Слике страница

The following cut represents those four states in the common catterpillar. THE EGGS, THE CATERPILLAR, CHRYSALIS,


[merged small][graphic]

XXI. Chemistry. 515. By Chemistry we ascertain the ingredients, component parts, or first principles of which all kinds of matter are composed.

DEFINITIONS. 1. Decomposition implies the separation of the elementary substances of which any compound substance is formed.

2. Pulverisation signifies the mere mechanical separation of bodies into smaller ones, without being decomposed into its elementary ingredients.

3. Chemical affinity affords proof that atoms are compounded in different forms, which coalesce' and dovetail together with more or less facility.

4. The sensible atoms appear attracted or repelled accordingly as they, or those of the media in which they are placed, are more or less mutually affected.

5. The substance which decomposes another, is called a chemical test or re-agent.

6. If a salt be dissolved in water, it is said to be in solution, and the water is called the menstruum.

7. When water will dissolve no more of any substance, the water is said to be saturated.

8. If we would extract the salt, we must evaporate the wa. ter by heat, with a still, a retort or alembic; and if the va. por from either of these pass through a spiral tube or worm, to the receiver, we shall have distilled water : and the salt will remain in the still

9. Solid substances are reduced into powders by tritura. tion, pulverization, and levigation ; brittle substances are pulverized by hammers, pestles and mortars, stones and mullers.

10. The separation of the finer parts of bodies from the coarser is performed by means of sufling or washing.

11. Filtration is a finer species of sifting, performed through the pores of paper, fannel, fine linen, sand, &o. It is employed only for separating fluids from solids.

12. Füsion, or the melting of a solid body, by the action of heat, requires, according to their several natures, crucibles of different kinds strong enough to resist the fire ; made of earthenware, porcelain, or a mixture of clay and powder of black-lead, or black-lead altogether.

13. Sometimes crucibles have covers made of earthenware, but in other cases the fused metal must be exposed to a current of air : for this purpose the crucibles are broad and shallow, and are called cupels.

14. Blow pipes are used for directing the flame of a candle or lamp against any piece of ore or other substance required to be examined : and when oxygon or hydrogen gas is used instead of common air, the heat is most powerful.

15. The various degrees of heat, or atomic motion which are required for the performance of chemical operations, render it necessary that the chemist should alo be possessed of a furnace.

16. Chemical combinations are more generally influenced by the agency of powers, called by the names of attraction and repulsion, but in truth consisting of various susceptibilities of motion in the atoms of bodies, and in the columns of the media in which they are placed.

17. When a new substance is produced from the combina. tions of two others, the operation is called synthesis. When that substance is decomposed, or resolved into its constituents. by the assistance of other chemical agents, the eration is termed analysis.

18. Elementary bodies are those which no art of modern chemistry has been able to decompose into other elements.

19. Aiomic motion produced by percussion, by friction, or by transfer, is the cause of all the varieties of heat, fire and caloric.

20. Temperature signifies the varied intensity or violence of intestine atomio motion, which, by increasing the distance of the particles or atoms increases the volume of bodies. A. Different bodies change their states at very different

temperatures or degrees of atomic motion. Thus mercury, which becomes solid at about 40 degrees below 0 in Farenheit, boils at about 660 degrees : sulphur, which becomes fluid at 218 degrees, boils at 579 degrees; other boils at 98 degrees.

22. Resistance, says Sir Richard Phillips, is a phenomenon of parting with received motion. A body said to be resisted, is merely parting with its motion to the atoms which it encounters in the media within which it moves; and, as it continues to part with its motion to the radiating atoms, its gradually diminished energy of motion is, in vulgar language, said to be destroyed by resistance.

23. Friction, says he, like resistance, is a mere phenomenon of parting with motion, but to a fixed body instead of a fluid : and being a variation of percussion, or of transfer of motion without change of place, it produces similar phenomena of intestine atomic motion or heat, which when continued or accelerated, produces all the other phenomena of accelerated atomic motion or heat.

24. Crystallisation, he says, is a mere effect of parting with atomic motion, in certain connections with, or relation to, the atoms of the surrounding media, in which the crys. tallized body is placed.

