ADDITIONAL NOTES.

Of Specific Gravity.

THE HE common method of taking the specific gravity of the metais, or of any solid body, is by comparing the difference which there is in their weight when weighed in air and in water; that is, to divide the absolute weight by the loss, and the quotient is the specific gravity. Thus, if a mineral, which weighs three ounces in air, weighs only two ounces when weighed in water, the specific gravity of such mineral is 3} that is, if water, as it generally is, be called 1.000, the substance now examined is 3.000; or, to make it plain to the young student, if a pint of water weigh one pound, the same bulk of the mineral will weigh three pounds. See page 35.

A more ready way to determine the specific gravity of solids is to fill a phial with water, and note the weight of the whole accurately in grains. Then weigh 100 grains of the mineral or other substance to be examined, and drop it gradually into the phial of water. The difference of weight of the bottle with its contents now, and when it was filled with water only, will give the specific gravity of the matter under examination, For example, if the bottle weighs 50 grains more than it did when it was filled with water only, it shows that 100 grains of the mineral displace only 50 grains of water, and consequently that it is twice the specific gravity of water. This method is said to have been discovered by Archimedes. See Note, page 32.

The specific gravity of fluids is generally determined by an areometer, which is a graduated glass tube with a bulb, so contrived that it may swim in the fluid in a perpendicular position. The specific gravity is shown by the degree to which this instrument sinks in the fluid to be examined, and this will consequently always be lower in proportion as the liquid is lighter.

The specific gravity of ardent spirits is generally ascertained by means of a bydrometer, of which various kinds are sold by the mathematical-instrument-makers.

The following method, which was contrived by Dr. Lewis, the celebrated author of the Commercium Philosophico-Techpicum, was communicated to me by the amiable Samuel Moore, esq. late secretary to the Society for the promotion of arts, manufactures, and commerce.

In this method the specific gravity of a liquor is estimated from the excess of the weight of a certain measure of it above that of an equal measure of distilled water. A set of weights is made for this purpose, called carats, or cadukes, in the following manner:

A convenient bottle being procured, the tare of the bottle is first taken; it is then completely filled with distilled water; the weight of the water is accurately divided into two equal parts, and weight made equal to one of these parts is marked 64; by continuing the division, are obtained the weights 32, 16, 8, 4, 2, 1, carats; so that a carat is the 128th part of the weight of the water. Another weight is then made which counterpoises the bottle when filled with water; and so many carats as the bottle filled with an alkaline ley, or any other li- quor, weighs more than this waterpoise, so many carats strong is the liquor said to be.

In taking the specific gravity of bodies, an attention should always be paid to their temperature, as the specific gravity of a body, when expanded by caloric, will always be less than it is when at a low temperature.

II.

Of a curious Property of Charcoal.

A property of which charcoal is very eminently possessed, and which may be regarded as a singular one, is that of absorbing, even when cold, aëriform fluids, and condensing them in its pores, in quantity exceeding many times its own bulk. A very interesting application may be made of this property. The gases which are absorbed suffering so great a condensation, two gases introduced at once into the pores of the charcoal may perhaps be made to combine. It appears from the 32d vol. of the Annales de Chimie, that water has actually been formed by this means. The subject deserves to be prosecuted; and if strong mechanical pressure were applied at the same time, it is probable such combinations might be diversified, and carried to a greater extent. More on this subject may be seen in Murray's Chemistry, 2d edition, vol, ii. page 486; and in the Jour nal de Physique for 1783.

III.

Of Ebullition.

Under an exhausted receiver, water will bɔil at the heat of 92° of Fahrenheit's thermometer; whereas, under the usual pressure of the atmosphere, it does not boil till it is heated to 212°; though, as the atmosphere varies in density, it occasions some little variation in this respect; for Sir George Shuckburgh found by experiment, that when the barometer was at 26 inches, water boiled at less than 205 degrees, but when it was at 31 inches it required to be heated to near 214° before it would boil.

Under the common pressure of the atmosphere

Dr. Black made experiments upon several liquids in vacuo, and found that in general they all boiled with about 140 degrees of heat less than when bearing the weight of the atmosphere. Vitriolic ether (if the pressure of the atmosphere be removed) will boil when 52 degrees below the cold sufficient for freezing water. See pages 49 and 97.

IV.

Of the Combinations of Caloric.

