drop into it small portions of the mixture, which burn without deflagrating, when too little saltpetre is not used. The mass is dissolved in some drops, or in as small a quantity of water as possible, then lime-water added in excess, and heated to boiling, by which the arseniate of lime is more easily collected and washed. The precipitate is collected, mixed with fresh burned charcoal powder, and put into a glass tube of the following form;

The tube is first gently mixture may have ab-`

so that the mixture comes to lie at a. heated to drive off any moisture the sorbed, and then the under part of a is kept in the flame of the blowpipe until the glass begins to melt. The arsenic is now reduced and collected in the neck b, where it is spread over so small a surface that the smallest quantity may be detected. Onetenth of a grain of sulphuret of arsenic is sufficient to afford a satisfactory and decisive reduction test. Even the arseniate of lime, which is obtained from one-sixth part of a grain of sulphuret of arsenic, can, if carefully collected, serve for three different reduction tests *.

In using these delicate tests, we must be sure that our reagents contain no arsenic. All the sulphuric acid which is not obtained from volcanic sulphur, but either from the sulphur from iron-pyrites, or immediately from iron-pyrites, contains arsenic, and affords, when it dissolves zinc or iron, an arseniuretted hydrogen gas. If the same acid is used in the preparation of sulphuretted hydrogen gas, we have to dread a mixture of arseniuretted hydrogen gas with the sulphuretted hydrogen gas, by

* The following more simple mode of obtaining metallic arsenic from sulphuret of arsenic has been lately proposed by Berzelius. A very small portion of sulphuret of arsenic is introduced into a tube, like that on page 338, and brought up to a. Then a piece of steel piano-forte wire (No. 11.), an inch in length, is inserted into the tube, so far as the surface of the sulphuret. The steel-wire is next to be heated in a spirit of wine lamp, and the heat gradually raised in such a manner that the sulphuret, in the state of vapour, passes along the surface of the glowing iron. In this way, sulphuret of iron, and sublimed metallic arsenic, are obtained. The operation ought to be conducted slowly. Shavings of iron will not answer, because the arsenic combines with them, without any sublimation.

which the precipitation of an arseniated sulphur may take place, because the hydrogen of both in the test-fluids becomes oxidized by means of the air. The muriatic acid obtained by means of such a sulphuric acid contains also arsenic. We must therefore use, in such experiments, distilled sulphuric acid, but not until we have previously tested it by means of sulphuretted hydrogen for arsenic. The same applies to the muriatic acid used in such experiments. In all cases of this kind we cannot use too much caution.

The reduction-test is the only certain one, and it renders all others superfluous. If this does not succeed, the result is always unsatisfactory. Even the garlick arsenical smell so much depended on, is not to be trusted without actual reduction, as such a smell is sometimes evolved from the animal matter from the stomach. We may conclude by remarking, that no chemist or medical man can conscientiously and legally appear in a public court, as an evidence in such a case, without he has actually himself taken the contents from the stomach, or has had them sent to him, under proper seals, by a trust-worthy medical man, who declares on oath that he has taken the same from the stomach *.

On a Chemical Composition of Zinkenite and Jamesonite. By H. ROSE, Member of the Royal Academy of Berlin. And Description and Analysis of Pyrochlore, a new Mineral. By F. WÖHLER.

I. On the Chemical composition of Zinkenite and Jamesonite.

THE Zinkenite, in its chemical composition, approaches more nearly to Jamesonite and red silver, than to any other minerals. I found it to contain the following constituent parts: Sulphur

* From the great delicacy of the reduction-test, it is evident that extremely minute portions of arsenic may be detected. This being the case, the court is entitled to demand of the chemist or medical man, on examination, whether or not he can prove that the articles of food, used by the deceased, did not contain minute portions of arsenic. Bread, for instance, is sometimes adulterated with alum; alum is prepared, at times, from aluminous rocks, containing iron-pyrites; and iron-pyrites, as mentioned by Berzelius, contains - arsenic. This view might be farther illustrated.-ED.

+ In the third analysis, the quantity of sulphur could not be determined. JANUARY-MARCH 1827.

Z

22.58, lead 31.34, antimony 44.39, copper 0.42=99.23. The Jamesonite, like the zinkenite, consists principally of sulphate of antimony and sulphate of lead, but in different proportions. Three analyses of Jamesonite afforded the following results :

34.40 parts of antimony in the first analysis, combined with 12.87 parts of sulphur, to form the sulphuret of antimony, and 40.75 parts of lead, with 6.33 parts of sulphur, to form sulphuret of lead. The excess of sulphur, 2.95 parts, is nearly sufficient to form, with the iron, sulphuret of iron. Although the quantity of sulphuret of iron in Jamesonite is considerable, I still consider it as accidental, because neither iron nor lead, in the oxidated or sulphuretted state, combine together; the crystallised Fahlerz, for example, in which sulphuret of iron occurs, never contains sulphuret of lead, even when the tetrahedrons of Fahlerz are imbedded in lead-glance. The true composition of Jamesonite may be expressed by the formula 3Pb Sa + 4Sb Sa; the sulphuret of antimony in it containing double the quantity of sulphur, as the sulphuret of lead.