25. The following principles should be remembered. 1. That all fluids are combinations of heat (or transferred motion), with various substances ;

2. That combustion arises from the action of heat, or motion on the parts of the combustible body: and that the process called burning, is nothing more than the oxygen df the atmosphere uniting with certain parts of the body;

3. That oxygen seems to be the acidifying principle: and that all acids are combinations of oxygen with other substances;

4. And that all salts are combination of an acid with othor substances, called the base of the salt.

Obs.-Sir Humphrey Davy, in the preliminary observations to his Elements to Chemistry, beautifully observes, that the forms and appearances of the beings and substances of the external world are almost infinitely various, and they are in a state of continued alteration. The whole surface of the earth even undergoes modifications. Acted on by moisture and air, it affords the food of plants: an immense number of vegetable productions arise friho apparently the same materials: these become the substance of animals: one species of animal matter is converted into another: the most perfect and beautiful of the forms of organized life ultimately decay, and are reso!ved into inorganic aggregates : and the same elemary substancés, differently arranged, are contained in the ert soil, or bloom, and emit fragrance in the flower: or become in animal, the active organs of mind and intelligence. In artificial operations, changes of the same order occur: substances having the characters. of earth, are converted into metals; clays and sands are united, so as to become porcelain ; earths and alkalies are combined into glass ; acrid and corrosive bodies are formed from tasteless substances; colours are fixed upon stuffs, or changed, or made to disappear; and the productions of the mineral, vegetable, and animal kingdoms, are converted into new forms, and made subs-rvient to the purposes of civilized life. To trace, in detail, these diversified and complicated phænomena, to arrange them, and deduce general laws from their analogies, is the business of Chemistry."

516. The ancients conceived that there were but four elements, or first principles--Air, Water, Earth, and Fire.

The moderns have analyzed these four elements, and have discovered other elements of those elements, viz. Oxygen, Hydrogen, Nitrogen, Chlorine, Carbon, Caloric, (or atomic motion) Sulphur, Phosphorus, nine Earths, and twenty-eight Metals.

Obs. 1.-Atmospheric Air is now found to be a compound of Nitrogen or Azote ; and Oxygen ; which are preserved in a gaseous state by Caloric.

WATER is found to be a compound of Oxygen and Hydrogen.

EARTA is a compound of nine different substances, now called Earttis. (See 526.)

And FIRE is found to consist of mere atomic motion,

2.-The forms of matter are well arranged into three distinct classes, by Sir H. DAVY. The first class consists of solids ; which compose the great known part of the globe. Solid bodies, when in small masses, retain whatever mecha· nical form is given to them: their parts are separated with difficulty, and cannot readily be made to unite after separation; some solid bodies yield to pressure, and do not recover their former figure when the compressing force is removed, and these are called non-elastic solids; others, that regain this form, are called elastic bodies. Solids differ in degrees of hardness ; in colour ; in degrees of opacily or transparency; in density, or in the weight afforded by equal volumes; and

when their forms are regular or crystallised, in the nature of these forms.

The second class consists of liquids ; of which there are much fewer varieties. Liquids, when in small passes, aša sume the spherical form; their parts possess freedom of motion; they differ in degrees of density and tenacity ; in colour and degrees of opacity, or transparency. They are usually regarded as incompressible ; at least a very great mechanical force is required, to make them occupy space perceptibly smaller.

The third class, elastic fuids or gases, exist free in the atmosphere ; but they may be confined by solids, or by solids and fluids, and their properties examined. Their parts are easily moveable; they are compressible and expansible ; and their volumes are inversely, as the weights compressing them. All known elastic fluids are transparent, and present only two or three varieties of colour; they differ materially in den. sity.

3.-Besides these forms of matter, which are easily submit ted to experiment, and the parts of which may be considered as in a state of apparent rest, there are other forms of matter which are known to us only in their states of motion when acting upon our organs of sense, or upon other matter, and which are not susceptible of being confided. They bave been sometimes called ethereal substances, which appears a more unexceptionable name than imponderable substances. It can. not be doubted that there is matter in motion in the space between the sun, the stars, and our globe ; though it is a subject of discussion, whether successions of particles be emitted from these heaveoly bodies; or motions communicated by them, to particles in their vicinity, and transmitted by successive impulses to other particles. Ethereal matter differs, either in its nature, or in its affections, from motion ; for it produces different effects-radiant heat and different kinds of light.

517. CALORIC, say many chemists, is a mere name of that element or principle, which, combined with various bodies, produces the sensation of heat; but, according to the theory of Sir Richard Phillips, there is no such element, and all the phenomena are mere effects of atomic-motions.

Obs. 1.-Body, says he, is susceptible of two varieties of motion : (1) a motion or impulse of an aggregate, which occasions it to change its place in regard to other aggregates;

« ПретходнаНастави »