When water is in the state of ice, it combines with caloric, without discovering by the thermometer any rise of temperature; but the moment the whole becomes fluid, every addition of caloric expands the mercury in the thermometer till the water arrives at 212°, when on a sudden the thermometer ceases to rise, though the water continues to receive and combine with a vast quantity of caloric. On these curious facts Mr. Murray has remarked, "We see no cause why, for a great extent of the thermometrical scale, bodies should receive caloric wit out entering with it into any intimate combination; why, when the temperature is raised to a certain point in each body, this combination should suddenly take place; and why, after another interval in the thermometrical scale, it should be renewed ;-for example, why water at 32°, and, under the

common atmospheric pressure, at 212°, and at those points only, should contract an intimate union with caloric." It is true, the cause which produces these effects is concealed from our investigation, but the uses of this order of things are many and important. See Notes, pages 82, 87, and 93. In addition to those facts, other important results, from these seeming anomalies of nature, must present themselves to every contemplative mind, affording it fresh assurances that nothing but beneficence united with consummate wisdom could have dictated such curious and efficacious deviations in the established laws of the universe.

"'Twas this that guided thy Almighty hand,

When formless chaos heard thy high command;
When, pleased, thy eye the matchless work review'd,
And goodness placid spoke that all was good.""

V.

Of Water.

In addition to what has been said in the note to chap. iii. p. 94, on the peculiar law which water observes during its change of temperature; it is worthy of notice, that as rivers, lakes, &c. give out caloric to the currents of cold air passing over them, the upper stratum of water becomes of greatet specific gravity, and therefore sinks, and that this occasions the rise of a portion of warmer water which gives out its caloric in like manner; and that this constant circulation very much contributes to moderate the rigour of winter throughout the temperate zones. In the ocean and other deep bodies of water, this circulation goes on for a considerable time, and an immense quantity of caloric is thus thrown into the atmosphere :—but if no limit had been put to this process, what would have be come of those innumerable tribes of creatures that inhabit this element? The exigency has however been admirably provided for by the Author of Nature; for whenever the whole mass of water arrives at the temperature of 42°-5, its specific gravity no longer increases by the further diminution of its temperature, and the circulation that we have been speaking of entirely ceases. How completely does this simple deviation from a general law answer all the purposes for which it was intended!

"Such the all-perfect hand

That pois'd, impels and rules the steady whole."

On the top of a hill near Lochness, in Scotland, whose perpendicular height is nearly two miles, there is a small lake of

fresh water, hitherto said to be unfathomable, which never freezes during the coldest winters. Here the depth is so great, that the circulation is not completed before the return of suinmer; whereas the Lochanwyn, or Green lake, 17 miles distant, is perpetually covered with ice.

"

VI.

Of Oxygen Gas.

Details of several surprising cures by means of oxygen gas have been given in the different volumes of Mr. Tilloch's Philosophical Magazine*. By the new invented gasometers, and the apparatus sold with them, any kind of factitious air may be accurately measured, and mixed with any portion of atmospheric air that the physician may prescribe. See page 58. And notwithstanding the ridiculous effects which have been attributed to pneumatic medicine, by the over sanguine practitioner, there can be no doubt but that the addition of more oxygen gas to atmospheric air, when aided by proper tonics, has in many instances been very efficacious, and peculiarly so in those female complaints which arise from want of sufficient tone and vigour in the system.

Thus the afflicted, feeble, sickly maid,

Whose spirits languish, and whose health declines,
Looks to pneumatic chemistry for aid,

And on that science every hope reclines.

Anxious she marks the mixture of the airs,
And joys to hear them bubble and expand †,
While fell Hysteric every sinew tears,
And overwhelms her with his leaden hand.

Trembling she grasps the cold metallic tube,
In fear mysterious, and with caution drinks;
While every pulse acquires a kinder throb,
And still increases as the column sinks.

See also an interesting "Essay on the Medicinal Properties of Factitious Airs," by Tiberius Cavallo, esq. 8vo. Dilly, 1798.

When the oxygen gas is prepared, it is thrown up through water into the gasometer in bubbles, where it expands and mixes with the atmospheric air, previously measured into it.

The gasometer is covered by an inverted cylindrical vessel which preserves the air from escaping; this is hung upon pulleys, and sinks lower and lower in the water, as the gas is inhaled from underneath it.