248 Determination of the Volume of Mercury in a Thermometer. 



when the object is immersed in a medium which differs 

 greatly from it in refractivity. Phosphorus, arsenic sul- 

 phide, and mercury-potassium iodide have been used for this 

 purpose, but they are all open to grave objections. None of 

 them are permanent in the air ; some are dangerously in- 

 flammable ; while most of them act on delicate organic struc- 

 tures, although available for such things as siliceous Dia- 

 tomacese. 



The most hopeful direction in which to look is undoubtedly 

 towards some of those complex organic compounds which 

 are now being built up by many workers in England and 

 Germany. 



XXXIII. Note on the Determination of the Volume of Mercury 

 in a Thermometer. By A. W. Clayden, M.A., F.G.S., 

 Bath College*. 



IN Professor Clark's paper on the Seat-capacity of a 

 Thermometer!, he shows that it may be calculated ap- 

 proximately if the specific gravity of the instrument, and that 

 of the glass of which it is made, be known. The first step in 

 his method is to determine the volume of the contained mer- 

 cury, which he points out may be ascertained by the following 

 formula, 



v _ V(S-S 2 ) > m 



y i- g^s, » w 



in which Y 1 = the volume of mercury, 

 Si = its specific gravity, 

 V = the volume of the instrument, 

 S = its specific gravity, 

 S 2 = the specific gravity of the glass. 



Now this expression will only be correct if the bulb and tube 

 of the thermometer be completely filled with mercury, as they 

 will be at a temperature (t°) slightly above the highest read- 

 ing of the scale. At all lower temperatures Y will be too 

 large and S too small ; an error being introduced, as Professor 

 Clark points out, by the empty space of unknown capacity 

 which occupies the unfilled portion of the tube. 



At this higher temperature (t), Y 1 becomes Vx(l+/3^), (3 

 being the absolute coefficient of expansion of mercury. Simi- 

 larly V becomes V(l+a£), a being the coefficient of cubical 

 expansion of the glass. 



* Communicated by the Physical Society : read January 23, 1886. 

 t Proc. Physical Soc. vol. vii. p. 113. [Phil. Mag. vol/xx. p. 48.] 



