What occasions the difference in these bodies? Liquid substances are nothing more than solids converted into liquids by heat,* a certain increase of which would convert the liquids into vapour.

solidity is the natural state of all bodies; for we are able to reduce most substances to a state of fluidity by the combination of caloric. In general, bodies treated in this way expand in all their dimensions, and the attraction of aggregation is so much weakened thereby, that the particles of the body slide over each other, and are put in motion by the slightest impulse. This is the only distinguishing character of fluidity that we are acquainted with.

On the contrary, the greatest number of liquid substances take a solid form by reduction of femperature. Thus water congeals, and forms ice. Even the gases show this disposition. Chlorine, or oxymuriatic acid gas, becomes concrete, and crystallizes at a temperature near to that at which water congeals. All the gaseous substances, when they have lost their elasticity by forming certain combinations, are disposed to assume the solid state, if the temperature allows it. Ammoniacal gas and carbonic acid gas become solid as soon as they enter into combination; and hydrogen gas, the most subtile of the ponderable elastic fluids, and 14 times lighter than the air we breathe, forms, with oxygen gas, the water which becomes ice. See more on this subject in the first volume of Berthollet's Chemical Statics.

Fluidity is owing to the matter of heat being interposed between the particles of the fluid; which heat would dissipate all fluids into the air, were it not for the pressure of the atmosphere, and the mutual attraction which subsists between those particles Were it not for this atmospheric pressure, water would not be known in any other states than those of ice and vapour; for, as soon as ice had acquired caloric enough to give it fluidity, it would evaporate, and be dispersed into the regions of space. This may be proved by direct experiment, as will be shown in the following chapter. The constitution of the world in this respect exhibits a beautiful instance of the harmony of nature, and of the exquisite contrivance of its divine Author.

On the other hand, could we totally abstract the matter of heat from any fluid, no doubt but that fluid would by that mean be changed to a solid; the lightest vapours being nothing more than solids combined with heat.

Not only fluids, but all those substances which are soft and ductile, owe these properties to the chemical combination of calorie. Metals owe their malleability and ductility to the

What other name is given to liquids?

They are likewise called fluids: we call the air also a fluid.*

Why is the air called a fluid?

Because it flows like a fluid;† because like a

same cause; for, in very intense artificial colds, the most ductile metals, such as gold, silver, and lead, lose their malleability and become brittle, as Van Mons has shown, Annales de Chimie, tom. xxix. 300.

The following experiment will prove that it is caloric which converts solids to fluids:-Expose a pound of water and a pound of ice, both at 32°, in a room the temperature of which is above the freezing point. The water will arrive at the temperature of the room several hours before the ice is melted. The caloric, therefore, which has all the time been entering into the ice, but is not to be found in it by the thermometer, must have become chemically combined with it in order to give it fluidity. The caloric appears to be lost; its properties are merged in the fluid, just as muriatic acid, by union with lime, loses all its characteristic properties. See this further explained in the chapter on caloric.

* Atmospheric air is one of the permanently elastic fluids. Steam is an elastic fluid; but atmospheric air in all states, and in all seasons, is permanently elastic. This elasticity arises from caloric being chemically combined with the solid substances of which it is composed. I say solid, because we have abundant evidence that oxygen and nitrogen are both capable of taking a solid form, and actually do, in many instances, exist in a state of solidity. Nitrogen is a component part of all animal substances, and exists in a solid state in all the ammoniacal salts. Oxygen takes the same state when it combines with metals and other combustibles; and in the composition of the nitrous salts they both take the same state of solidity. These facts surely evince that atmospheric air owes its fluidity to caloric.

One of the general laws discovered by Dr. Black, and which be laid down as a chemical axiom, was, that "whenever a body changes its state, it either combines with caloric, or separates from caloric." The great number of natural appearances which are explained by this general law renders it important, and it ought to be remembered by all who wish to make a progress in the knowledge of the science.

The air is also known to be a fluid, by the easy conveyance which it affords to sound.

In chemical laboratories a vacuum is often formed in the retorts and other glass vessels, which occasions explosions, and

fluid it presses in every direction, and because light substances will swim in it.

What is the cause of bodies swimming in fluids? All substances will swim in fluids if they be lighter than the fluids they swim in.*

Is this universally the case?

Yes; this is an established law of nature.† Thus a cork swims upon water, while a stone

sinks in it.

Have you a clear idea of the cause of these substances sinking and swimming in fluids?

The first swims because it is lighter, and the other sinks, being heavier than water.

sometimes dreadful accidents. These are frequently produced by a torrent of cold atmospheric air rushing into these vessels. If the air did not possess the common properties of fluids, these effects could not take place. It is a property of fluids to press in all directions, upwards as well as downwards; so does atmospheric air.

