the thermometer the highest temperature which would be attained, since during the operation some heat is lost by radiation. The true maximum temperature can be calcu- lated by many methods which are given in works on thermo-chemistry (especially in BertheloVs ' Calorim6trie pratique ').
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Regnault-Pfaundleb's Method of Correction
Let us suppose that in a calorimetric operation we have observed the temperature after intervals of 20 seconds during the three following periods :
1. Before the reaction. The variations of the surface of the mercury are regular, and sensibly proportional to the interval of time between two observations.
2. During the reaction. The mercury rises rapidly at first, then more slowly until a certain maximum is reached, after which it descends slowly at first, then :
8. The downward progress of the mercury becomes regular, and the variation of the position of the surface again becomes proportional to the time. This indicates the end of the reaction.
Let us suppose that during the first period 10 readings have been taken after intervals of twenty seconds. Let us denote these readings by the letter t with the interval affixed. By direct observation, we shall know t , t lf t 2y t z up to £ 9 . At the moment t l0 the reacting sub- stances are mixed, so that we are not able to read this is equal to t 9 increased by the mean of the changes of temperature observed during the preceding nine intervals. From this we get the equation
- io — * 9 t — q —
In the second period (during the reaction), the readings l n , hi U P to £ so are taken. Here we must assume
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