236 LECTURE XXVI. 



For the speedy examination of a variety of fluids, differing but little in 

 specific gravity from some known standard, an hydrometer may be very" 

 conveniently employed. This instrument is said to have been invented by 

 Archimedes : it consists of a hollow ball, with a weight below it, and a 

 slender stem above, so graduated as to express the specific gravity of the 

 fluid by the degree to which it sinks. Sometimes the instrument is sunk 

 to a certain mark, by means of weights placed in a dish at the end of the 

 stem ; or different weights are fixed to it below, while the graduations of 

 the scale are still observed ; and it may even be applied to finding the 

 specific gravities of solids, the solid being first placed in the dish at the end 

 of the stem, and then in a second dish which is suspended from the bulb 

 below the water. (Plate XXI. Fig. 278.) 



Another mode of ascertaining the specific gravities of fluids differing but 

 little from each other in density, is to have a series of globules of glass, so 

 loaded as to correspond to the specific gravities indicated by as many 

 numbers, which are marked on them ; and throwing several of them 

 together into the fluid, to observe which of them remains nearly stationary 

 without either rising to the surface or sinking. This method, though not 

 expeditious, appears to be very secure from error : the globules are sold by 

 patent, adapted for the measurement of the strength of spirituous liquors. 



In whatever manner we compare the specific gravities of bodies with 

 that of water, it is necessary, for very accurate experiments, either that the 

 water be employed at the temperature of the air when moderately warm, 

 or that a proper correction should be made for its change of bulk at dif- 

 ferent temperatures. Platina, the densest known substance, is 23 times as 

 heavy as distilled water, gold 19J, mercury 13|, lead 11, silver 11, copper 

 9, iron and steel 71 , stony substances usually about 2, rectified spirits , 

 naphtha, the lightest liquid, ^ cork about , common air -g-j-^, steam s , 

 and pure hydrogen gas -r^^nr* From this comparison the weight of a cubic 

 foot of any of these substances may be easily determined ; since a cubic 

 foot of water weighs nearly 1000 ounces avoirdupois, or more nearly 998 ; 

 thus a cubic foot of gold would weigh about 195,000 ounces, and be worth 

 above 60,000 pounds sterling ; a cubic foot of iron weighs 7750 ounces, and 

 a cubic foot of common stone about 2500. 



The method of measuring the bulk of solid bodies by immersing them in 

 a fluid, was applied, by its inventor Archimedes, to the detection of a fraud 

 in the composition of a mixed metal :* and at present the principal use of 

 hydrometers is for ascertaining, by the specific gravity of a compound of 

 alcohol and water, the proportional quantities of its ingredients. But in all 

 experiments of this kind, it is necessary to be aware, that a considerable 

 change of the joint bulk of two substances is often produced by their mix- 

 ture : and that in general their dimensions are considerably contracted. 

 Thus, 18 gallons of water, and 18 of alcohol, instead of 36 gallons, make 

 only 35, consequently the specific gravity of the compound is one 35th 

 greater than the mean of the specific gravities of the ingredients. And in 

 some cases the whole dimensions of a single substance may even be co'u- 



* Vitruvius, Architect. 1. ix, c. 13. 