*This may be shown to a child at the breakfast-table, by placing a tea-cup upon a basin of water, and informing him that it swims there because it is specifically lighter than a body of water of its own bulk. Water may then be gradually poured from the tea-pot into the cup; and he may be directed to observe how the cup sinks in the basin as it becomes loaded with water, until the united weights of the water and the cup are too great for the water in the basin to support, and the whole sinks.

Here the pupil may be informed that it is on this principle that ships and other vessels are constructed, and that it is this property of fluids which enables us to float a vessel from one country to another.

The weight of goods in a vessel is indicated by the depth to which the vessel sinks in the water. In canal boats this is shown by graduated metallic plates affixed to them. An account of a curious method of ascertaining the tonnage of ships hydrostatically, may be seen in the first number of the Retrospect. It is founded on the different draught which a ship will have in salt and in fresh water, owing to the different specific gravity of the two fluids. That nautical men should he ac

1

What do you really mean when you say that a stone is heavier than water ?*

Not that the stone is heavier than the whole of the water in the vessel; for, if it be heavier than a portion of water of its own bulk, it must sink.t

What term is made use of to express the relative weight of bodies?

Specific gravity. Thus the specific gravity of one body may be much greater than that of

quainted with this hydrostatical axiom, is certainly of importance; for, should a captain load his vessel with a full cargo at any sea port, his vessel would inevitably sink when brought into the Thames.

* Some of the questions in this introductory chapter may perhaps be thought trifling and insignificant; but the reader is requested to consider nothing unimportant that tends to lay a just and stable foundation for the superstructure of an interesting science. The generality of elementary writers presume too much when they suppose that what they omit is universally known. Many things which they consider to be of public notoriety, are known perhaps to few who have not paid a partiéular attention to the subject. Hence, many works which are called elementary disappoint the expectation of the student, and are laid by with distaste, for want of the first rudiments of the · science being detailed with minuteness and simplicity.

The pupil should be informed that all stones are not heavier than water, for that these natural bodies differ very much in specific gravity; that, though the specific gravity of sulphate of barytes is 4.40, or nearly 44 times as heavy as an equal bulk of water, some species of the asbestus are lighter than water. Some kinds of pumice stone are also much lighter than that fluid.

It is an axiom in hydrostatics, that every substance which swims on water displaces so much of the water as is exactly equal to its own weight; whereas, when a substance sinks in water, it displaces water equal to its bulk.

Take a piece of hard wood, balance it accurately in a pair of scales with water, and then place it gently on the surface of water in a vessel exactly filled with that fluid, and it will displace a portion of the water, which will flow over the top of the vessel. If the wood be now taken out with care, it will be

another, though their absolute weights be the same.*

How do you explain the difference in the specific gravity of different bodies?

When one body is larger, or takes up more room, than another of the same weight, the first is said to be specifically lighter than the other, and vice versa.

found that the water in the scale will exactly fill the vacancy left by the wood.

* The specific gravity of bodies is denoted in chemical writings by comparing it with the specific gravity of pure water, in decimal figures, water being always considered as 1.000. Thus the specific gravity of the strongest sulphuric acid of commerce is said to be 1.900, or nine-tenths heavier than water. Iron is 7.650, or more than 74 times heavier than water; that is, a cubic inch of iron, if put into a scale, would require 74 inches of water to balance it; silver is 10.478; gold, 19.300; and platinum 23.000, or 23 times heavier than water. The specific gravity of bodies is noted in the same way throughout this work.

†The nations of antiquity were unacquainted with any method of ascertaining the specific gravity of bodies. A singular event was the cause of its being discovered by Archimedes 200 years before Christ. Having reason to believe that an unprincipled goldsmith had greatly debased the golden crown of Hiero II. king of Syracuse, he was anxious to ascertain the fact; but was perplexed by not knowing how to effect it. However, one day while bathing, the difference in the weight of his own body, when in the water and when out of it, gave him the idea that he might adopt that method for discovering the specific gravity of the king's crown; and it is related of him that he was so rejoiced at the discovery, that he leaped from the bath in an ecstasy, and ran through the streets of Syracuse, crying I have found it! I have found it! It would be a pleasing, and not altogether an unprofitable employment, to endeavour to recount the mathematical, philosophical, and chemical truths, that we are in possession of, which tend to promote the happiness and civilization of man, and which the ancients were unacquainted with. In these enlightened times every youth may find avocations suited to his taste, and even in his pleasures will be at no loss to select amusements which are both rational and respectable.