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VI. JS T otes on Thermometry. By F. D. Brown, B.Sc, 



Demonstrator of Chemistry at the University Museum, Oxford*. 



[Plate II.] 



SOME years ago, when I determined to try and find out 

 something about the attractive forces which the atoms 

 and molecules seem to possess, by studying the effects of heat 

 upon chemical substances and upon mixtures of such sub- 

 stances, I was led to the conviction that, if the work which I 

 proposed to do was to be of any permanent use, I should be 

 obliged to take many and minute precautions regarding the 

 measurement of temperatures — a measurement which, owing 

 to the peculiarities of mercurial and other thermometers, is so 

 liable to error. In order to learn how best to use my thermo- 

 meters, and how to refer their readings to a satisfactory 

 standard, I made a considerable number of experiments. At 

 the time when these experiments were made I imagined that 

 the subject of thermometry, although presenting many diffi- 

 culties to my mind, had been thoroughly worked out by 

 others, and therefore that a printed record of my observations 

 would be generally deemed to be of little utility. The recent 

 publication of a paper by Dr. E. J. Mills (Edin. Roy. Soc. 

 Trans. 1880), of one by Professors T. E. Thorpe and Riicker 

 (Phil. Mag. [5] xii. p. 1), and more especially of a report by 

 M. Pernet (Mdm. et Travaux du Bur. inter, des poids et mes. 

 i. 1881, pp. 1-52), has led me to change my opinion, and to 

 think that there still remain many points connected with 

 thermometers about which not only I, but others also, would 

 be glad to have more certain information. Acting upon this 

 belief, I have put together in the following pages some of the 

 results of my experiments. 



The Mercurial Thermometer as a Standard. 



I was soon convinced that any attempt to express tempera- 

 tures in degrees of an ideal absolute thermometer, or even to 

 refer them correctly to the readings of an air-thermometer, 

 would involve a most extensive and wearisome investigation, 

 which would postpone indefinitely the work I wished to do. 

 To avoid this substitution of the means for the end, I decided 

 to construct a mercurial thermometer and to use it as a stan- 

 dard, keeping it until such time as the progress of our know- 

 ledge should render its comparison with the air-thermometer 

 a matter of less difficulty. 



As a mercurial thermometer is very liable to be broken, I 

 first wanted to know whether this instrument fulfilled the 

 primary condition of a true standard, of being capable of 

 * Communicated by the Physical Society. 



