How do chemists examine the properties of bodies?

The chemical examination of bodies is in general effected by producing a change in the nature or state of the body under examination ".

By what means do chemists effect a change in the qualities or states of natural bodies?

This is generally effected by means of heat, or

a This change is frequently effected by the addition of some other substance which can form a combination with a part of the substance under examination, and leave the other part in a detached state. On this principle re-agents, or chemical tests, are employed, the operation of which will be explained as we proceed.

To the eye, many substances appear similar to other substances, though they possess different, and perhaps opposite, qualities; it therefore became necessary to discover the means of analysing these substances, and of ascertaining wherein their difference consists. These we find in chemical re-agents.

It may, with few exceptions, be considered as an axiom in the science of which we are treating, that whenever chemical action takes place, a real change is produced in the substance operated upon, and that its identity is destroyed. An example will place this in a clear point of view. If a little carbonate of lime (powdered chalk) be put into a glass of water, the chalk will sink to the bottom of the vessel. Though it should be mixed with the water, if left at rest it will soon subside. No chemical action has taken place; therefore the water and the carbonate of lime both remain unaltered. But if a small quantity of diluted sulphuric acid be added to a glass of chalk and water, a violent effervescence will commence the moment they come into contact with each other; a chemical union of the two substances will be the consequence of this chemical action; the identity of each ubstance will be destroyed; and sulphate of lime (a body entirely different from either of the substances employed) will be produced.

Heat has a tendency to separate the particles of all bodies from each other. Hence nothing more is necessary to effect the decomposition of many bodies than to apply heat, and collect the substances which are separated by that means. The mixture of two or more compounds often produces a decomposition in each by means of chemical affinity, a property of bodies which will be more fully explained hereafter.

a

by the mixture of some other matter with the matter intended to be examined ".

How does the application of heat and mixture enable chemists to examine the properties of bodies?

By these means we effect the decomposition of a compound body, and thus acquire a knowledge of the nature of its ingredients.

a It is owing to the laws of affinity that we are enabled to examine natural bodies by means of re-agents, or chemical tests. Some of these are simple, and act by single affinity; others, which are compound, act by producing a double decomposition. See chap. xiii,

b Some idea of this mode of examination may be given by the following experiment:-Put a small piece of solid camphor into a phial half filled with diluted alcohol; in a short time the camphor will be dissolved in the fluid, and the spirit will be as transparènt as at first. This solution is owing to the affinity which subsists between these two substances. If water be now added (which has a greater affinity for the alcohol than the camphor has) the water will unite with the ardent spirit, and the camphor will be precipitated. In this way, the camphor may be nearly all recovered, as at first. By distillation the alcohol may also be separated from the water, and exhibited in a separate statė.

This is also called analysis. It is distinguished by chemists into the simple and the complicated analysis. The former is effected whenever a body is so decomposed that its elements, may be exhibited in a separate state, and the body reproduced by reuniting these elements. Where the elements of a body form new compounds during its decomposition, and cannot be reunited to reproduce a similar substance to that which has been operated upon, it is an instance of complicated analysis.

In order to ascertain the exact nature of bodies, we have often recourse to synthesis as well as analysis. Whenever the component parts of any body are reunited in order to form a similar substance, and a similar substance is actually produced, the nature of that body is said to be proved by synthesis. When a body adimits of being examined in both these ways, the result is as satisfactory as can be desired.

The examination of those substances which we receive from the hand of Nature, if conducted on these principles, may sometimes be tedious, but the consequences will be pleasing; the

What is meant by decomposition?

In chemical language, decomposition means the art of dividing a body into its simple elements. Thus water may be decomposed, and reduced into oxygen and hydrogen, which are simple substances, incapable of further decompo

sition.

What are the different states of natural bodies? All bodies are either solid, liquid, or aëriform.

processes may be slow, but they will be sure; and the acquisition of truth, by patient investigation, and by means of our own exertion, affords to a cultivated mind the most delightful gratification.

"Nature, exhaustless still, has power to warm,

And ev'ry change presents a novel charm."

Take sulphate of magnesia (Epsom salt) as an instance of the decomposition and re-formation of a substance by chemical means. Make a solution of this salt in boiling water, and pour into it a little of a solution of carbonate of soda: the soda will precipitate a white powder, which on examination will be found to be carbonate of magnesia. When settled, decant the supernatant liquor, evaporate it till a pellicle rises on its surface, and set it aside to crystallize. When cold, crystals of sulphate of soda (Glauber's salt) will be found in the vessel. In this decomposition, the sulphuric acid of the Epsom salt combines with the soda to form sulphate of soda, and the carbonic acid of the soda combines with the magnesia to form carbonate of magnesia. Thus Epsom salt may be analysed, and shown to consist of sulphuric acid and magnesia. In order to prove the composition of this salt by synthesis, dissolve magnesia in diluted sulphuric acid, saturate the liquor, and crystallize. Epsom salt will be the result.

It may be remarked, that chemists have not only the power of decomposing natural bodies, but of producing, by certain combinations, various other substances, such as are not found in the kingdoms of Nature. Alcohol and ether are both of this class.

When attraction prevails in bodies, they become solid; when caloric or the matter of heat prevails, they become gas: fluidity seems to be a medium between the two. The ultimate particles of matter existing in these separate states escape the cognizance

Chap. 1.]

AND MISCELLANEOUS.

What do you mean by a solid body?

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Solidity is that quality of bodies whereby their parts cohere firmly, so as to resist impression". What are liquid substances?

Liquid substances are those whose parts do not cohere firmly, but readily yield to any impression, and have free motion among themselves ".

of our senses. They are so infinitely small, as not only to evade the scrutiny of the highest magnifying powers in glasses, but even imagination itself is incapable of forming any idea of the size of an original particle of matter. When we have reduced matter to the most impalpable powder, we are far, very far, from the atoms which compose that powder.

A new theory to account for the phænomena of matter has been proposed by the celebrated Boscovich, and admitted by many philosophers. An account of this system may be seen in the Supplement to the Encyclopædia Britannica, under the article Boscovich.

a The force of cohesion increases in a substance in proportion as its moleculæ are brought nearer together. Thus alumine, which has shrunk considerably in bulk by being submitted to a high degree of heat, has not only experienced much mechanical cohesion, but has thereby acquired the power of resisting acids and alkalies. See Berthollet's Chemical Statics, vol. i. 3.

b Sir Isaac Newton has said, that the primary particles of all bodies are hard, whether solid or fluid; and that if the particles be so disposed or fitted to each other as to touch in large surfaces, such body will be hard; otherwise it will be soft. Perhaps, it would be more philosophical to say, that when the attraction of aggregation is strong enough to resist the motion of the particles of a body among themselves, that body will be solid, otherwise it will be soft.

Lavoisier has explained solidity thus:-"The particles of all bodies," says he, may be considered as subject to the action of two opposite powers, repulsion and attraction, between which they remain in equilibrio. So long as the attractive force remains stronger, the body must continue in a state of solidity; but if, on the contrary, heat has so far removed these particles from each other as to place them beyond the sphere of attrac tion, they lose the cohesion they before had with each other, and the body ceases to be solid."

We have no reason to suppose that fluidity is an essential property of any liquid substance whatever; but rather that

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 temperature. 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.

a 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 har mony 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 caloric. Metals owe their malleability and ductility to the