140 THE THERMOMETER. 



by the atmospheric pressure, or to select some determinate pressure of the at- 

 mosphere, or height of the barometer, at which the fixed temperature must be 

 taken. An alloy of two parts of lead, three of tin, and five of bismuth, was 

 found by Newton to be fused at a fixed temperature nearly equal to that of boil- 

 ing water. As this fusion is not affected by the atmospheric pressure, it might 

 be taken as the means of determining the boiling point on a thermometer ; but 

 it is more convenient to note the temperature of boiling water, and at the same 

 time to observe the height of the barometer. If it be agreed that the boiling 

 point be taken when the barometer stands at a given altitude, as at 30 inches, 

 then, by knowing the law at which the temperature of boiling water varies, 

 with reference to the variation in the pressure of the atmosphere, it will be 

 easy to reduce the boiling temperature under any pressure to that with the 

 pressure agreed upon. The pressure recommended in r the directions published 

 by the Royal Society for the construction of thermometers, is that of the atmo- 

 sphere when the barometer stands at 29-8 inches. 



The temperature at which water boils is varied, in some degree, according 

 to the material of the vessels which contain it, and also according to solid sub- 

 stances which may be mixed with it, though they may not be held in solution. 

 If distilled water be boiled in a vessel of glass, the process will be observed to 

 go on irregularly, and with apparent difficulty. When the fire is removed, and 

 the temperature lowered, it may be restored to the state of ebullition by throw- 

 ing into it some iron filings. Nevertheless, though it thus boils, its tempera- 

 ture is lower than that which it had when boiled in the glass before the iron 

 filings were introduced. In determining the boiling point on the thermometric 

 scale, the water should, therefore, be free from any solid admixture, and should 

 be boiled in a metallic vessel. 



In observing these fixed points of temperature, the thermometer, when im- 

 mersed in melting ice, should be completely submerged, not only as to the 

 bulb, but as to the tube, in order that every part of the mercury should take the 

 same temperature. If the bulb alone were immersed, the mercury in the bulb 

 would have the temperature of the melting ice, while the mercury in the 

 tube would have the temperature of the surrounding air ; consequently, the 

 column would stand at a greater altitude than that which it would have were 

 it all at the same temperature. It is possible, by calculation, to allow for this 

 difference ; but it is more effectual, and more conducive to accuracy, to im- 

 merse the whole thermometer in the fluid. 



The accurate determination of the boiling point requires still further precau- 

 tions. 



When water contained in the vessel boils, the strata at different depths have 

 different temperatures ; and if the instrument be immersed vertically, the mer- 

 cury in the bulb will have a higher temperature than the mercury in the tube. 

 It is necessary, therefore, if the thermometer be immersed in the fluid, that it 

 should be placed in a horizontal position, and not immersed to a greater depth 

 than is necessary to cover the bulb and tube. This position, however, is one 

 which renders it extremely difficult to observe with accuracy the height of the 

 column. The fact, which will be proved hereafter, that steam raised from wa- 

 ter has the same temperature with the water from which it proceeds, furnishes 

 an easy means of fixing the boiling point. Let the thermometer tube be in- 

 serted in the neck of a vessel, so that the bulb shall reach nearly to the surface 

 of the water, and let another orifice be provided through which the steam may 

 escape into the atmosphere. This done, let the water be boiled until the 

 space in the vessel above its surface is completely filled with steam, as will be 

 shown by the rapid escape of the steam from the orifice provided for that pur- 

 pose. The thermometer, including the tube and bulb, is now surrounded by 



