weighted by means of a leaden collar, and the graduation on the stem allows of its being adjusted with great accuracy.

In the case of solids soluble in water, some other liquid, like naphtha or oil of turpentine, the specific gravity of which is already known, may be used. The specific gravity of liquids may be determined, as already indicated, by the balance. A special form of specific gravity balance that admits of rapid de

termination and great accuracy at the same time is the Westphal balance, illustrated in Figs. 9 and 10. The graduated arm of the balance has suspended from it a glass plummet carrying a thermometer, the weight of which is so adjusted that when sunk in distilled water at 15° C. and loaded with the rider L,, the arm of the balance is exactly horizontal. The other adjustments of the weights necessary to establish equilibrium when the liquids used

are heavier or lighter than water are shown in Fig. 10. In practice specific gravity is more conveniently determined by the aid of what are termed hydrometers. These are glass tubes loaded at the lower end with mercury or shot, so that they will float upright when immersed in a liquid. On the stem is marked a scale of degrees or equal parts. It is obvious that if one of these hydrometers sink to a certain depth in water, it will sink

still deeper in a liquid lighter than water, or float higher in one

FIG. 10.

heavier than water. If

then the point to which

M

Readings of the Westphal balance.

1 3683

17427

15522

it sinks in water be marked 1 of the scale, the distance above this would be marked in decimal fractions less than 1, while the distance below would be marked in fractions greater than 1. In order to mark slight differences more accurately, a number of hydrometers are used in a set, of which several are weighted and graduated for liquids lighter than water and several for liquids heavier than water. Arbitrary scales are also in use for hydrometers, such as that of Beaumé for liquids heavier than water, that of Beaumé for liquids 0.8642 lighter than water, and those of Tralles, Twaddle, Gay Lussac, Beck, . etc. Special forms for particular liquids are

1.0460

also used, as alcoholometers, salimeters, saccharometers, lactometers, etc.

C. SPECIAL PROPERTIES OF GASES.

1. Attraction and Repulsion in Gases.

Expansion and Compressibility of Gases.-Gases have already been referred to as showing in the highest degree the tendency to expand because of the repulsion which exists between the molecules, causing them to separate more and more widely. Hence the volume of a gas is always dependent upon the tem

perature and pressure to which it is subjected at the time. This may be readily illustrated by placing a small rubber balloon partially distended with air or gas, but securely closed to prevent the escape of the gas, under the receiver of an air-pump (see p. 43). Upon exhausting the air from within the receiver, the balloon immediately distends and swells to several times its original bulk. This is, of course, due to the expansion of the gas under diminished pressure, as when the air is admitted again to the receiver of the pump the balloon contracts to its original dimensions.

The compressibility of gases under increased pressure is also capable of ready illustration by the aid of the pneumatic syringe. This consists of a tube of heavy glass closed at one end and provided with a tight-fitting solid piston. When this is pushed down the air may be compressed to a fraction of the original volume; but upon withdrawing this pressure the piston immediately rises and the air expands to its original volume.

An important fact to be noted is that there are limits to this contraction in volume under the influence of pressure. After a certain pressure has been reached (known as the critical pressure), the contraction of the gas causes a change of physical condition, and the gas becomes a liquid. Every gas has thus its critical pressure and temperature, the passing of which causes it to liquefy. This change of form will be referred to more fully under Heat.

Diffusion of Gases.-Gases possess the power of diffusing, whereby two gases of different densities rapidly become a homogeneous mixture, in the highest degree, as might be expected from the freedom.

FIG. II.

of motion possessed by their molecules.

Diffusion of gases.

Porous diaphragms of

parchment, rubber, or unglazed pottery all allow of this diffusion.

It may be illustrated by the apparatus shown in Fig. 11. In this case, the porous cell shown fitted to the upright glass tube is filled with air, while a bell-jar containing hydrogen is held for a moment depressed around it. Gaseous diffusion immediately takes place through the walls of the porous cup, and the result is a mixture of hydrogen and air, both within and without the porous dividing wall. But hydrogen, as the lighter gas, diffuses more rapidly than the air, and the result is an excess of the gaseous mixture within the cup, and, in consequence, a bubbling out at the bottom of the upright glass tube.

The investigation of these phenomena of diffusion by Graham has shown that they take place according to a very simple law. He found that the. rapidity of the diffusion of two gases of different densities was inversely as the square root of the densities of the gases. Thus, as hydrogen is about 14.5 times lighter than air, it will diffuse nearly four times faster than air.

Absorption of Gases by Liquids and Solids.-These two cases, while showing some points of resemblance, are generally based upon different physical operations. In the case of the absorption by liquids, we must suppose that, under the influence of a powerful attraction, the gas which is absorbed is first liquefied, and then taken up in a physical admixture. This attraction is dependent to a considerable degree upon temperature, but with that limitation it is fixed and uniform for the particular gas and the liquid which acts as its solvent. Thus, the nitrogen and oxygen of the atmosphere are soluble in water in different proportions from those in which they exist admixed in air.

Numerous examples of the solubility of gases in liquids will be found later in the discussion of the chemical elements and their compounds. In some cases the action is believed to be purely a physical one, as in the case of oxygen just mentioned; in other cases it is probably due, in large part, to the forming of a chemical combination, such as a hydrate, as in the case of the absorption of ammonia, chlorine, carbon dioxide, etc. The absorption of gases by solids, on the other hand, is considered to be due, in the main, to the attraction which causes the gases to deposit in a more or less thick layer of condensed gas upon the surface of the solid. Porous bodies which expose a large surface are, therefore, especially adapted for the illustration of this. Freshly prepared wood charcoal, for instance, at ordinary temperatures, will absorb ninety volumes of ammonia gas and smaller amounts of other gases. It is interesting to note that, in general,