Imatges de pÓgina

miles per second, from the sun and other luminous bodies; according to others, the phænomena are explained by supposing a series of vibrations excited in an elastic medium diffused through all space. What is this supposed medium called? Æther. How is colour accounted for according to the latter theory? By the different degrees of rapidity with which the æther vibrates. How many vibrations in a second of time pro duces in our minds the sensation of red? Four hundred and fifty eight billions. Can philosophers really count so many vibrations? They tell us very gravely that though they cannot count them, they can by calculation determine their number. What do you understand by the refraction of light? When light passes from air into water, glass, or other transparent medium, it is bent from its rectilineal course, and this is called the refraction of light. When light falls obliquely on a polished surface, what is the law of reflection? It makes the angle of incidence equal to the angle of reflection. On what principles are telescopes constructed? Reflecting telescopes are constructed on the law of reflection, and refracting telescopes on that of refraction. What parts of the eye are principally concerned in vision? The cornea, the crystalline lens, and retina. What are the uses of the cornea and crystalline lens? To collect the rays of light into a point.


If a piece of amber be rubbed against cloth, what property does it acquire? The property of attracting light bodies, such as bits of straw, paper, and feathers. Why are the phænomena of attraction and repulsion called electrical? From electron, the Greek name for

amber. Into how many classes are bodies divided? Into good and bad conductors of electricity. Name the best conductors. Metals and most liquids, except oils. Name the worst conductors. Silk, glass, wax, and all resinous substances. Is the electric fluid simple or compound? According to Dr. Franklin simple, and others compound. If you suppose it simple, when is a body said to be electrified positively? When it has received more than its natural share of electricity. When is it electrified negatively? When a part of its natural share of electricity has been taken away. According to the latter theory, when is a body said to be electrified positively? When that portion of the electric fluid called negative or resinous has been removed. When is a body electrified negatively? When its positive or vitreous electricity has been removed. When do two bodies attract each other? When they are differently electrified. When do they repel each other? When they are similarly electrified. Who was the first person that found lightning and electricity to be the same? Dr. Franklin, by means of a kite raised to a great height. How would you prove them to be identical? Because they produce the same effects; as, for example, giving the shock, melting metals, and setting fire to inflammable substances. What do you understand by zig-zag lightning? That which strikes between a cloud and the earth in a zig-zag form. What is sheet lightning? That which strikes from one cloud to another. Which is the most dangerous? Zig-zag lightning. What ought you to do if in the country in a thunder storm? Avoid trees; and if there be apparent danger, lie down flat on the ground. What kind of electricity is that called voltaic? That which is produced by combining two metals, as zinc and copper with water, and an acid, so as to form a voltaic battery. When this kind of elec

tricity is employed for giving shocks and acting on animals, whether dead or alive, what is it called? Galvanism, from the discoveries of Dr. Galvani. What is magnetism? That property communicated to hard iron or steel by which it attracts iron. When a magnetic needle is suspended freely, does it point to the north pole of the earth? No; it points at present to 23° W. of the N., this is called the variation of the compass. Into how many parts is the mariner's compass divided? Into thirty-two parts; the north, south, east and west being called the four principal or cardinal points. Can electricity be produced by magnets? By certain arrangements magnets may be made to generate electricity so as to give shocks and produce other effects. Can a magnet be produced by means of electricity? Yes; by rolling copper wire about a piece of soft iron, and connecting the ends of the wire with a battery. Does lightning sometimes injure the compass needle of a ship? Yes; it sometimes reverses the very poles. Can electricity be formed by heat? Yes, by joining different metals and heating the alternate points of contact. What property is possessed by a certain kind of fish? When it is touched it produces a peculiar electric effect. What are those shocks similar to? Similar to those produced from a Leyden phial or by a voltaic battery. Do these fishes affect an electrometer, or has it been possible to elicit sparks from them? They do affect an electrometer, but it has not been possible to elicit sparks from them. Can you tell me the names of the fishes which belong to the kind just described? The Gymnotus Electricus or electric eel, the Rayed Torpedo, the Silurus Electricus, and Triurus Indicus.


Among all the sciences cultivated by man, none is held in higher estimation than chemistry, both from the brilliant phænomena that it exhibits, and the practical application to which its discoveries lead.

The whole process of metallurgy is essentially chemical, from the reduction of the ore to the polishing of the instrument; dyeing and calico printing, tanning and soap making, depend entirely on chemistry for their skilful production, and so also do baking, brewing, and every culinary art. On the face of nature we cannot cast our eye without seeing chemical principles exhibited in her grand laboratory. Every drop of dew that falls, and every particle of moisture that is evaporated, are examples of chemical principles, and it is by it alone that we can explain the cause of the lightning's flash and the thunder's roar.

In the animal economy also we discover remarkable instances of chemical decomposition. The food is digested in the stomach and assimilated to the blood in a manner which, if the physiologist cannot fully explain, he at least can by chemistry imitate for digestion proceeds in the stomach of a newly killed rabbit, when slight galvanic shocks are made to pass through that organ. The compositions that the chemist forms are no less wonderful than the simple elements that he obtains. He can produce explosive mixtures of such powerful violence, that gunpowder itself may be considered as an innocuous body: but he also, from the bones of animals, can separate a substance so combustible that it becomes luminous when exposed to air, and from the ashes of wood, a metal lighter than water, and which water sets on fire. Perhaps objections may be

raised, regarding the advantage of teaching chemistry to the young; but if the science be useful, its principles cannot be too early inculcated, and we would only wish to introduce it as a relaxation after more severe studies, or as a reward when other tasks have been satisfactorily performed. This method we have tried and have found it answer admirably. The best method of teaching we consider to be that which has been so ably recommended by Dr. Reid; and that is, that each pupil ought to have a small chemical box, and should repeat on a small scale, those experiments which the teacher has performed before him. In making gases, this method, however, will not apply; but in testing, exhibiting crystallization, and other similar simple operations, the system works well.

How do you distinguish a chemical from a physical phænomenon? Natural philosophy treats of the action of masses, whilst chemistry investigates the action of atoms. In physical phænomena there is only change of place, whereas in chemical a new body or bodies result. Give instances of chemical action. Upon a few thin slices of phosphorus drop a piece of iodine; combustion ensues, and ioduret of phosphorus is formed. Upon a piece of common chalk pour dilute muriatic acid; carbonic acid is evolved, and muriate of lime remains in solution. If the piece of chalk were triturated in a mortar, each particle would retain the same properties as the mass from which it was separated; whereas by adding dilute muriatic acid we obtain a new compound essentially different from the chalk and acid with which we experimented. Hence trituration is a mechanical process, and the decomposition of chalk a chemical. What do you mean by a gas? Any body that retains the form of air at ordinary temperatures. Steam is a vapour. Atmospheric air, as it is permanently elastic,

« AnteriorContinua »