this purpose. The drug is rendered soluble by depriving it of a portion of its oxygen, which element is again absorbed in the subsequent exposure of the printed fabric to the atmosphere; and thus the insolubility necessary to fixation is restored.

By diluting the mordants the depth of shade is varied, and by their appropriate admixture compound colours are obtained.

But quite enough has been said to give a general idea of the principles on which this art is founded, and those who seek more detailed information are referred to Dr. Ure's Dictionary of Arts, Manufactures, and Mines,-article, "Calico Printing." The subject is there treated in a very comprehensive manner, and will amply repay the trouble of a perusal.

We now proceed to discuss the new application of voltaic electricity to the purpose already stated. It would appear almost unnecessary, in the present advanced state of public intelligence, to enter into any description of the method adopted for the development of galvanic phenomena; but a few words on this subject may perhaps be acceptable to some of our readers.

If we take a plate of copper and a plate of zinc, and immerse them simultaneously in a vessel of dilute sulphuric acid, the zinc will immediately begin to be dissolved, and bubbles of hydrogen gas, resulting from the decomposition of the water, will be extricated from its surface; but no effect whatever will be produced upon the copper so long as the two metals do not touch each other: the moment, however, that they are brought into contact, nearly the whole of the hydrogen gas makes its appearance upon the surface of the copper, though the zinc is still the only metal which continues to be dissolved.

There is, in fact, a very extraordinary force thrown into circulation by the contact of these metals under the circumstances described. This force is termed galvanic, and in the line of its passage from the zinc to the acid, from the acid to the copper, and from the copper back again to the zinc, it produces a very singular class of effects. By combining together a number of these simple arrangements in consecutive order, so that the electricity generated by each pair of plates shall circulate through the whole series, the energy of the power which is thus called into action becomes highly exalted, and we have what is termed a compound voltaic battery. By attaching wires or slips of flexible metal to the two ends of the battery, a facility is afforded of transmitting a current of electricity through any body upon which we may wish to operate.

Suppose we pass this current through muriate of soda, or any other saline solution, decomposition takes place, accompanied by a definite transfer of the resulting elements to the two poles or terminating wires of the battery. The hydrogen of the water and the alkali of the salt make their appearance at what is termed the negative pole, and the oxygen and the acid at the positive pole; and if

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In a subject of this nature the introduction of some few technicalities is almost unavoidable; and their explanation is, at the same time, impossible in so limited a paper.

the metal forming this latter have a moderate affinity for oxygen, it will be chemically acted upon and dissolved. The extension of this one fact is all that is requisite for the production of the most elaborate designs by voltaic electricity.

Let it be required to print in two colours, blue and brown. A compound pattern must be formed in such a manner as to present to the cloth, or other substance to receive the impression, different metals in different parts of its surface. The metals in this case would be iron and copper, and the mode of printing such a pattern will be readily understood from the following description:

Upon a smooth plate of metal, in connexion with the negative pole of an active battery, two or three thicknesses of calico are to be placed, having been previously moistened with a mixed mordant of nitrate of soda and ferrocyanate of potassa. On the calico is laid the metallic design. This contact, however, produces no effect until the upper plate is touched with the positive wire; but the moment the electric circuit is thus completed, a decomposition of the interposed solution takes place; hydrogen, potassa, and soda pass to the negative pole, whilst oxygen, with nitric and ferrocyanic acids, are disengaged at the positive pole; where, acting upon the metals thus presented to them, an accurate copy of the design, in its proper colours, is instantaneously produced. The cause of this is obvious: ferrocyanate of iron is blue, and ferrocyanate of copper is brown, and thus the two pigments required are produced wherever the two metals touch the cloth.

The introduction of nitrate of soda into the mordant is for the purpose of facilitating the passage of the electricity, and of preventing the incrustation of insoluble matter upon the metals, which, without this aid, would inevitably take place, and totally check the operation after two or three impressions.

But let us take another example.

To print in red and black, the cloth must be soaked in an aluminous mordant, and the design made of one metal alone-iron. This is touched with the positive pole of the apparatus, as in the instance just given; and, after the current of electricity has passed through the cloth, the latter is to be immersed in a decoction of madder. Wherever the pattern comes in contact with the cloth there will be a black impression developed, whilst the remaining portions will be dyed red.

Discharges may also be produced by this means, or a topical alteration of colour obtained when required.

If a zinc plate be placed upon calico, already dyed with Prussian blue and moistened with nitrate of soda, and lying on the positive pole of a battery, the moment the zinc is touched with the negative wire the blue is changed to a beautiful brown in those parts of the cloth which transmit the electricity; for the alkali no sooner reaches the negative plate than it decomposes the Prussian blue, taking up the acid, and precipitating peroxide of iron.

A great variety of these effects may be obtained. Cloth dyed

with indigo, and moistened with a solution of common salt, slightly acidulated with muriatic acid, is bleached by the battery at the positive pole, which, in this case, should be made of platina.

It must not be forgotten that in conducting these experiments it is necessary to use starch, or some other thickener, with the mordants, as otherwise the colours would run.

The principles which have been advanced and exemplified in this brief account, are susceptible of considerable modification; and when the light of science has been brought to bear more fully upon the subject, it is confidently anticipated that a wide field of application will disclose itself in our arts and manufactures. Electricity may possibly entirely dispense with that complicated machinery at present required in the process of topical dyeing, and the many tedious operations attendant upon the system now in use will be considerably abridged by the introduction of this wonderful agent.

