56 Chemical and Physical Notes 



generation of steam was varied from the highest to the lowest 

 which was possible with the lamp. The thermometer divided 

 into fiftieths of a degree Centigrade was used, and the mercury 

 stood at ioo'i8 ; that is the top of the mercury was exactly 

 in line with the centre of the division on the stem marking 

 ioo c< i8 C. It occupied this position when steam was being 

 generated at its highest rate of 8'4 grammes per minute, and 

 never varied by a fraction of the width of a division-line until 

 the rate of steam generation had been brought so low that it 

 issued as an exhalation, and not as steam. The rate was then 

 2'5 grammes per minute, and the top of the mercury fell to 

 the lower edge of the line marking ioo'i8 C., which may be 

 taken to represent a temperature of ioo'i79 C. 



In this experiment the rate of passage of steam was varied 

 from 2'5 to 8'4 grammes per minute, or over three-fold, and it 

 produced no effect on the thermometer ; therefore, the exit 

 tube efficiently removed whatever amount of steam entered 

 it, and offered no sensible resistance to it. No better evidence 

 than this could be furnished of the perfect efficiency of the 

 instrument for the purpose for which it was designed. 



With regard to the relative advantages of the thermometer 

 and the barometer in hypsometric work, this thermometer 

 may be taken as an example. It is divided into fiftieths of 

 a Celsius degree, the length of the degree being 35 mm. The 

 tension of saturated vapour, of 100 C., is 760 mm., and of 

 99 C. it is 733*305 mm., giving a difference of 26*695 mm. 

 pressure for a difference of i C. temperature. Thus, 267 mm. 

 on the barometer are represented by 35 mm. on the thermo- 

 meter. Converting the readings of the thermometer into the 

 corresponding ones of the barometer, each division would 

 correspond to about half a millimetre. At lower pressures 

 the effect of change of pressure on the temperature of satura- 

 tion is greater, and at higher pressures it is less. Thus, from 

 Regnault's experiments, we have in Table IV the difference of 

 pressure which causes a difference of i C. in the temperature 

 of saturated steam at different parts of the scale. 



This table shows how rapidly the tension of saturated 

 steam rises, as compared with its temperature. It also shows 



