168 
DR. H. T. BARNES ON THE CAPACITY FOR HEAT OF WATER 
reason to believe is the true Clark-cell value, two mean crystal cells in series would 
be '28 mv. lower. Therefore, cells X 3 and X 10 in series are lower than the true 
value by ‘15 mv. During 1899, cell X 3 was on an average ‘15 mv. above the mean 
crystal cells. Neglecting X 5 but not X : this difference is reduced to ‘10. X 10 was 
also -f‘15 or, neglecting X 5 , ‘10. This gives for X, + X 10 the value +‘20 mv., 
or different from the true Clark-cell value by —‘08 mv. During the early part of 
1900, cell Xj commenced to go down more rapidly, partially through the introduction 
of cell X 3 again into the mean. If we take the comparisons on February 5th and 
March 24th, we find for X 2 the value +‘16 mv., and for X 10 +‘12 mv. Neglecting 
X 1 as well as X- from the mean, the value of X 2 + X 10 becomes -f '06 + '02, or 
equal to -j- '08 mv., the difference from the true Clark-cell value being now 
— '20 mv. Summarizing we have, if e represents the true E.M.F. of the Clark cell 
in volts 
In 1898 X 2 + X ]0 2 Xe—'00015 ] 
In 1899 ,, —’00008 Mean value —'00014. 
In 1900 „ „ -'00020 J 
This gives the mean error, if we assume from 1898 to 1900 the mean value 
2 X e —'00014 as representing the true E.M.F. of the two Clark cells used in these 
measurements, as in 
1898, + 4 X 10 -6 ; 1899, — 2 X 10“ 5 ; 1900, + 2 X 10“ 5 ; 
all of which are less than 1 part in 10,000, and outside the possible limits of error 
of the other measurements. 
Whenever the temperature of the Clark cell was other than 15°C.,the E.M.F. 
was calculated, assuming a value at 15°, by the temperature formula obtained 
between 0° and 30° for a mean temperature of 15° which has already been given. 
As a matter of verification of this formula, which was deduced from the old crystal 
cells, I have summarized in Table V. the observations that have been made since, 
both by myself and the students under my supervision. 
Table V. 
Type of cell. 
Change in millivolts between— 
15° and 0°. 
15° and 30°. 
Old B.O.T. crystal. 
1896 
+ 16-62 
- 19-48 
Sealed. 
1896 
+ 16-62 
-19-58 
Portable B.O.T. crystal. 
1897 
+ 16-60 
-19-40 
Inverted. 
1898 
+ 16-67 
-19-54 
Portable B.O.T. crystal. 
1899 
+ 16-60 
-19-45 
Temperature formula. 
— 
+ 16-60 
-19-40 
Values obtained by Dr. Kahle 
— 
+ 16-40 
- 19-40 
