in sand; and some select as their mansion an empty univalve shell. Such is the practice of the species here represented, (Fig. 43, Sipunculus Bernhardus), resem

Fig. 43.

bling in this respect the Hermit crabs. Its colour is white; the animal can extend itself to a length of three inches, can retract the entire proboscis at pleasure, and change at will the proportions of the body itself.

We should hardly expect that animals so lowly in their organization, so harmless to man in their habits, as the Echinodermata, would be made the objects of either superstitious fears or practices. Yet when Dr. Drummond, the talented author of First Steps to Botany, was drying some specimens of the common Star-fishes or Five

*

Fig. 44.

fingers, in a little garden at Bangor, (Co. Down,) he heard some children on the other

side of the hedge say, "What's the gentleman doing with the bad man's hands; is he ganging to eat the bad man's hands, do ye think?" It appears that the name they are known by there, is that of the Devil'sfingers, and the Devil's-hands, and that children have a superstitious dread of touching them.

There is another species, distinguished by the great regularity of its outline-the Butthorn, (Asterias aurantiaca, Fig. 44,) and pretty generally distributed round our coasts. Of this Mr. Bean of Scarborough communicates to Professor Forbes the following singular superstition. "Our fishermen call this species a Butthorn. The first taken is carefully made a prisoner, and placed on a seat at the stern of the boat. When they hook a but (holibut) they immediately give the poor star-fish its liberty, and commit it to its native element; but if their fishery is unsuccessful, it is left to perish, and may eventually enrich the cabinet of some industrious collector."

FAMILIAR LECTURES ON

CHEMISTRY.

LECTURE XI.-COMPOUNDS OF SULPHUR AND OXYGEN.

Sulphurous Acid, SO,.-This compound has already been mentioned as the product of the combustion of sulphur in the air or in oxygen gas. It is also formed by the action of certain metals and other deoxidizing agents on sulphuric acid, at a high temperature: thus, copper or mercury heated in contact with strong sulphuric acid, takes away part of its oxygen and converts it into sulphuric acid, which escapes in the form of gas. This is the most convenient mode of preparing the gas. If you put some cuttings of sheet copper, or waste picces of copper wire, into a Florence flask, mounted as in Fig. 18, p. 192, pour in a quantity of oil of vitriol, sufficient to cover the copper, and apply a gentle heat by means of a spirit lamp or a gas burner, sulphurous acid will be given off, and may be collected either over mercury, or by simple displacement, like chlorine. Water absorbs it rapidly.

Sulphurous acid, under ordinary circumstances, is a transparent and colourless

gas, having a very suffocating odour, the same as that of burning sulphur, (p. 61.) When inhaled, it excites coughing, and a very painful sense of oppression at the chest. Its specific gravity is 2 247, that is to say, it is twice as heavy as oxygen.* When passed through a tube surrounded with a mixture of ice and salt, which produces a temperature of 0° Fahr., it condenses into a transparent and colourless liquid. This liquid boils at 14°, that is to say, at 18° below the freezing point of water; hence, to preserve it at ordinary temperatures, it must be enclosed in sealed tubes. At a very low temperature, about 110° Fahr., it solidifies in white flakes.

Water, at ordinary temperatures, absorbs fifty times its volume of sulphurous acid gas. The solution may be prepared by means of the apparatus represented in Fig. 19, p. 194, the wash-bottle, as in the case there mentioned, containing oil of vitriol. If very large quantities are required, the process may be made cheaper by using charcoal, wood-shavings, straw, or other waste organic substances, to deoxidize the sulphuric acid. The gas thus obtained, is not pure, being mixed with a considerable quantity of carbonic acid; but for preparing the aqueous solution, the presence of this latter compound is immaterial.

Sulphurous acid gas, when perfectly dry, has no action on vegetable colours; but the moist gas and its aqueous solution redden litmus powerfully. Some vegetable and animal colours, however, are completely bleached by sulphurous acid; thus, roses or violets introduced into the gas, or held over burning sulphur, are quickly turned white. Hence this acid is much used as a bleaching agent, the articles to be bleached being hung up in a large chamber, on the floor of which sulphur is burned in an earthen pan. This is the method adopted for bleaching silk and woollen goods, which cannot be bleached by chlorine, as that sub* The number 2-247 is somewhat more than double the specific gravity of oxygen, which is 1.106. There are many reasons, however, for supposing that the specific gravities of gases bear very simple relations to each other; and, on the other hand, the exact experimental determination of their specific gravities is a matter of great difficulty. Hence,

when experiment shows a near approach to a sim

ple ratio in the specific gravities of two gases, that

simple ratio is assumed to be true, the deviation

from it in the observed results being attributed

to errors of experiment.

stance destroys their texture. Linen and cotton goods are bleached by chlorine, as described on page 196.

Sulphurous acid is a powerful deoxidizing agent; it precipitates gold and silver from their solutions in the metallic state, and reduces many metallic oxides to a lower degree of oxidation; thus the sesquioxide of iron, Fe 03, is reduced by it to the state of protoxide, Fe 0. It likewise abstracts oxygen from iodic acid, and separates the iodine.

