Royal Society committee on thermometry

In 1776 the Royal Society of London appointed an illustrious seven-member committee to make definite recommendations about the fixed points of thermometers. The Royal Society Committee did take it for granted that the two water points should be used, but addressed the widespread doubts that existed about their true fixity, particularly regarding the boiling point. The Committee's published report started by noting that the existing thermometers, even those made by the "best artists," differed amongst themselves in their specifications of the boiling point. The differences easily amounted to 2-3 degrees Fahrenheit. Two causes of variation were clearly identified, and successfully dealt with (Cavendish et al. 1777, esp. 816-818, 853-855). First, the boiling temperature was by then widely known to vary with the atmospheric pressure, and the Committee specified a standard pressure of 29.8 English inches (roughly 757mm) of mercury, and also gave a formula for adjusting the boiling point according to pressure, in case it was not convenient to wait for the atmosphere to assume the standard pressure. The second major cause of variation was that the mercury in the stem of the thermometer was not necessarily at the same temperature as the mercury in the thermometer bulb. This was also dealt with in a straightforward manner, by means of a setup in which the entire mercury column was submerged in boiling water (or in steam coming off the boiling water). Thus the Royal Society Committee identified two major problems, and solved both of them satisfactorily.

However, the Committee's report also mentioned other, much less tractable questions. Cavendish himself had addressed the question of whether there was a temperature difference between "fast" and "slow" boiling (Cavendish [1766] 1921, 351). The notion that there are different temperatures associated with different "degree of boiling" can be traced back to Newton ([1701] 1935, 125), who recorded that water began to boil at 33° of his scale and boiled vehemently at 34° to 34.5° (on Fahrenheit's scale, this corresponded to a range of about 5-8°F). Similar observations were made by Jean-André de Luc, who was a key member of the Royal Society Committee and perhaps the leading European authority in thermometry in the late 18th century.

The Royal Society Committee investigated this issue carefully, which is not surprising given that its two leading members, Cavendish and De Luc, had been concerned by it previously. The Committee's findings were somewhat reassuring for the stability of the boiling point:

"For the most part there was very little difference whether the water boiled fast or very gently; and what difference there was, was not always the same way, as the thermometer sometimes stood higher when the water boiled fast, and sometimes lower. The difference, however, seldom amounted to more than 1/10th of a degree." (Cavendish et al. 1777, 819-820)

Still, some doubts remained. The trials were made in metallic pots and it seemed to matter whether the pots were heated only from the bottom, or from the sides as well:

"In some trials which we made with the short thermometers in the short pot, with near four inches of the side of the vessel exposed to the fire, they constantly stood lower when the water boiled fast than when slow, and the height was in general greater than when only the bottom of the pot was exposed to the fire." (ibid., 820)

Not only was that result in disagreement with the other trials made by the Committee, but it was the direct opposite of the observations by Adams and De Luc, according to which water boiling vigorously had a higher temperature than water boiling gently.

There were other factors to worry about as well. One was the depth of the boiling water: "if the ball be immersed deep in the water, it will be surrounded by water which will be compressed by more than the weight of the atmosphere, and on that account will be rather hotter than it ought to be." (ibid., 817-818) Experiments did vindicate this worry, revealing a variation of about 0.06° per inch in the depth of the water above the ball of the thermometer. However, the Committee was reluctant to advance that observation as a general rule. For one thing, though this effect clearly seemed to be caused by the changes of pressure, it was only half as large as the effect caused by changes in the atmospheric pressure. Even more baffling was the fact that "the boiling point was in some measure increased by having a great depth of water below the ball . . . [T]his last effect, however, did not always take place." (ibid., 821-822; emphasis added) Although the Committee made fairly definite recommendations on how to fix the boiling point in the end, its report also revealed a lingering sense of uncertainty:

"Yet there was a very sensible difference between the trials made on different days, even when reduced to the same height of the barometer, though the observations were always made either with rain or distilled water. . . . We do not at all know what this difference could be owing to. . . ." (ibid., 826-827)

The work of the Royal Society Committee on the boiling point is a lively testimony to the shakiness of the cutting-edge knowledge of the phenomenon of boiling in the late 18th century.

Interestingly, the Committee's lasting contribution was in fact in taking thermometry away from the boiling point altogether, as it recommended using the temperature of the boiled-off steam, not the boiling water (read more about the use of the steam point).

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