Chemical and Physical Notes 87 



n agree very closely, the extremes being 0-484 and 0*472, and 

 the mean 0-476. The factor used in the rule, page 83, is 

 0-475, which is a more convenient number than 0-476, and it 

 has now been shown how it is arrived at. 



The third part of the table gives the centesimal composi- 

 tion by volume of the bulbs of the four thermometers. The 

 thermometers are by different makers, and the quantity of 

 mercury in each shows how different they must be in pattern ; 

 yet there is great resemblance in their composition by volume. 

 The mean composition is 61 volumes of mercury to 39 volumes 

 of glass, and we shall never be far wrong if we take tlie volumes 

 of the mercury and glass in a thermometer bulb to be in the pro- 

 portion 3 : 2. 



Method of Determining the External Volume or Displace- 

 ment of the Bulb. We have shown how the factor 0-475 is 

 arrived at ; it now remains to show how the volume or 

 displacement of the bulb is determined. It can be roughly 

 ascertained by actual displacement of water in a graduated 

 vessel, but this method is not sufficiently delicate. When the 

 thermometer has a spherical bulb, its- diameter must be 

 measured with calipers. The result is not usually satisfactory, 

 because the length to be measured is very short, and it is 

 never certain that the bulb is truly spherical. Fortunately it 

 is usual nowadays to make thermometers with cylindrical 

 bulbs, and their volume is easily measured. Instead of using 

 calipers a fine thread is wound ten, twenty, thirty or more 

 times round the bulb, then unwound and its length measured ; 

 this divided by the number of turns gives the circumference 

 of the cylinder. The length of the cylinder is then measured, 

 and its ends are assumed to be hemispherical. With the 

 circumference thus determined, the diameter and sectional 

 area are calculated, or, more conveniently, taken from tables. 

 The length of the cylinder multiplied by the sectional area 

 gives the volume of the cylindrical portion. The two hemi- 

 spherical ends make up a sphere of the same diameter as that 

 of the cylinder, and its volume is likewise calculated, or taken 

 from tables. It is added to that of the cylinder, and the sum 

 is the volume, or displacement of the bulb. It is convenient 



