690 POPULAR SCIENCE MONTHLY, 



graduated thermometer, the temperature of boiling water is found 

 to be invariable. Not only does the thermometer immersed in 

 water keep for any number of hours of boiling the height it had 

 reached when the first bubbles came up, but it ascends to the same 

 point every time it is placed in boiling water. If Amontons had 

 added the proviso that the pressure of the atmosphere should be 

 the same in all the experiments, which we know now is indispen- 

 sable, he would have been rigorously exact. 



When we take a bulb of air connected with a manometer, mark 

 carefully the pressure which it sustains when it is plunged into 

 boiling water, and then the pressure at which, under other circum- 

 stances, it reaches the same volume, the ratio of that pressure to the 

 former may be regarded as expressing the ratio between the tem- 

 perature to which the air was raised under the latter condition to the 

 fixed temperature of boiling water. This ratio will be the same, 

 whatever thermometer, constructed in the same way, we may use. 

 In this way we have a sure means of obtaining instruments that can 

 be compared with one another. 



Amontons proposed for a thermometer, as Drebbel did, a mass 

 of air maintained at a constant volume under a variable pressure. 

 The rule by which he attached a certain degree of temperature to 

 each degree of heat and cold, or a larger number for more intense 

 heat and a smaller for cold, is the same rule to which Desormes and 

 Clement on the one hand, and Laplace on the other, returned a cen- 

 tury afterward; and is the rule proposed in the works of Sadi 

 Carnot, Clausius, and Lord Kelvin as the measure of the absolute 

 temperature. 



The profound reasons which cause us to prefer the definition 

 of temperature proposed by Amontons to every other could not be 

 divined at the beginning of the eighteenth century. The large 

 size and inconvenient shape of Amontons's instrument, and the 

 necessity of taking account of the variations of atmospheric pressure 

 in interpreting its indications, prevented its general adoption; and 

 the Florence thermometer was still preferred. Spirit thermometers, 

 that could be compared with one another, were in demand. Keau- 

 mur furnished them. 



Keaumur observed, in 1730, that a thermometer placed in freez- 

 ing water went down to a certain degree, and remained fixed there 

 as long as the water was not wholly solidified. The temperature 

 of water in process of congelation was therefore always the same, and 

 fixed. As physics has advanced, some corrections have been made 

 in this law, and causes have been discovered that make the point 

 of congelation of water vary; and physicists have been induced, in 

 view of it, to take as their fixed temperature, instead of the freez- 



