very generally is. The more capable the coal is of swelling (the third class), the more does the proportion of oil gas increase in the gaseous mixture.

It is only in those coals of the first and second classes in which the quantity of charcoal is small, that a decomposition of the combustible is effected before it has experienced a red heat; and even in these coals the decomposition does not make a marked progress at a low temperature.

The oily substance never begins to be developed until the heat has attained the degree of deep red. To all the coals of the two first classes, as well as those of the third, which contain much carbon, a low red heat must be applied to begin the decomposition, and a very strong red heat to terminate it. All the varieties of coal, besides oil and gas, also disengage water, on being distilled in the dry way.

In the ordinary trials of coals, the object of which is to determine the quantity and kind of coke or charcoal which they are capable of furnishing by dry distillation, the coals are usually employed in a state of desiccation in the air. This method is sufficient for common purposes; but it does not answer for chemical analysis, properly so called. In this latter case, M. Karsten found it necessary to dry, at the temperature of boiling water, the various combustibles which he intended to analyse chemically, with the view of comparing the results of the analysis, with the effects which the same coals produce on being submitted to dry distillation.

The author had at first presumed, that all varieties of coal, taken in their ordinary state of desiccation in the air, and such as they are employed for carbonisation, would not undergo a great loss of weight at any temperature below that of boiling water; or that, at least, this loss of weight would be nearly equal in all. But in order to attain his object, he found himself obliged to enquire what loss of weight coals experience from desiccation, at the temperature of boiling water. Hence a series of comparative trials which M. Karsten also extended to some other substances.

All these matters reduced to powder, were first exposed during five days, under the same circumstances, to a temperature of from 11 to 12 degrees of Reaumur's thermometer. When they JANUARY-MARCH 1827.

U

were all thus reduced to the same degree of desiccation, an equal quantity of each of them was weighed, and then dried at the temperature of boiling water; the matter, while still warm, was afterwards weighed a second time, and the difference of weight ascertained. At this high temperature, no decomposition of the bodies under trial had taken place, as was proved by all these substances resuming their original weight, after being exposed to the air for 36 hours.

The following table shews the weight, after desiccation, at the temperature of boiling water, of several substances experimentupon, viz:

ed

Substances submitted to Desiccation at the Tem

perature of Boiling Water, their original Weight
being represented by 100.

Weight retained after Dessiccation.

Sharings of common hornbeam,

Wood charcoal,

Fossil wood, passing into brown coal, of the country of
Aix-la-Chapelle,

Columnar coal (stangenkohle) of Mount Meissner in
Hesse,

Brown coal of Uttweiler, right bank of the Rhine,
Mineral charcoal (faserkohle) from Ibbenbuhren in Prussia,
(Westphalia),

Fibrous brown coal (surterbrand) of Iceland,

Compact coal (kennelkohle) of Lancashire, with highly intumesced coke,

90.7

91.6

80.2

97.2

95.05

99.1

86.95

98.4

Coal of the country of Essen and Werden, with intumesced

The same, with pulverulent coke,

Coal of Upper Silesia, with pulverulent coke,

Coal of the Canton of Bardenburg, country of Aix-la-Cha-
pelle, with pulverulent coke,

Coal of Sulzbach, near Dultweiler, with intumesced coke,
Coal of the country of Saarbruck, with conglutinated coke,

98.2

98.

(Table continued.)

Substances submitted to Desiccation at the Tem

perature of Boiling Water, their original Weight
being represented by 100.

Coal of Loebejun, in the circle of the Saale in Prussia, with pulverulent coke,

Piciform coal of Planitz, kingdom of Saxony, with conglutinated coke,

Weight retain

ed after Desic cation.

99.

94.3

Coal of Pottschapel, near Dresden, with intumesced coke,
Glance coal of the country of Tecklenburg, Lingen,
Glance coal (glanzkohle) pretended anthracite of Schonfeld

94.4

98.3

in Saxony, with pulverulent coke,

95.95

Glance coal of Lischwitz, near Jena, in Saxony,
Conchoidal anthracite of Rhode Island, United States,

94.8

94.9

Pretended anthracite of La Motte, department of the

Isère, with pulverulent coke,

95.5

Coal of the country of Waldenburg, Lower Silesia, with

Another variety of the same, with pulverulent coke,

97.1

Coal of the country of Waldenburg, passing from coal with vesicular coke, to coal with intumesced coke,

