The Boy's Playbook of Science Part 17

The iodized collodion is now poured on, and the excess returned to the bottle. Collodion can be made very easily, but if prepared without due precautions, it cannot be used afterwards, and reminds one of the old story of the enthusiastic son, who, when asking his father's permission to espouse the beloved, enumerated amongst her other accomplishments, the fact that she _could_ make a pudding, and was answered by the bluff question, "But can you eat it afterwards?" So it is with collodion: a great deal of messing and loss of time is saved by purchasing it of the various makers, amongst whom may be specially noticed Mr. Richard Thomas, of 10, Pall Mall, who has devoted the whole of his attention to the preparation of this important photographic chemical, and with a success which his numerous patrons can well testify. The collodion is sold either mixed with the iodizing solution, or the two can be obtained separately, with directions on the bottles as to the quant.i.ties to be mixed together.

The plate covered with the iodized collodion is quickly transferred to a bath containing a solution prepared in the following manner:--Dissolve four ounces of nitrate of silver in eight ounces of water, and to this add twenty grains of iodide of pota.s.sium in one ounce of water; shake them together, and then pour the whole into fifty-six ounces of distilled water, and in half an hour add one ounce of alcohol and half an ounce of ether; agitate the whole and filter the next morning. The collodion plate is kept in this solution for a certain period, only learnt by experience, and should be occasionally lifted out to see if a uniform transparency is obtained; say that the immersion may be continued for five minutes, it is now ready for the camera, and may be exposed from about one to two minutes, or more if the light is deficient; the time of exposure is also a matter of _practice_, mere directions can be of no use in this stage of the process.

The picture is developed on a levelled stand, with a solution of three [Page 148] grains of pyrogallic acid in three ounces of water, to which sixty drops of glacial acetic acid have been added. When fully developed the plate is washed with water and fixed with a solution of hyposulphite of soda, consisting of one part of the saturated solution to eight of water, again thoroughly but gently washed, so as not to endanger the separation of the film from the gla.s.s; it is allowed to dry spontaneously, and being coated with amber varnish (a solution of amber in chloroform) is now ready to print from. (Fig. 123.) It is, perhaps, hardly necessary to add, that the sensitizing and developing processes must be performed in a dark room.

[Ill.u.s.tration: Fig. 142. A. Gla.s.s or gutta-percha bath to hold the sensitizing solution. B. Gla.s.s, with piece cemented on the end to hold the prepared gla.s.s plate, C, whilst dipped in the bath, A. The plate C has a cross in one corner to show prepared side.]

[Ill.u.s.tration: Fig. 143. First effect of peripatetic photography on the rural population.]

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BROMINE.

Bromine ([Greek: _bromos_], a bad odour). Symbol, Br. Combining proportion, 80. Specific gravity, 2.966.

In a previous portion of this work, the connexion between chlorine, iodine, and bromine has been pointed out; and as we have to notice the colour of the element bromine, the chromatic union of the triad may be alluded to. These elements present very nearly all the colours of the spectrum:

Bromine red to orange.

Chlorine yellow to green.

Iodine blue, indigo, violet.

These three elements also furnish examples of the three conditions of matter; iodine being a solid, bromine a fluid, chlorine a gas; the relation of their combining proportions is also curious: as might be expected, the fluid bromine takes an intermediate position, and (according to the axiom that half the sum of the extremes is equal to the mean) by dividing the combining proportions of iodine and chlorine, and adding them together, we have, as nearly as possible, the combining proportion of bromine:

Chlorine 35 2 = 17.75 Iodine 126 2 = 63 ----- 80.75

The combining proportion of bromine is 80, but 80.75 is so near, that it may reasonably be conjectured future experiments will reduce the number of the three elements, and may prove that they are only modifications of a single one. This is the only kind of alchemy which is tolerated in the nineteenth century, and any philosopher who will reduce the number of elements, and prove that some of them are only modifications of others, will achieve a renown that must transcend the _eclat_ of all previous discoverers.

Bromine was discovered by Balard, in 1826, and, like chlorine and iodine, is a const.i.tuent of sea water. The chief source of bromine is a mineral spring at Kreutznach, in Germany. The process by which it is obtained offers a good example of chemical affinity; the water of the mineral spring is evaporated, all crystallizable salts removed, and a current of chlorine gas pa.s.sed through the remaining solution, which changes to a yellow colour, in consequence of the liberation of the bromine by the combinations of chlorine with the bases previously united with the former; the liquid is then shaken with ether, which dissolves out the bromine. In the next place, the etherial solution is agitated with strong solution of pota.s.sa, and is thus obliged to part with the bromine which is converted into bromate of pota.s.sa; this is ultimately changed by fusion to bromide of pota.s.sium; and by distillation with black oxide of manganese and sulphuric acid, the bromine is finally obtained. Six [Page 150] processes are therefore necessary before the small quant.i.ty of bromine contained in the mineral spring-water, is separated.

_First Experiment._

Bromine is a very heavy fluid, which should be preserved by keeping it in a bottle covered with water; when required, a few drops may be removed by means of a small tube, and dropped into a warm bottle, which is quickly filled with the orange-red vapour. If some phosphorus is placed in a deflagrating spoon, and exposed to the action of bromine vapour, it takes fire spontaneously.

