Chemical and Physical Notes 73 



the temperature of the thermometer above the temperature 

 of the air is /, and in any interval of time dB it falls to 

 (t dt), then after any number n of such intervals (dB) the 

 logarithm of the fraction of excess temperature remaining is 



n log f j ; and by multiplying the number corresponding 



to this logarithm by /, the excess of temperature of the 

 thermometer after n intervals of time, each equal to dB, is 

 given. 



The law which has just been explained is Newton's law 

 of cooling, often called the logarithmic law. It is worthy of 

 remark that Newton looked upon this law as axiomatic and 

 self-evident the moment it is stated, and he did not think that 

 it required experimental demonstration. It did not escape 

 question, notably by Amontons ; and Lambert, in vindication 

 of Newton, although he had held that the law was self-evident, 

 carried out a beautiful series of experiments, which are 

 detailed at length in the third part of his classical work on 

 Pyrometry 1 and the measurement of heat. They completely 

 bore out Newton's law. 



The method of making the experiment has. just been 

 described, and as it is important that every one should be 

 familiar with the practice of it, we give an actual example. 

 (See Table IX.) 



The experiment here recorded was made under very 

 favourable circumstances in the month of September in 

 a large room the temperature of which was sensibly the 

 same as that of the air outside, namely 2O'2 C, and this 

 remained quite constant for a much longer time than was 

 required for the experiment ; indeed, it hardly varied at all 

 during the day. The results are instructive, because they 

 give a good idea of the kind of agreement between observa- 

 tion and theory which we have a right to expect. The 

 temperature was observed at every ten seconds. The initial 

 excess of the temperature of the thermometer over that of the 

 air is 8x>, and log 8-0 = 09031. After the first interval of 



1 Pyrometrie oder vom Maasse des Fetters und der IVarme, von Joliann 

 Heinrich Lambert. 410. Berlin, 1779. 



