tioned above, the same in both terms, are omitted, as also the constant linear degree of the common scale.

Let the temperature be reckoned on AB, as on the common scale of an airthermometer commencing at A or 448° F and let CF be a line of such a nature, that every ordinate as BC EF, &c. may be proportional to the specific heat of air under a constant

E

B

F

G

volume, at the respective temperatures B, E, &c. So that the intercepted areas will denote the corresponding variations in the quantity of heat under a constant volume. But if the specific heat of air under a constant pressure exceed that under a constant volume, in the constant ratio of K to 1, and if these ordinates be every where increased in that ratio, another line GD, passing through their extremities, must be of the same nature with CF, and the intercepted areas to the former as K to 1.

1

Again, let the specific heat of a mass of air under a constant pressure be BD x 1°; and let its temperature be raised from B to E under the same pressure; then the area BDGE will denote the increase of heat, and EG x 1 the specific heat under a constant pressure at the temperature E. Now EG: EF :: K: 1, wherefore EF x 1° is the specific heat of the dilated mass at the temperature E, under a constant volume. But EF x 1° would still have been the specific heat, had the air under its original volume been raised to the temperature E; and because EF :[ EG :: 1: K, its specific heat at the temperature E under a constant pressure would have been EG x 1°, as before. Hence, the constant ratio of the specific heats renders them independent p

of the actual density or pressure, and, therefore d ę

and

d p

are constant quantities. It thus appears, that the above expressions for the specific heats answering to a degree on the common scale, vary inversely as 1+ at; or, that any ordinate BD, or BC is inversely as AB, which is the well known property of the hyperbola; and, therefore, CF and DG are both hyperbolas, having A for their centre, and AE for an asymptote. We have, then, without going through the process of integrat

+

ing a partial differential equation, arrived at the same construétion as was used on page 337, vol. i., and which represents the relation between the common and true scales of temperature, viz. that when the variations on the latter are uniform, those on the former follow a geometrical progression.

On the Detection of Arsenic in cases of Poisoning. By J. L. BERZELIUS.

7

In cases of poisoning with arsenic, the individual may have

N

taken the deadly poison, either in the pulverulent form, or in a state of solution. In the first case, we can almost always detect visible particles of arsenic in the contents of the stomach, or on the inner coat of the stomach, where they are distinguished by dark red spots, on which they are to be looked for. The nature of these particles, although much under the one-tenth of a grain in weight, may be ascertained with great care and perfect certainty by the process or test of reduction. The following method I employ in the use of this test:-A glass tube, from one-tenth to one-seventh of an inch in diameter, is drawn out, at one extremity, into a fine point, from two to three inches in length, which ought not to be wider internally than the thickness of a coarse knitting needle, and is then hermetically closed at the extremity.

The particle of arsenic (even the one-hundredth part of a grain in weight is more than is necessary), is moved upwards to a, and covered with charcoal powder, which has been previously exposed to the flame of the blowpipe, to drive off any moisture it might contain, to b. The tube is then brought, in a horizontal position, into the flame of a spirit of wine lamp; and in such a way, that a, where the grain of arsenic lies, remains beyond the flame.. 'As soon as the charcoal at b is heated to redness, a is brought into the flame, by which the arsenious acid is converted into gas; and, during its passage through the glowing charcoal, is reduced. The metallic arsenic is condensed in the small tube, at the line

where it is beyond the flame, in the shape of a shining, dark metallic ring, which, by gentle heating, can be driven farther forward; and thus more is accumulated, by which it acquires a higher lustre. The small diameter of the tube prevents all circulation of air, so that no part of the metal is reduced. It only: remains to determine the arsenic by its smell. This is effected, if we cut the tube between the charcoal and the metal, then heat it gently in the place where the metal rests, while we hold our nose over it but at a little distance.

The second case occurs, when no visible grains of arsenic are present, as in those instances where death has been caused either by solution of arsenic, or by finely pounded arsenious acid. When the poisoning has been caused by the solution of arsenic, it is often impossible to detect the arsenic, because the solution has been carried off before death. If, however, some portion of it still remains, it is discovered by heating the contents of the stomach, at a boiling heat, with caustic potash, and then with muriatic acid. The filtered fluid is reduced, by evaporation, to a smaller volume; and, if necessary, again filtered, and then a stream of sulphuretted hydrogen passed through it. The fluid is now heated, to cause the precipitate to collect, or evaporated, if it does not subside until it does, and then filtered *. If the quantity of precipitate is so small that it cannot be mechanically removed from the filter, it must-be removed from the paper by means of caustic ammonia, and the fluid evaporated in a watch-glass. The sulphuret of arsenic can be oxidized in two ways; either it is dissolved in a little aqua regia, until all the arsenic is converted into arsenic acid, the fluid freed from sulphur, dried by a gentle heat, then the residuum dissolved in a drop of water, and supersaturated with lime-water: Or, better, we mix the sulphuret of arsenic with saltpetre and deflagrate the mixture at the end of a hermetically sealed glass tube. We first melt a little saltpetre in the tube, and then gradually

* If the quantity of arsenic is very small, the fluid becomes yellow, without precipitation; but if it is then evaporated, the sulphuret of arsenic falls. in proportion as the acid concentrates during evaporation. If the fluid becomes yellow, without any precipitation of sulphuret of arsenic, during the evaporation, it cannot be considered as a sign of the presence of arsenic. This colour almost always occurs when the fluid contains nitric acid, which reduced to the state of nitrous acid, colours the dissolved animal substances yellow.

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 ;

b a

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 sulp. uric 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 sto mach *.

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 adul, “ terated with alum; alum is prepared, at times, from aluminous rocks, con-' taining iron-pyrites; and iron-pyrites, as mentioned by Berzelius, contains arsenic. This view might be farther illustrated.-En.

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

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