Mr. F. D. Brown's Notes on Thermometry. 69 



the error of observation, and shows no tendency to increase 

 when X increases. It may be noted that with both the above 

 thermometers the mean value of m is greater than '0001545, 

 the value usually assigned to it, but that it differs from that 

 number by so little that the error committed by substituting 

 the one for the other in the calculation of the correction C will 

 rarely amount to more than o, 02 C. 



The above experiments were made at 100°, because this is 

 the only temperature which can be maintained absolutely 

 constant for an hour without the use of a quantity of compli- 

 cated apparatus ; and it is evident that the slightest variation 

 in the temperature would entirely spoil the series of observa- 

 tions. At higher temperatures the sources of error which 

 beset the readings of thermometers increase so rapidly that 

 the exact value of the coefficient m becomes of less and less 

 importance as the temperature rises, notwithstanding the 

 fact that the correction C iucreases in amount. Since there 

 is no reason whatever to suppose that any different results 

 would be obtained at such higher temperatures, I thought 

 it unnecessary to make any further experiments, more espe- 

 cially as those given above yielded precisely those numbers 

 which the ordinary laws of expansion predicted. 



There is another point connected with thermometry, to 

 which I devoted attention some years ago. It has been 

 suggested that when a thermometer is placed in a vapour at 

 maximum tension, as in the ordinary chemical process of dis- 

 tillation, it does not truly indicate the temperature of the 

 vapour. This suggestion owes its origin to the fact that drops 

 are seen to accumulate and drop off the end of the thermometer. 

 It has been supposed that this condensation of the vapour on a 

 surface which should be as hot itself, is due to the molecular 

 attraction of the glass for the vapour. If this be the case, the 

 heat evolved by the vapour during liquefaction on the thermo- 

 meter-bulb would raise the temperature of the latter. The 

 thermometer would thus indicate a higher temperature than 

 that of the mass of the vapour. The experiments which I 

 made upon this subject, like those instituted by others, were 

 inconclusive. I possess, however, an apparatus which seems 

 to me eminently suited to answer the question satisfactorily. 

 It is at present being employed for other purposes : but 

 I trust that, when it is at liberty, I shall be able to put it to 

 this not unimportant use. 



