MECHANICS. 43 
between the divisions is greater in proportion to the thickness of the tube: 
to: avoid making them of inconvenient length, they are not made to be of 
universal application, but for particular liquids, or for liquids that are lighter 
or heavier than water. The zero of volumeters intended for liquids lighter 
than water is placed at the lower end of the scale, that for those heavier 
than water at the:upper part; and the filling of the ball, c, is to be adjusted 
so that the tube @ may sink to the proper point. The scale, which for every 
good. instrument’ must be made especially, is generally on a slip of paper 
placed inside of the tube, which is then hermetically sealed above it. There 
are other areometers, which, more conveniently, give the specific gravity 
directly :' in these: the scale is not equally graduated, but the divisions 
increase from below upwards. For practical purposes, such areometers are 
much used for particular liquids, as alcohol, solutions of salt, milk, &c., 
giving the proportions in which they are mixed with other substances. 
They receive particular names, according to the fluid for which they 
are destined: Alcoholmeter, Saccharometer, Lactometer, Hydrometer, 
Salometer, &c. — 
c. Attraction between Solids and Liquids. 
If the extremity of a fine tube be immersed in a liquid, the level of the 
latter will be higher or lower inside the tube than outside of it, according 
as the tube is moistened by the liquid or not; thus, in a glass tube immersed in 
water, it will be higher ( pl. 18, fig. 17), and immersed in mercury it will be 
lower (fig. 18). The-force which causes these phenomena of elevation or 
depression is called capillarity, or capillary attraction, and comes into play 
whenever solids and fluids are brought into contact. In such cases, the heights 
of elevation or depression of the liquid are inversely as the diameters of the 
tubes ; the finer these are, therefore, the higher is the rise or fall of the 
liquid. For the empirical determination of this law, a very accurate direct 
measurement of the place of the liquid in the tube becomes necessary ; and 
for this, the apparatus invented by Gay Lussac answers very well. In this, 
apparatus (fig. 19), the height of the liquid in the tube can be ascertained by 
means of a small telescope, g, moved up and down a graduated post, and capa- 
ble of being fixed at any elevation. Having fixed the post of the telescope in 
a vertical position by means of the adjusting screws and the plummet f, the 
height of the liquid in the tube is to be noted, the tube then moved aside, 
and the plate h, through which passes with some friction a finely-pointed 
rod, k, laid upon the vessel a. The point of this rod is to be brought in 
exact contact with the surface of the liquid, and the height read off by 
means of the telescope. The difference of these heights will be the height 
of the column of liquid in the interior of the tube. 
It must not be forgotten that whenever a liquid rises or falls in a narrow 
tube, the summit of the column is not perfectly flat, but concave in the first 
case, as in fig. 20, and convex in the second (/fig.21), the radius of 
217 
