wish to 

 use different instruments for different cases and to compare their 

 indications. Further, we wish to compare the indications of our thermo- 

 meters with those of instruments used by other experimenters on other 

 occasions. We must, therefore, have a definite scale, as nearly as 

 possible the same on all instruments. This is secured by the use of 

 definite " fixed points," the same on all instruments, each corresponding 

 to a definite fixed temperature. The two fixed points universally used 

 are 1st, the temperature of ice when just melting under the atmospheric 

 pressure of 760 mm. ; and 2nd, the temperature of steam from water 

 boiling normally under the same atmospheric pressure. The height of 

 the mercury column of a thermometer placed under constant conditions 

 in different vessels containing melting ice will remain invariable. Placed 

 under similar conditions in the steam from water boiling normally at a 

 pressure of 760 mm. in different vessels, it will again remain invariable, 

 but at a point much higher in the stem. The volume of the tube or, 

 if it is of even bore, the length between these two fixed points is 

 divided into a number of equal parts, and each part indicates a degree. 



The Centigrade Scale* On the centigrade scale, now universally 

 employed for scientific purposes, the temperature of melting ice is 0, 

 and that of boiling water under the stated conditions is 100, and the 

 interval is divided into 100 parts. The scale is indicated by writing C. 

 after the temperature, as C. 



The Fahrerilieit Scale. On the Fahrenheit scale, the temperature of 

 melting ice is 32, and that of boiling water is 212, the interval being 

 divided into 180 equal parts. A Fahrenheit temperature is indicated by 

 writing F. after the temperature, as 212 F. 



This scale was arranged by Fahrenheit early in the eighteenth century. 

 He found that a mixture, of which he did not state the proportions, of 

 ice, water, and sal-ammoniac, or sea-salt instead of sal-ammoniac, gave 

 a very low definite temperature, which ho took as 0. He found that 



* Some interesting notes on the history of therinometry will be found in Bolton's 

 Evolution of the Thermometer, 



