DETERMINATION OF THE SPECIFIC HEAT OF WATER. 429 
For the present we cannot do better than use thermometers made either of standard 
Jena or French glass, as their behaviours have been thoroughly studied. By making 
use of the published investigation, we may. then refer our measurements. to the 
thermometers of Berlin or Sévres. 
In calorimetric researches it is often irmaselila or inadvisable to dgaunine the 
zero after each measurement. The behaviour of standard glass is so regular that, 
after the thermometer has been in air for a few years, we may predict with consider- 
able accuracy the position of the zero from a knowledge of the temperature to which 
the instrument has been exposed. The following Table will illustrate this point. 








TABLE I], 
1. Dee war oS ee 5. @ 
-| | E | | 
Observed zero | | : 
Date ore Duet | Ae reduced to Caleulated Difference. 
‘ | : observed. 760 millims. Zero. Was 
1893. ° : oats | ssi 
Dee. 19 | 12°47 | 0167 0194. | 0198 |;  +:0001 
sp ee SLPS: 0194. | 0183 | 0193 - —-0010 
oy 2 | 13°28 | 0162, || ‘0185 | _ 0186 —-0001 
ny 14-14 0150 ‘0187 | ‘0178 +-:0009 
ry ES | -0080 | 0104 - | 0102 _ - +:0002 
pe. 1} 22°46 “0086 | ‘0123 ‘0101 | +:0022 
ny lS BPRS | ~ -0085 | 0115 | “0100 +0015 


The second column gives the temperature to which our standard thermometer had 
been exposed previous to the zero determination; the third column gives the 
observed zero, measured by an eye-piece micrometer, the fourth place of decimals 
having no significance; the fourth column gives the observed zero corrected for 
change due to the varying pressure on the bulb. According to (GUILLAUME, the zero 
of a thermometer after exposure to a temperature t, should be capable of expression 
in the form z — 000092 (t° — 15°) where z is the zero corresponding to the tempera- 
ture of 15°. The fifth column gives the zero reading calculated according to this 
formula, taking for z the value 0°0170; the sixth column shows the difference 
between the calculated and the observed reduced values. 
The agreement is satisfactory, especially for the lower temperatures. The experi- 
ment on December 12 was made after exposure of the thermometer to the tempe- 
rature of 22° during four hours, and the zero is seen to be exactly that calculated 
from the formula. In the two last observations the thermometer had only been kept 
in the water for about two and a half hours; and we conclude from the observations 
that the time was not quite sufficient to lower the zero through its full amount. 
A little consideration is necessary to decide how far the uncertainty of the zero 
