CHEMICAL AND PHYSICAL NOTES. 111 
tilled water displaced when the instrument is floating at 49 mm. and 
98 mm. respectively, and we have only to perform the division as 
before. The two pairs of weights are 183-273 and 187°283, and 
183°7630 and 187°8830, which give the quotients 1:021879 and 
1:021877. Working according to this plan and using no weights 
lighter than 0:1 grm., we can easily get two parallel series of nine 
independent determinations each, of the weights of equal volumes of 
any two liquids and especially of distilled water and sea water 
having the same temperature. These depend on nothing but the 
weight of the hydrometer. The determination of a weight is one of 
the physical operations which can be performed with the greatest 
exactness. 
It may be here pointed out that the weight which causes the 
displacement of the hydrometer is its own weight plus the weight 
of the liquid meniscus which it carries on the stem at the line of 
flotation. It is impossible to weigh or measure this exactly, but we 
are only concerned with the question whether and in how far it can 
effect the exactness of our determination of the specific gravity of 
sea-water. It may be taken that the volume of the meniscus of 
distilled water does not differ by a measurable amount from the 
volume of the sea-water meniscus, therefore, the weights of the 
meniscuses will be in the proportion of their specific gravities. 
Imagine for one moment that the volume of the meniscus is 
loc. or equal to the volume of the whole of the stem of the 
hydrometer ; then, in our example, we should have for the 
displacing weight in distilled water, 182°783 + 1 = 183-783 grms., 
and for that in sea-water, 186°783 + 1:022 germ. = 187°805 
grms., and the quotient of these is 1:0218845. When no allowance 
was made for the weights of the meniscuses, the quotient was 
1°0218839; there is, therefore, a difference of six in the seventh 
decimal place. The volume of the meniscus is not more than about 
0°01 c.c., so that the error due to neglect of the weights of the 
meniscuses affects no higher place than the ninth. 
The specific gravity of a sea-water, as determined by the absolute- 
weight hydrometer, is a fraction of which both the numerator and 
the denominator consist, for the most part, of the weight of the 
glass instrument. It follows, that there may be a considerable 
error in the determination of this weight, without the exactness of 
the quotient or the specific gravity being sensibly impaired. For 
instance, let us imagine that an error of 1 grm. has been made in the 
determination of the weight of the instrument, so that it weighs 
181°783 grms, in place of 182:783 grms.; then, continuing the 
