On the Steam Calorimeter, 



225 



On comparing the result deduced as the mean of these eight obser- 

 vations with the value tabulated opposite the pressure of 765 mm. in 

 Table II, it is seen that the experimental value is practically identical 

 with that derived from Zeuner's formula. From the experiments 

 0-000609, from the formula 0'000610. I have thought, then, Zeuner's 

 results probably the safest to adhere to of the many estimations that 

 have been advanced for the density of steam at atmospheric pressures. 



The method of using Table II is obvious. The volume of the 

 substance estimated in cubic centimetres is multiplied by the suitable 

 value taken from the table. This is the displacement in steam. The 

 displacement in air must also be estimated for the prevailing tem- 

 perature, t Y , and pressure, by reference to a table of air densities, as 

 the difference of the two is, of course, that which affects the observa- 

 tion of the weight of the substance transferred to an atmosphere of 

 steam. 



If it be desired to secure the observations from error as far as 

 possible at all points, then two further corrections on the value of w 

 are necessary: — (1.) An allowance for the change of volume of the 

 substance due to thermal expansion in passing from air temperature 

 to steam temperature. This may be considerable in the case of metal 

 vessels or large masses of metal. This correction is additive to the 

 value of w. (2.) A correction for the displacement in steam of the 

 precipitated water, i.e., the reduction to vacuo of the weight of water 

 w. This is also an additive correction. 



Both these corrections are included in the following equation for 

 the true weight of condensation, w, 



w - — r+y — ' w 



volume of the substance at £ T , 



density of air at t Y and prevailing pressure, 



,, steam at prevailing pressure, 

 the weight added during experiment. 



This formula departs from strict accuracy only in so far as it 

 assumes unit mass of water to occupy unit volume at the temperature 

 of the steam. 



Approximate Correction for Displacement. — In a great many cases, 

 the vast majority of cases, indeed, it will be sufficient to substitute 

 for the foregoing a far simpler correction based on the density of 

 steam relative to air. Assuming a mean pressure of 760 mm. : — 



s 2 



where V 1 = 

 V 2 = 

 D = 

 b = 



w i = 