98.5

Coal of Upper Silesia, with intumesced coke,

97.1

Coal of the country of Waldenburg, with pulverulent coke,

96.4

Coal of Upper Silesia, with conglutinated coke,

95.9

Coal of the neighbourhood of Beuthen, Upper Silesia, with pulverulent coke,

93.1

Coal of the country of Saarbruck, with intumesced coke, Coal of Eschweiler, country of Aix-la-Chapelle, with intumesced coke,

95.1

99.1

Coal of Eschweiler, another bed, with intumesced coke,

Coal of Wellesweiler, country of Saarbruck, with intumesced coke,

97.85

Coal of the country of Waldenburg, Lower Silesia, with

The loss of weight indicated by this table, whatever differences it may present, does not appear to have any relation to the properties of the varieties of coal, and in general of the matters subjected to experiment. The greatest loss was experienced by the fossil wood and by the coal with conglutinated coke having but a small proportion of charcoal. The former substance losses 19.8, and the latter 6 per cent. The more the quantity of charcoal increases, the smaller does the loss of weight become. M. Karsten, however, was surprised to see that a coal analogous to anthracite, and anthracite itself, experienced a considerable loss (from 5 to 6 per cent.), which would not have been presumed from their hardness and semimetallic lustre.

In general, the lightness, that is to say the porous and loose state of a body, does not appear to have any influence upon this loss of weight, or at least it does not always exert an influence upon it; for if it did so, mineral charcoal, which, of all the substances submitted to trial, is the lightest and loosest, perhaps, without excepting even wood charcoal, would have experienced the greatest loss. The charcoal of mineral wood, however, does not lose more than 1, while the hard and shining anthracite of Rhode Island loses upwards of 5 per cent. On the other hand, graphite, rendered very loose by bruising and pulverization, preserves its weight unaltered. Are the loss of weight which charcoals experience, and their subsequent increase on exposure to the atmosphere, owing to the emission and absorption of atmospheric air and humidity, or of humidity only? The author has not entered upon this inquiry; but he thinks, that, with the view of elucidating the cause of the differences which are observed in the manner in which mineral combustibles comport themselves, it would be interesting to try them thus at the moment of their being taken from the mine, and particularly those which in the open air increase considerably in weight. With regard to such coals as experience a very considerable diminution of weight, on being dried at the temperature of boiling water, their produce in coke by carbonization ought to be very small, and not to agree with the results of chemical analysis, if, as is commonly done, coals dried in the air be employed in the carbonization, and in the chemical analysis, coals dried at the temperature of boiling water.

(To be continued.)

297)

Considerations regarding the shining of the Eyes of the Cat, and several other Animals. By M. BENEDICT PREVOST.

EVERY body knows that the eyes of the cat shine in the dark.

Our domestic cats afford us so frequent opportunities of observing this phenomenon, that it seems peculiar to them; but there are several other animals which equally present it, and I have seen it in the dog, the sheep, the cow, the horse, the polecat, and even in several serpents, and in some insects, among others in the species of sphynx commonly known by the name of the Death's-head Moth.

Buffon says that "the eyes of the cat shine in the dark somewhat like diamonds, which throw out, during the night, the light with which they were in a manner impregnated during the day." Valmont de Bomare says, that "the pupil of the cat is during the night still deeply imbued with the light of the day," and some lines lower he adds," the eyes of the cat are during the night so imbued with light, that they then appear very shining and luminous." Spallanzani says that "the eyes of cats, polecats, and several other animals, shine in the dark like two small tapers, and that this light is phosphoric." M. Dessaignes, in his memoir on phosphorescence, which was crowned by the Institute on the 5th April 1809, says that "the eyes of certain animals have the faculty of inflaming, and of appearing like a fire in the dark.

4

Thus the most eminent naturalists and philosophers are of one mind with the vulgar in regard to this fact, that the eyes of cats and some other animals shine in the dark with a light which is peculiar to them, or with which they have been impregnated during the day. I myself, also, was long in the habit of acquiescing in this opinion, taking the matter partly upon authority of others, and partly observing the phenome myself in the vague way in which every person somet serves things, and men of science as well as others; whic not, however, be productive of great inconveni more importance attached to the citing such ob the making them. Every body," says Mc ject to say foolish things; the misfortune i ously."

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