198 PROCEEDINGS OF THE AMERICAN ACADEMY 



Baudin thermometers seem to contain lead as far as one can tell from 

 the blackening in a gas flame, but they stand very much above the air 

 thermometer at 40°. I have since tried some of the Baudin ther- 

 mometers up to 300°, and find that they stand below the air thermom- 

 eter between 100° and 240° ; they coincide at aboid 240°, and stand 

 above between 240° and 300°. This is very nearly what Regnault 

 found for " Verre Ordinaire." It is to be noted that the formula 

 obtained from experiments below 100° makes them coincide at 233°, 

 which is remarkably close to the result of actual experiment, especially 

 as it would require a long series of experiments to determine the 

 point within 10°. 



The comparison of thermometers also shows that all thermometers 

 in accurate investigations should be used as thermometers with arbitrary 

 scales, neither the position of the zero point nor the interval between 

 the 0° and 100° points being assumed correct. The text books only 

 give the correction for the zero point, but my observations show that 

 the interval between the 0° and 100° points is also subject to a secular 

 change as well as to the temporary change due to heating. Of all 

 the thermometers used, the Geissler is the worst in this as in other 

 respects, except accuracy of calibration, in which it is equal to most of 

 the others. 



The experiments on the specific heat of water show an undoubted 

 decrease as the temperature rises, a fact which will undoubtedly sur- 

 prise most physicists as much as it surprised me. Indeed, the dis- 

 covery of this fact put back the completion of this paper many months, 

 as I wished to make certain of it. There is now no doubt in my mind, 

 and I put the fact forth as proved. The only way in which an error 

 accounting for this decrease could have been made appears to me to 

 be in the determination of m in " Thermometry." The determination 

 of m rests upon the determination of a difference of only 0.°05 C. be- 

 tween the air thermometer and the mercurial, the 0° and 40° point8 

 coinciding, and also upon the comparison of the thermometers with 

 others whose value of m was known, as in the Appendix. Although 

 the quantity to be measured is small, yet there can be no doubt at least 

 that m is larger than zero ; and if so, the specific heat of water 

 certainly has a minimum at about 30°. 



One point that might be made against the fact is that the Kew 

 standard, Table L., gives less change than the others. But the cali- 

 bration of the Kew standard, although excellent, could hardly be 

 trusted to 0°.02 or 0°.03 O, as the graduation was only to i° F. In 

 drawing the curve for the difference between the Ivew standard and 