_Second Experiment._

Powdered antimony sprinkled into the vapour of bromine immediately takes fire.

_Third Experiment._

A burning taper immersed in a bottle containing the vapour of bromine is gradually extinguished.

_Fourth Experiment._

Liquid bromine exposed to a freezing mixture of ice and salt, or reduced to a temperature of about eight degrees below zero, solidifies into a yellowish-brown, brittle, crystalline ma.s.s.

_Fifth Experiment._

A solution of indigo shaken with a small quant.i.ty of the vapour of bromine is quickly bleached. Many substances, when brought in contact with liquid bromine, combine with explosive violence, and therefore experiments with liquid bromine are not recommended, as all the most instructive and conclusive results can be obtained by the use of the vapour of bromine, which is easily procured by allowing a few drops to fall into a warm, dry bottle.

Bromine, as already mentioned, is used in the art of photography.

FLUORINE.

Symbol, F. Combining proportion, 19.

This singular element seems almost to embody the ancient idea of the alchemists, being a sort of _alkahest_, or universal solvent; or in plainer language, its affinities for other bodies are so powerful, that it attacks every substance (not even excepting gold), at the moment of its liberation, and combines therewith, so that its isolation has not yet been effected. Chemists who a.s.sert that they have been able to obtain fluorine in the elementary condition, p.r.o.nounce it to be a gas which possesses the colour of chlorine; but the experiments, as. .h.i.therto conducted, render that statement extremely doubtful.

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The only interesting fact connected with fluorine, is the remarkable property of attacking gla.s.s and other silicious bodies, belonging to its combination with hydrogen gas, called hydrofluoric acid. This acid is easily obtained and used by placing some powdered fluorspar in a leaden tray six inches square and two inches deep. If sulphuric acid is now mixed with the powdered spar, so as to form a thin paste, and heat applied, the vapour of the hydrofluoric acid quickly rises, and can be employed to etch a gla.s.s plate upon which a drawing may have been previously traced by scratching away the wax, with which it is first coated. By heating the gla.s.s plate before a fire, a sufficient quant.i.ty of wax is soon melted on to it by merely rubbing the wax against the gla.s.s plate; any excess should be avoided, if a well-executed drawing is required to be etched on its surface. (Fig. 144.)

[Ill.u.s.tration: Fig. 144. A A A. The gla.s.s plate, with the waxed side downwards, placed on the leaden tray containing the fluorspar and sulphuric acid. B. Spirit lamp.]

The wax plate must not remain too long over the leaden tray, as the heat is apt to melt the wax, when the acid not only attacks those parts from which the wax has been removed by the etching needle, but also the surface of the gla.s.s generally, and thus the clearness of the design is spoilt. After exposure--and it is as well to prepare two or three gla.s.s plates for the experiment--the wax is quickly removed by rubbing and was.h.i.+ng with oil of turpentine, and the design (beautifully etched into the gla.s.s) is then apparent.

CHAPTER XII.

CARBON, BORON, SILICON, SELENIUM, SULPHUR, PHOSPHORUS.

This group of non-metallic elements has been frequently styled "Metalloids," meaning substances allied to, but not possessing, all the properties belonging to a metallic substance; and therefore perhaps the expression, non-metallic solids, is the best that can be adopted. They may be subdivided into two cla.s.ses of three each, which have properties more or less allied to each other--viz.,

Carbon, Boron, Silicon; and Selenium, Sulphur, Phosphorus.

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CARBON.

Symbol, C; Combining Proportion, 6.

This element has almost the property of ubiquity, and is to be found not only in all animal and vegetable substances, in common air, sea, and fresh water, but also in various stones and minerals, and especially in chalk and limestone.

There is, perhaps, no element which offers a greater variety of amusing experiments and elementary facts than carbon, whether it be considered either in its simple or combined state.

A piece of carbon, in the shape of the Koh-i-Noor, was one of the chief attractions at the first Exhibition in Hyde Park. The diamond is the hardest and most beautiful form of charcoal; how it was made in the great laboratory of nature, or how its particles came together, seems to be a mystery which up to the present time has not yet been solved, at all events no artificial process has yet produced the diamond.

Sir D. Brewster, speaking of the Koh-i-Noor, remarks that on placing it under a microscope, he observed several minute cavities surrounded with sectors of polarized light, which could only have been produced by the expansive action of a _compressed gas or fluid_, that had existed in the cavities when the diamond was in the _soft_ state.

Now it is known that bamboo, which is of a highly silicious nature, has the property of depositing in its joints a peculiar form of silica, called tabasheer. Silicon is one of the triad with carbon--_i.e._, it is allied to carbon on account of certain a.n.a.logies; may it not then be supposed that, in times gone by, ages past, when the atmosphere was known to be highly charged with carbonic acid gas, there might possibly have existed some peculiar tree which had not only the power of decomposing carbonic acid (possessed by all plants at the present period), but was enabled, like the bamboo, to deposit, not silica, which is the oxide of silicium, but carbon, the purest form of charcoal--viz., the diamond? Speculation in these matters is ever more rife than stern proof, and it may be stated, that all attempts to manufacture this precious gem (like those of the alchemists with gold and silver) have most signally failed.