5. Never omit to pa.s.s a broad camel-hair brush over the plate just before pouring on the collodion.

6. Bear in mind that, as light is the producing agent, so will it prove a destructive one: not less than four folds of yellow calico should be used to obstruct white light; and in that case the aperture covered should be no larger than is necessary to admit sufficient light for working by. Examine occasionally the yellow calico: when this material is used to exclude white light, it becomes bleached by constant exposure. Do not trust alone to any coloured gla.s.s; no gla.s.s yet made is anti-actinic under all aspects of light and conditions of exposure.

7. When the negative requires intensifying, carefully wash off all traces of the first developing solution before proceeding to intensify.

This operation may be performed either before or after the iodide is removed by fixing.

8. Gla.s.s baths are preferable to porcelain, ebonite, or gutta-percha baths for solution of nitrate of silver.

9. In using either spirit or amber varnish, before pouring it off, keep the plate horizontal a few seconds. This gives time for soaking in, and prevents the formation of a dull surface arising from too thin a coating.

10. Rub the lenses occasionally with a soft and clean wash-leather; the rapidity of action is much influenced by the brightness of the lenses: their surfaces are constantly affected by moisture in the atmosphere, which condensing, destroys the brilliancy of the image.

11. The white blotting-paper used for some photographic purposes is not suitable for filtering solutions; that only should be employed which is made for this purpose, and is sold under the name of filtering-paper.

12. _Hyposulphate of soda._--A great deal of rubbish is sold under the name of this salt. As a test of its quality, 1 drachms should entirely dissolve in 1 drachm of water, and this solution should dissolve rather more than 4 grains of iodide of silver.

13. _Chemicals._--The purity of photographic chemicals cannot be too strongly urged; the cheapest are not always the most economical. The commercial preparations are generally not to be depended upon, as these, though perhaps unadulterated, are, strictly speaking, not chemically pure. It is best to procure them from well-known chemists, who understand the purpose for which they are intended, and make the preparation of these substances peculiarly a branch of their business.

14. Never leave chemical solutions exposed in dishes: when done with, pour them back into gla.s.s-stoppered bottles, and decant for use from any deposit, or filter if necessary.

15. In all photographic processes it is absolutely necessary to be chemically clean; and this sometimes is not easy. As a rule, never be satisfied with cleanly appearances only, but take such measures as shall insure the absence of all extraneous matter in preparing the solutions, cleaning the gla.s.ses, dishes, &c.

16. All stains on the hands, linen, &c. may be removed by means of cyanogen soap or cyanide of pota.s.sium, which should be applied without water at first, then thoroughly washed off. To a.s.sist the operation, the hands may be now gently rubbed with a fine piece of pumice-stone, when the stains quickly disappear.

For more perfect and complete directions the reader is referred to a most excellent work, "The Modern Practice of Photography," by R. W.

Thomas, F.C.S.

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PNEUMATICS.

"There is a tricksy spirit in the air That plays sad gambols."--BEN JONSON.

The branch of the physical sciences which relates to the air and its various phenomena is called Pneumatics. By it we learn many curious particulars. By it we find that the air has weight and pressure, colour, density, elasticity, compressibility, and some other properties with which we shall endeavour to make the young reader acquainted by many pleasing experiments, earnestly impressing upon him to lose no opportunity of making physical science his study.

The common leather sucker by which boys raise stones will show the pressure of the atmosphere. It consists of a piece of soft but firm leather, having a piece of string drawn through its centre. The leather is made quite wet and pliable, and then its under part is placed on the stone and stamped down by the foot. This pressing excludes the air from between the leather and the stone, and by pulling the string a vacuum is left underneath its centre; consequently the leather is firmly attached to the stone, which enables you to lift it.

WEIGHT OF THE AIR PROVED BY A PAIR OF BELLOWS.

Shut the nozzle and valve-hole of a pair of bellows, and after having squeezed the air out of them, if they are perfectly air-tight, we shall find that a very great force, even some hundreds of pounds, is necessary for separating the boards. They are kept together by the weight of the air which surrounds them in the same manner as if they were surrounded by water.

