INTRODUCTION.

EXPERIMENTAL SCIENCE may be conveniently divided into PHYSICS and CHEMISTRY.

PHYSICAL SCIENCE treats of the changes of matter, without any regard whatever to its internal constitution.

Thus, the laws of gravitation and cohesion, which belong to physical science, only concern matter irrespective of its composition.

CHEMISTRY, on the other hand, makes us acquainted with the composition of different forms of matter, and with the changes which they are capable of inducing in each other.

Water, considered with regard to its physical properties, is a colorless, mobile liquid, boiling at 212°, and freezing at 32°, almost incapable of compression, and so forth. Chemically speaking, water is described as a compound of so much hydrogen and oxygen, capable of entering into certain combinations, and of inducing certain changes in other forms of matter.

The science of chemistry is usually divided into two branches-inorganic and organic chemistry. As a convenient mode of classifying our knowledge this division is useful, but as a natural and absolute separation it has no existence; for the two classes of substances, inorganic and organic, so merge into each other-so many so-called organic substances are found capable of being prepared by inorganic methods, that the boundary-line is day by day becoming fainter, and will, perhaps, in time, vanish altogether. Probably one of the safest definitions that can be given of organic chemistry, as distinguished from inorganic, is contained in the statement, that the former branch of the science treats of those substances which are the products, either directly or indirectly, of the vital process in animals or vegetables; and such a definition will be tacitly admitted throughout this work.

SPECIFIC GRAVITY.

§ 1. The specific gravity of a substance is the term used to express the relation which exists between the weights of equal volumes of this substance, and of some standard body arbitrarily selected.

Pure water at the temperature of 60° F. (15°.5 C.) is the standard to which the specific gravities of solids and liquids are referred, whilst gases are compared with pure and dry atmospheric air.

Since we have here to compare the weights of equal volumes, and as alterations of volume always attend upon variations of temperature and barometric pressure, it is of course highly important that these conditions (the latter of the two, especially in the case of gases) be taken into consideration.

The determination of the speciße gravities of substanees is an operation of considerable importance to the practical chemist, and it will not, therefore, be out of place to describe, at the cutset, the methods of ascertaining the specifie gravities of solids, Squids, and gases.

DETERMINATION OF THE SPECIFIC GRAVITY OF A SOLID

MASS INSOLUBLE IN WATER.

§ 2. The mass is accurately weighed in the crdinary baline... It is then attached to a fine silken thread which may be ecvered with the sin of w to prevent variation in weight or a horsehair, and sospended to the book at the bottom of the specide gravity-pan of the balance; the latter is then brought into equiibrium by adding the requisite weights; the surface of the miss having been carefully wetted with a brush to remove all air-babbles in, which is better in some cases, having been heated in the water and allowed to encl below the surface, it is now completely immersed in pure water at 60°F 15°5 CO, and the balance again brengen inte equilibrium; the weight which it has been found requisite to remove fe this purpose is that of an amount of water equal in volume to the mass; mi the speelde, gravity may now be maliziated by the Stilowing simple proportion

of the solid to be operated upon) at 60° F. (15°.5 C.), and the weight again ascertained. The liquid should be poured upon the powder by small portions at a time, and well agitated, to remove air-bubbles (or, in some cases, it might be slightly heated, and allowed to cool before weighing). By subtracting the weight of the liquid which has been poured into the bottle, from the total weight of the liquid which the latter is known to be capable of containing, we obtain the weight of a volume of liquid equal to that of the powder employed, and from this datum the specific gravity is calculated as usual.

DETERMINATION OF THE SPECIFIC GRAVITY OF A LIQUID.

5. The instrument employed for determining the specific gravity of a liquid. is a small light bottle (the weight of which is known), capable of containing a known weight of water at 60° F. (15°.5 C.) when filled to a certain level previously ascertained, and carefully marked. The bottle sold by the instrumentmakers for this purpose, is generally provided with a perforated stopper through which the excess of liquid escapes. The operation by which the specific gravity is obtained is exceedingly simple. The bottle is filled to the required level with the liquid to be examined, at the temperature of 60° F. (15°.5 C.), and weighed. (When the temperature of the liquid is below this point, it is generally raised by clasping the bottle in the hand, or immersing it in warm water; if the temperature be higher than 60°, the bottle may be surrounded with a strip of wet blotting-paper, and cooled in a current of air.) To obtain the specific gravity, it is merely necessary to divide the weight of the liquid by the known weight of water which the bottle is capable of containing. (It is scarcely necessary to remark, that the presence of bubbles of air upon the sides of the bottle must be carefully avoided, and that the exterior of the latter must be well wiped before weighing.)

Fig. 1.

Fig. 2.

In order to obtain quickly an approximation to the specific gravity of a liquid, an instrument known as the hydrometer is frequently employed; it consists of a glass tube of small diameter, forming the stem, to the lower extremity of which two bulbs are attached. The upper of these bulbs is simply filled with air, whilst the lower contains some heavy substance (mercury or shot), the weight of which is regulated according to the purpose for which the bydrometer is intended, being, of course, heavier when the hydrometer is to be employed for liquids of high density. When the hydrometer is placed in any liquid, it will assume an upright position, with more or less of its stem projecting above the surface; it is obvious that, following the ordinary law with regard to floating bodies, the height of the stem above the surface will depend upon the specific gravity of the liquid, and it is only necessary that the stem be graduated, in order that we may at once read off the specific gravity. The zero of the scale is usually obtained by floating the hydrometer in pure distilled water at 60° F. (15°.5 C.), and marking upon its stem (or upon a scale attached to it) the level to which it sinks; the same experiment being repeated with another liquid of known specific gravity, a second starting-point is obtained, and the space between these two points is divided into an arbitrary number of degrees, to which those on all parts of the scale are of course equal. Since the stem of the hydrometer of extensive range would be inconveniently long, separate instruments are adapted to different portions of the scale. In the graduation commonly used in this country, the degrees correspond to grains, and