If, however, the atmosphere had been composed of two gases, each having widely dissimilar specific gravities, there would have been a difference. No permanency of tone could then have been depended on—the pitch of every original note would have been continually varying as its transmission might be propagated by the vibrations of ever-varying mixtures of the two gases of dissimilar specific gravities. All that studied arrangement of defined notes, which constitutes the art of music, would have been lost to us for ever, had we been enveloped by such an atmosphere. In illustration of this circum

stance, I will perform two or three simple experiments. Striking this little bell you will recognise, and you will bear in mind its peculiar note. This is its natural musical sound-the sound evolved by striking it in atmospheric air. I now immerse the same bell in a suspended glass jar containing hydrogen, a gas which is

much lighter than the atmosphere, as you are aware. Remark how different is the musical pitch of the note now evolved.

Varying the experiment, I will now cause a tuning fork in the key of C to vibrate over a small glass jar, which, when made to resound, emits the same note, and

is therefore in union with the tuning fork. The glass jar vibrates, as you will perceive, to the same note C; but now if I fill this jar with hydrogen, and inverting it to prevent all escape of the gas, cause the tuning fork to vibrate opposite its mouth once more, the unison is destroyed, and the jar no longer sounds responsive to the note C.

Thus you will, I trust, have been impressed with some of the leading points or qualities of the substance nitrogen-its beautiful adaptation to the various necessities of the terrestrial universe. You will be able to contrast in your minds its qualities with those of the elements which have formed the subject of my previous lectures you will see that its most striking peculiarities consist in the formation of so many varied results of energetic power out of an element so apparently impassive and devoid of energy-in the indirect mode by which its combinations are effected-in their varied, often opposed, chemical qualities when formed-in the vast accession of power which they gain by combination -in the prevalence of nitrogen throughout all the higher organised forms-and in the indifference it displays to the current of passing electricity.

INTRODUCTION TO LECTURE V.

SULPHUR-SYNONYMES, HISTORY, NATURAL HISTORY, PREPARATION, AND QUALITIES.

SYNONYMES.-Brimstone, Sulphur.

HISTORY.-Sulphur has been known from periods of great antiquity, by all nations of whom we possess records, having been employed not only in the arts, but as a medicinal agent. Pliny (lib. xxxv. chap. 15) mentions its use in medicine; also that its fumes (sulphurous acid) were employed in the bleaching of wool.

NATURAL HISTORY.-Sulphur is very extensively distributed in both kingdoms of nature,-most largely as a mineral product, but widely and in small quantities as a constituent of animals and vegetables. Its commercial supply is furnished from volcanic countries, where it occurs crystallised. Occasionally iron pyrites (sulphuret of iron) are employed as a source of sulphur, and sulphur derivatives—namely, sulphurous acid, and indirectly sulphuric acid; but in pyrites, sulphur is

usually contaminated with arsenic. Native sulphur is brought for the most part from Sicily, where it occurs in beds of a blue clay formation. On the spot it is subjected to a rough purification by fusing, and being cast into oblong masses or blocks, it is then brought into commerce. Roll sulphur, which is another usual commercial form, is generally the produce of roasting copper or iron pyrites, and collecting the evolved fumes of sulphur in a long chamber of brick-work, and fusing the latter into sticks. Tablund sulphur, or flowers of sulphur, is the third commercial state which the element is made to assume, amd is produced by distilling sulphur and condensing the vapour. The appellation sulphur vivum designates native sulphur as first dug from its beds.

In the organic kingdom sulphur is found extensively, perhaps universally, diffused throughout animals, and entering in little minute proportions into the greater number of vegetables. Its detection in this minute veiled condition is rendered easy by means of the solution of oxide of lead in potash, as described in the substance of the ensuing lecture.

PROPERTIES.-Sulphur, being one of the substances which can assume not only two series of crystalline forms, each referable to a different system (dimorphism), but also two well-marked phases of amorphous condition (allotropism), must be described in reference to these characteristics.

If the crystallisation of sulphur have been effected at a temperature below 232° F., it belongs to the right square prismatic system of Weiss and Mohs; if at 232° F., the crystalline form assumed is the oblique prismatic system.

The allotropic amorphous variations of sulphur are commented on, and explained in the ensuing lecture.

The specific gravity of sulphur is from 1970 to 2.080. It becomes negatively electrified by heat and by friction, and is a non-conductor of electricity. Insoluble in water, soluble in ten times its weight of boiling oil of turpentine at 316°, forming a solution which remains clear at 800 (Ure), slightly soluble in alcohol, but more soluble in ether. The density of its vapour is about 6·6 (Dumas, Mitscherlich Ann. de Chem. et Phys., 1. 170, and lv. 5); hence 100 cubic inches of its vapour at mean temperature and pressure would weigh 206 grains.

PHOSPHORUS-ITS ETYMOLOGY, HISTORY, PREPARATION,

AND QUALITIES.

ETYMOLOGY.-pws, light; pɛpev, to bear; on account of its luminosity in the dark.

HISTORY.-Phosphorus is one of the numerous chemical substances discovered by the alchemists during their vain endeavours to turn the baser metals into gold. Brandt, a merchant of Hamburgh, is the person to whom the credit of this discovery belongs, in 1669. Almost