THE PRESSURE OF THE AIR SHOWN BY A WINE-GLa.s.s.

Place a card on a wine-gla.s.s filled with water, then invert the gla.s.s: the water will not escape, the pressure of the atmosphere on the outside of the card being sufficient to support the water.

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ANOTHER.

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Invert a tall gla.s.s jar in a dish of water, and place a lighted taper under it; as the taper consumes the air in the jar, the water, from the pressure without, _rises up_ to supply the place of the oxygen removed by the combustion. In the operation of cupping the operator holds the flame of a lamp under a bell-shaped gla.s.s. The air within this being rarefied and expanded, a considerable portion is given off. In this state the gla.s.s is placed upon the flesh, and as the air within it cools it contracts, and the gla.s.s adheres to the flesh by the difference of the pressure of the internal and external air.

ELASTICITY OF THE AIR.

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This can be shown by a beautiful philosophical toy, which may easily be constructed. Procure a gla.s.s jar such as is here represented, and put water into it. Then mould three or four little figures in wax, and make them hollow within, and having each a minute opening at the heel, by which water may pa.s.s in and out. Place them in the jar, as seen in the figure, and adjust them by the quant.i.ty of water admitted to them, so that in specific gravity they differ a little from each other. The mouth of the jar should now be covered with a piece of skin or india-rubber, and then, if the hand be pressed upon the top or mouth of the jar, the figures will be seen to rise or descend as the pressure is gentle or heavy; rising and falling, or standing still, according to the pressure made.

REASON FOR THIS.

The reason of this is, that the pressure on the top of the jar condenses the air between the cover and the water surface; this condensation then presses on the water below, and influences it through its whole extent, compressing also the air in the figures, forcing as much more water into them as to render them heavier than water, and therefore heavy enough to sink.

THE AIR-PUMP.

The time was, and that not very long ago, when the air-pump was only obtainable by the philosophical professor or by persons of enlarged means. But now, owing to our "cheap way of doing things," a small air-pump may be obtained for about a guinea, and we would strongly advise our young friends to procure one, as it will be a source of endless amus.e.m.e.nt to them; and, supposing that they take our advice, we suggest the following experiments.

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The air-pump consists of a bell gla.s.s, called the receiver, A, and a stand, upon which is a perforated plate, B. The hole in this plate is connected with two pistons, the rods of which are moved by a wheel handle backwards and forwards, and thus pump the air out of the receiver. When the air is thus taken out, a stop-c.o.c.k is turned, and then the experiments may be performed.

Under the receiver of an air-pump, when the air has been thoroughly exhausted, light and heavy bodies fall with the same swiftness. Animals quickly die for want of air, combustion ceases, a bell sounds faint, and water and other fluids change to vapour.

TO PROVE THAT AIR HAS WEIGHT.

Take a florence flask, fitted up with a screw and fine oiled silk valve.

Screw the flask on the plate of the air-pump, exhaust the air, take it off the plate, and weigh it. Then let in the air, and again weigh the whole, and it will be found to have increased by several grains.

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TO PROVE AIR ELASTIC.

Place a bladder out of which all the air has apparently been squeezed under the receiver, upon it lay a weight, exhaust the air, and it will be seen that the small quant.i.ty of air left within the bladder will so expand itself as to lift the weight. Put a corked bottle into the receiver, exhaust the air, and the cork will fly out.

SOVEREIGN AND FEATHER.

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Place a nicely-adjusted pair of forceps at the top of the receiver, communicating with the top of the outside through a hole, so that they may be opened by the fingers. Then place on each of the little plates a _sovereign_ and a _feather_. Exhaust the air from the receiver; and having done so, detach the objects, so that they may fall. In the open air the sovereign will fall long before the feather, but in vacuo, as in the receiver now exhausted of its air, they will fall both together, and reach the bottom of the gla.s.s at the same instant.