II. On Pyrochlore, a new mineral species.

Pyrochlore occurs in the neighbourhood of Friederichschwärn in Norway, in zircon-syenite, where it was first found by Dr Tank. Dr Wöhler, during his journey with Berzelius and Brongniart, met with this mineral near to Laurvig, in veins in zircon-syenite. Berzelius proposes to name it Pyrochlore, in order to distinguish it from Polymignite, which, before the blowpipe, retains its black colour, while the pyrochlore becomes yellow. Its colour is reddish-brown, like brown titanite, and on the fresh fracture appears almost black :-in thin splinters, is translucent; in thicker pieces opaque. It crystallises in regular octahedrons. It is generally imbedded in felspar, sometimes in Elaolite. Its specific gravity = 4.2064.216,-Rose. scratches fluor-spar, but is scratched by felspar. Its streak is

It

brown. The fracture is conchoidal, without any trace of cleavage. The surface of the crystal is shining and smooth, but the fracture surface splendent; and lustre between vitreous and resinous. Its constituent parts are as follows: Titanic acid 62.75, lime 12.85, oxide of uranium 5.18, oxide of cerium 6.80, oxide of manganese 2.75, oxide of iron 2.16, oxide of zinc 0.61, water 4.20, fluoric acid, quantity not determined, magnesia a trace, 97.30.

The Law of the Preservation of Species, illustrated by the Phenomena of the Seed of the Stipa pennata. By Mr JOHN MACVICAR, Lecturer on Natural History in St Andrew's. (With a Plate.) Communicated by the Author.

ALTHOUGH

ALTHOUGH it cannot be said that the primary object of nature, in reference to a species, is to prevent its destruction, yet its existence is an essential condition to that end, whatever it may be, and accordingly, nowhere do we observe a more admirable mechanism, than in those organs which are most eminently conservative or reproductive. The general law by which their developement and efficiency are regulated, may perhaps be thus stated, that, in proportion as the causes operating to destroy a species increase, so also do the organs or functions operating to preserve it.

Thus, as we descend the scale of animated beings, the successive species become more and more restricted in their faculties, their cunning, or swiftness, or force, by which they may meet their enemies, the number of which is also increased, or in those resources by which they may survive the violent action of the elements, which beat upon their more minute and simple structures, as rudely, and as boisterously, as upon the more perfect animals. Their liability to destruction, then, becomes greater as we descend. But to counterbalance this, we find that, in obedience to the law which has been stated, the very degradation of their structures becomes subservient to their existence. For, by a collateral diminution of sympathies, the life of the individual becomes more independent of partial in

juries, and a tenacity is imparted to it, which would even be ridiculous in the higher animals. Thus, it is very absurd to think of a man continuing to live after his head had been cut off; yet low in the scale, we find many species which, when decapitated, can serve themselves with new heads, as efficient as those of which they had been deprived, and scarcely differing from them, but in their paler complexion. Of this circumstance Mr Dalyel availed himself, in his very interesting investigation of the Planariæ. For when he wished to know how many eyes the Planaria nigra possessed, not being able to distinguish them on account of the black colour of the animal, he decapitated several, and was then able to count the eyes in the pale reproduced heads. As to legs, the amputation of one of which without surgical aid, would prove inevitably fatal to a man, there are many animals which seem to part with them without much inconvenience; while there are others (as the crabs), which, according to recent observations, seem to scorn the possession of a leg when injured, casting it triumphantly from them.

If we descend still farther among animals still more beset by enemies and accidents, we find species which really seem to be "immortal under the edge of the knife," which to cut in pieces, is only to give being to so many individuals as perfect as that which was attacked.

The action of the same admirable law is illustrated in the reproduction of the race. Thus in the most perfect animals, the species is divided into two groups, only one of which is capable of producing offspring. As we descend, this bisexual character is obliterated, and every individual, often without the presence of another, acquires this power. Still lower, not only do we find each animal provided with a specific apparatus for this purpose, but the same end accomplished in other ways also, as by gems and spontaneous division.

In the vegetable economy, which runs parallel to that of animals, we observe the same law to operate. Thus the oak, which cannot easily be destroyed, the individual life of which survives the sweep of many ages, can only be reared from an acorn; while the tender moss, which springs up among the turf beneath which its roots are spread, or the parasitic lichen on its trunk and branches, the lives of which are subject to a multitude of