Chemical and Physical Notes 7 1 



in particular cases, as for persons who are short-sighted, the 

 telescope is not absolutely indispensable. The bulb of the 

 thermometer is then warmed in any convenient way, as by 

 the heat of the hand through a fine cloth, to prevent soiling 

 the glass. The temperature of the thermometer should be 

 raised from fifteen to twenty degrees above that of the air. 

 It is then allowed to cool while hanging quite motionless in 

 the air. When its temperature has fallen a few degrees, time 

 is taken to the nearest second when the mercury passes a 

 given division. It is then observed as it falls, at regular 

 equal intervals of time. The length of these intervals of time 

 is regulated by the rapidity with which the thermometer cools, 

 and may conveniently be 5, 10, 20, 30, or even 60 seconds. 

 The observations should be continued until the temperature 

 of the thermometer has fallen to about two degrees above 

 that of the air. This should not have changed during the 

 operation. If there has been any sensible variation the 

 observations should be rejected, and the operation should 

 be repeated when there is no variation. 



The principle which should find interpretation in the 

 observations is that in equal intervals of time the bulb of the 

 thermometer loses equal fractions of the heat which it possessed 

 at the beginning of the interval. Here heat is understood to 

 mean excess of heat, or the heat corresponding to tempera- 

 tures above that of the medium in which the thermometer is 

 cooling. 



If time has been taken when the temperature of the 

 thermometer is exactly 10 above that of the air, and it is 

 found that in the 1st second it falls to 9 -9, then at the end of 

 the ist second the heat remaining is only -ffo of the initial 

 amount which we may represent by unity, and in the interval 

 it has lost y^ of the amount that it had at the beginning of 

 the interval. If the above principle holds good, it must lose 

 in the 2nd second T ^ of the amount which it has at the 

 beginning of that second, and at the end of the 2nd second 

 the heat remaining will be -$>$ of the amount which was 

 present at the beginning of that second. But the amount 

 present at the beginning of the 2nd second was $fc of the 



