SECTION III.

1. "How does the intensity of the light of a candle vary with the distance of the illuminated surface from the flame? Illustrate your meaning by a numerical comparison in some supposed instance."

The light received at different distances from any luminary is in the inverse proportion of the square of the distances. Thus, of three objects placed at 1, 2, 3 feet respectively, from a candle, the light of the first being the standard of comparison, that of the second would be inversely as 1 is to 22 or 4; that of the third inversely as 1 is to 32 or 9, in respect to the first, and inversely as 4 is to 9 in respect to the second.

2. "If a ray of light pass from air into water, what change takes place in its direction, and according to what law? Under what circumstance is it possible for a ray to pass out of water into air?"

A ray of light passing in an oblique direction from air into water, suffers a change in its previous direction. Instead of pursuing a straight line from the object which emits it to the point of contact with the water, and thence through the water, it is refracted or broken back at the point where it enters the water, and in a direction towards a perpendicular from the same point. The amount of this refraction varies with the media through which the ray passes; glass has a very considerable refracting power, oil less, alcohol less than glass, and water less than alcohol.

The angle between the incident or striking ray and a perpendicular from the point where the ray enters the water is called the angle of incidence, the angle between the same perpendicular continued downwards, and the refracted ray, is called the angle of refraction. When light passes out of a rare medium, as air, into a dense medium, as water, the angle of incidence is greater than

the angle of refraction. The sine of the angle of incidence is to the sine of the angle of refraction as 1.336 is to 1. For other bodies we should obtain different degrees of refraction; and the number, which in each case is the antecedent of 1 in the above ratio is called the index of refraction, or the refractive power of the medium in question. Thus, 1.336 is the index of refraction of water, and the ratio of that index to 1 is the constant ratio of the sines, a technical phrase in optics.

When light emerges from water it is deflected from its previous course towards the surface of the water, and the amount of this deflection is always the same. Hence, if the ray from beneath the water strike the surface at an angle equal to the amount of deflection, the ray will not pass out of the water; if it strike at any greater angle it will emerge.

3. "Explain the decomposition of light by means of a prism."

To decompose light by means of a prism, attach a prism to a small aperture in the shutter of a darkened apartment, so that it may intercept the ray or pencil of rays of the sun's light that would otherwise pass downwards to the floor or other object, and appear as a spot of white light. Place a white screen in such a position as to intercept the ray after it has passed through the prism and been refracted. The white spot of light that was received by the interposed prism will diverge and form on the screen an oblong image of the sun, containing seven colours, viz.: red, orange, yellow, green, blue, indigo, and violet. This elongated image is termed the prismatic spectrum, or, solar spectrum. By other experiments the seven coloured rays may be again conjoined or recomposed, when they are found to form a single beam of white light.

See Brewster's Optics.

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4. "Show distinctly, how it is that a lens may be used, first as a burning-glass, and secondly, to correct defective sight."

A convex lens, such as is used in spectacles, converges all the rays of the sun that may be collected on it, into one intense ray, whose heat is sufficient to set fire to inflammable objects. Some large lenses of this kind concentrate a sufficiently intense pencil of the sun's rays to fuse metals.

The same description of lens, but of smaller size, is used to correct some forms of defective sight. At times changes occur in the crystalline lens of the eye, affecting its density, form, and refracting power. The consequence is an inability to see small objects, as the eye cannot converge the rays of light proceeding from them when they are sufficiently near to be seen in the ordinary state of vision. But this defect is remedied by the aid of a convex lens, which converges light from small distances to distinct foci on the retina, thus compensating the flatness of the crystalline.

SECTION IV.

1. "What solid substances, and what gases, form the organic part of all vegetable and animal substances?"

The solid constituents of the organic parts of vegetable and animal substances are, 1, protein and its numerous compounds, as albumen, fibrin, cassein; 2, cellulose, with which are associated starch, gums, and sugar; 3, wax, and the fats known as stearin, margarin, and olein; 4, chlorophyle, or the green colouring mat

The original form of the above question is, "What solid substance, and what three gases, form the organic parts of all vegetable and animal substances?" Ans. Carbon, Oxygen, Hydrogen, and Nitrogen.

ter of plants; 5, gelatine. The compounds formed from the above are very numerous, and chemists are continually adding to the list. The gases found in combination with the preceding solids are oxygen, hydrogen, and nitrogen.

2. "Describe the function of the leaf in plants by night and by day."

At night the leaves of plants emit carbonic acid gas and absorb oxygen from the surrounding atmosphere, thus vitiating the air by a double process. By day, and chiefly in sunshine, they, on the contrary, imbibe carbonic acid gas from the air and evolve oxygen, which is supposed to be derived from the carbonic acid which they absorb, the solid carbon being appropriated to the nutrition of the plant.

3. From considerations founded on a knowledge of the inorganic constituents of plants, account for the fact that one crop will grow where another fails; that mixed crops grow well together; that a rotation of crops is necessary."

Some plants require more than others of particular inorganic substances, and, in the absence of such substances from a soil, would not flourish so well as other plants which abstract a less proportion of the same aliments. Thus, land which contains a very little lime might be suitable for the growth of rye-grass, but would afford a poor crop of clover or lucerne, as the latter plants require much lime. Similarly, some mixed crops grow well together, because each of the mixed plants abstract more of some and less of other inorganic constituents than is required by the other. The impolicy of continuing the same kind of cropping on the same land, arises from the fact that the land will soon be impoverished from the continued abstraction of those substances which the crop particularly

needs. Thus, grain crops require much magnesia and phosphoric acid, and only flourish when these ingredients exist in a fair proportion in a soil. Hence, if a field had been sown with wheat many successive years, the soil would become exhausted of these constituents, which must be restored by the application of such manures as contain them, or a change of crop must be resorted to.

4. "What are the chemical constituents of lactic acid? What relation does it bear to milk of sugar? In what way does the formation of lactic acid determine the separation of curds in milk? How may these be made to disappear, and in what way does the rennet act in cheesemaking?"

Lactic acid is composed of 6 equivalents of carbon, 4 of hydrogen, and 4 of water; or, according to Johnston (Agric. Ch. p. 943), of 6 of each of those elements. It is produced from the milk of sugar through the influence of the cassein or cheesy matter of the milk. Under the influence of the cassein the elements of a portion of the milk of sugar are made to assume a new arrangement, and lactic acid is the result. The simple

contact of the cassein causes the elements of the milk of sugar to break up their old connection and to arrange. themselves anew in another prescribed order. In milk the sugar and curd are naturally intermixed; but the cheesy matter, acting slowly on the sugar, induces a transformation in the arrangement of the elements of the sugar which is entirely displaced by the new compound from its own constituents, viz. lactic acid. But this lactic acid will not, like milk of sugar, remain intermixed with the curd, which, therefore, appears in a separate form. The acid, in fact, enters into composition with the soda of the cassein, or cheesy matter, forms lactate of soda, and leaves, in the form of curd,