The composition of sulphurous acid is determined by burning sulphur in pure oxygen gas. For this purpose a glass globe, (Fig. 26), filled with pure oxygen, is placed over mercury, and a small porcelain dish containing sulphur is introduced into it by means of a stout wire. The sulphur being

Fig. 26.

then set on fire by a burning glass, and the whole of the oxygen thereby converted into sulphurous acid, it is found, when the combustion is over and the apparatus has cooled down, that the volume of sulphurous acid produced is exactly the same as that of the original oxygen. Hence it appears that sulphurous acid contains its own volume of oxygen. But sulphurous acid is twice as heavy as oxygen, bulk for bulk; consequently, half its weight must be made up of the oxygen which it contains, and the other half must be that of the sulphur. We conclude, then, that sulphurous acid gas is composed of equal weights of sulphur and oxygen, or of 16 parts (1 eq.) of sulphur, and 2 x 8 parts (2 eq.) of oxygen; so that its formula is SO. Further, since sulphur vapour is also twice as heavy as Oxygen, it follows that sulphurous acid gas contains half its volume of that vapour; or, in other words, 2 volumes of oxygen and 1 volume of sulphur vapour make up 2 volumes of sulphurous acid gas.

The salts of sulphurous acid are called Sulphites, a term which you must be careful not to confound with Sulphide. If you pass sulphurous acid gas into a solution of

large leaden pans, to drive off the greater part of the water, and bring the acid to a greater degree of concentration. When, however, this process has reached a certain point, the acid begins to dissolve the lead, whereby the vessels are destroyed, and the acid itself contaminated with sulphate of lead. The concentration is therefore completed in vessels of platinum. In these, it may be carried on till the liquid attains a specific gravity of 1.845, and boils at 620°. This is the highest degree of concentration that can be attained; if the heat be further continued, the acid evaporates unchanged. Acid of this particular strength is called Oil of vitriol.

The preparation of sulphuric acid is a most important branch of chemical manufacture, and is conducted on a very extensive scale. The quantity of the acid consumed in chemical processes is enormous. So extensive and varied indeed are its applications, that the progress of a nation in the arts of civilized life might fairly be estimated by the quantity of sulphuric acid consumed in it. On account of this large consumption, every attention is paid to render the preparation as complete and economical as possible. The construction of the leaden chamber is a very important matter. The chief object to be attained is to keep the several gaseous substances, sulphurous acid, nitric acid, watery vapour, and air, in contact with each other as long as possible, and to mix them well together. For this purpose, the chamber is sometimes divided into compartments by means of leaden partitions, so arranged that the vapours shall enter at the upper part of the first, pass into the second at the bottom, thence into the third at the top, and so on. The arrangement is shown in Fig. 27, in which a is the water-boiler, d

Fig. 27.

b the furnace in which the sulphur is burned and the nitric acid evolved. The aqueous vapour, nitric acid, and sulphurous acid, enter the leaden chamber at c, pass through its several divisions in the direction hown by the arrows, and the nitrogen and

other uncondensed gases make their escape at d. Sometimes a series of chambers are used, connected alternately at top and bottom by pipes; sometimes, again, the chamber is made of very large dimensions, and without any divisions whatever.

Sulphuric acid may likewise be obtained by a totally different process, viz. by the action of heat on the salt commonly called green vitriol or copperas. This salt is a sulphate of iron, containing a large quantity of water of crystallization, (p. 179.) When moderately heated, it gives off the greater part of its water, and is reduced to a powder, which, at a higher temperature, is completely decomposed, giving off its sulphuric acid, together with the small quantity of water not expelled at the lower temperature. The decomposition is effected by heating the dried salt to redness. in earthen retorts, to which are adapted earthen receivers containing a little water: or common oil of vitriol, to facilitate the condensation of the acid vapours. The sulphuric acid and water then distil over together, and condense in the receivers in the form of a dark brown liquid. Part of the sulphuric acid, however, is resolved into sulphurous acid, which escapes as gas, and oxygen, which remains behind in combination with the iron. This is the process by which sulphuric acid was first obtained; it is now practised only at Bleyl in Bohemia, and at Nordhausen in Saxony. The acid which it yields is called Nordhausen sulphuric acid. This product is much more costly than common sulphuric acid, and is used for only one purpose in the arts-viz., for dissolving indigo. It fumes in the air at ordinary temperatures; and when distilled in a retort at a gentle heat, yields a colourless vapour, which if passed into a perfectly dry receiver surrounded with ice, solidifies in beautiful, white, feathery crystals, consisting of pure anhydrous sulphuric acid, (p. 178.) The residue in the retort is common oil of vitriol; in fact, the Nordhausen acid may be regarded as a solution of the anhydrous acid in that liquid.

Anhydrous sulphuric acid melts at 77°, and at a somewhat higher temperature is converted into a vapour, which is transparent and invisible when pure, but forms dense white fumes on coming in contact with moist air. This vapour, when passed