36 
Proceedings of the Royal Society 
A tall glass jar, capable of holding a litre, is rinsed out with the 
water the density of which is to be determined and which has 
the temperature of the laboratory, and then filled up to the 800 c.c. 
mark. A thermometer, the error of which is exactly known, is 
hung in the water, read after being immersed for 1 minute, 
removed and the hydrometer put in, sunk up to the top of the 
scale, 1 centimetre from the top of the stem, and allowed to rise. 
A weight, sufficient to sink the instrument until not more than 
one-third of the stem is submerged, is then put on, and after it 
has come to rest, the scale is read to half a millimetre. Another 
weight is added, so that not less than two-thirds of the stem are 
immersed ; after coming to rest the readings are taken, the weights 
removed, the hydrometer dried and replaced in its box, and the 
thermometer again hung in the liquid for a minute. The tempera- 
ture being noted, the thermometer is withdrawn, and the water 
poured back into the bottle. The whole operation occupies about 
ten minutes, and gives data for two distinct values of the density. 
The hydrometer is not read closer than to half a division, although 
with care it might be taken to one-third. 
A table giving the volume of the hydrometer at every tenth of 
a degree centigrade, from 0° to 25°, has been drawn up, and from 
this table the volume of the body at the observed temperature is 
taken; the volume of the stem immersed is got from a table 
which gives the value for each half-millimetre from 0 to 100. 
These added together give the immersed volume, and the weight 
being taken from another table and divided by this volume 
gives the density at the observed temperature. The mean of the 
two densities is taken, and reduced from the mean of the two 
corrected observed temperatures to 15° *5 6 C. by means of Dittmar’s 
table (Chall. Rep. Phys . Chem., vol. i. pt. i. p. 70). 
The Specific Gravity Observation Book contains a copy of the 
observations (see p. 34) on the upper half of each page, and below it 
the following particulars : — 
Date and hour of determination. 
Temperature of Water (t\) and its correction. 
1st Weight on hydrometer. 
Hydrometer Reading. 
‘Ind Weight on hydrometer. , „ 
