Chemical and Physical Notes 89 



The area of this surface is the area of the outlet for heat, 

 and, other things being equal, it determines the rate and term 

 of cooling. We found the total volume of the bulb to be 

 2'875 c.c., and its water value i - 366 grms. It is clear that, if 

 the area of the external surface remains constant and the 

 volume or the water value is increased, other things remaining 

 the same, the term of cooling will be increased and, of course, 

 the rate diminished. It is quite analogous to the case of a 

 water-cistern. If the area of outlet remains the same the 

 time which it will take for the cistern to empty itself will 

 depend on the quantity of water in it, while, if the size of 

 the cistern and the quantity of water in it remain the same, 

 the time which it will take to empty itself will depend on the 

 smattness of the outlet. Other things being equal, the term 

 of cooling of a thermometer will depend on the magnitude of 

 the ratio, volume of bulb : area of external surface. 



In this case the ratio is 12*547 : 2>8 75 =4'3^4- This may 

 be called the virtual cooling radius. In the case of a cooling 

 sphere it is the radius of the sphere. 



The physical and thermal data relating to particular 

 thermometers have been dwelt upon at great length because 

 they convey a much better idea of the nature and propaga- 

 tion of heat than more voluminous general statements. It 

 may be well here to remind the reader that in the department 

 of science which has been called geo-physics, when it is a 

 question of specific heat it is nearly always specific heat 

 per unit volume that is required, and the thermal study of 

 thermometers is helpful from this point of view. 



Calorimetric Constants of a Thermograph. It was interesting 

 to know what could be obtained with a recording thermometer 

 of ordinary type, and in Table XIV the results of some observa- 

 tions made at Cambridge with a Richard's recorder are given. 



The figures in this table are taken from the curves drawn 

 by the instrument on a drum revolving once in forty-eight 

 minutes. The instrument was allowed to take the tempera- 

 ture of the room, then exposed in the shade in the open air 

 when a fresh breeze was blowing, and allowed to remain there 

 until it had taken the temperature of the air. It was then 