NOTE.-Mr. Baggs, the discoverer of this beautiful application of voltaic electricity, during a late visit to Manchester, very obligingly presented me with patterns in several kinds of metal, necessary for illustrating this mode of printing; and he also performed several successful experiments, in the Lecture Room of the Royal Victoria Gallery, which I have since availed myself of in several lectures on this branch of electricity. Since Mr. Baggs left Manchester, I have been successful in applying magnetic electricity to the same purpose; and I find, also, that sparks from the prime conductor of a common machine will produce some of the colours.-W. STURGEON.

Hints on a New Theory of the Phenomena of the Daguerreotype. By J. RYAN, Esq., M.D., &c., &c.*

IT is a source of much regret to the scientific world that the rationale of an art, which seems almost to realize in our days the Promethean fable of the ancients, should remain so long in obscurity.

We are well aware that numerous hypotheses have been advanced, both by our continental neighbours and by our own countrymen, in order to account for the phenomena of the daguerreotype; but, at the same time, we have to confess our conviction of the futility of each and all which have hitherto been brought forward.

While any art, whatever may be its aim and character, remains in a state of empiricism, we cannot expect any very rapid or marked progression. Each step in advance is taken in the dark, amid uncertainty and fear; and ultimate success, whether immediate or remote, is the result of chance rather than of induction.

We are not ignorant of the avowal of some, that a condition like this is favourable rather than otherwise to the success of an art or science, giving a stimulus to repeated experiment, and preventing

the substitution of theory for practical result. But our readers will agree with us, that while it would be wrong to build up theories and then torture our experiments into an accordance with our preconceived notions, yet the true philosophic mode is to pass, step by step, from experiment to experiment, by the light of inductive reasoning; to aggregate facts, certainly, but not to rest content until we have so classified and arranged, and investigated them, as to leave no part of our process a matter of doubt. Until this is accomplished, any art or science is in a state of illegitimacy, having no claim to the honourable distinctions of true philosophy.

Our readers are aware that the favourite theory of the daguerreotype is at present the chemical one, founded upon a pure hypothesis-namely, "that the gold-coloured pellicle, formed by the vapour of iodine upon the silver plate, is a compound of iodine and silver." M. Donne and others contend, that the chemical action between the silver and iodine, results simply in the production of the common iodide of silver. This, however, will not bear the test of experiment; and if there be chemical action at all, the compound resulting is possessed of properties not appertaining to the well known iodide.

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The next step in the chemical theory is the decomposition of this newly formed compound of iodine and silver by means of light; so that wherever the light has fallen the silver surface becomes exposed, and prepared for the action of the mercurial vapour in the third step of the process. This explains," says M. Donne, in his letter to M. Arago, "the extreme inconvenience, pointed out by M. Daguerre, of leaving the silver plate too long exposed to the vapour of iodine-until, for instance, a violet tint be produced; indeed, in such case, two couches of iodine are formed, the one of a violet colour and superficial, the other more solid and yellow. Thus, when the light has acted on the first, it does not affect the second, and the latter does not admit of the action of the mercury. This fact may be confirmed by removing with the finger the first couch of iodine acted on by the light, below which presents itself the couch of yellow, intact. According to these experiments, the image produced by the process of the daguerreotype would be thus formed the light parts by the mercury in globules, and probably amalgamated with the silver; and the shadows by the simple metallic surface, without any foreign deposit or production of any combination. It is, in fact, that which is left-when, after the operation, all remaining traces of the iodide have been removed by a wash of the solution of the hyposulphite of soda. The black or shaded parts are bare, and reflect the light like polished bodies and glasses, while the light parts are covered with a couch of greyish white, which soils the fingers, is easily removed, and in which the microscope exhibits a quantity of mercurial globules: hence the necessity of a perfect burnish of silver, as M. Daguerre recommends."

The chemical theory, of which the above is a condensed, yet fair view, is wanting in two very material points. In the first place, it

is founded upon a simple hypothesis; and secondly, it fails to account, in any way, for the cause of the action of light upon the iodide of silver-if such it be. At any rate, it has to set at rest the longdisputed point of the existence of chemical rays of light, ere we can admit the decomposition of the new compound by light.

A highly valued friend of the writer has lately put him in possession of some new hints on the theory of the daguerreotype, and as they are based upon well known data, and seem worthy of further consideration, he has ventured to bring them before the public eye, leaving the further investigation of this important subject to those who have sufficient opportunity for that purpose. The propositions embodying the new theory have been in the possession of the writer for some months, but owing to the multiplicity of his engagements, he has been hitherto unable to follow out by experiment, as he once intended, the valuable hints therein embodied.

The phenomena of the daguerreotype (says Mr. Baggs), are evidently referable to electricity; and in proceeding to offer an explanation of the subject, I prefer the adoption of that line of argument which is the most open to contradiction, if any trace of sophistry should be manifested. The apparent rationale is, therefore, embodied in a series of progressive propositions, every one of which will, I hope, bear the test of the closest investigation. Wherever I have deemed it necessary, I have quoted an authority in support of my propositions; but where the facts are so well known or so self-evident as to render this unnecessary, I have, of course, spared myself the trouble of doing so. Thus, of the three first theorems, I would only remark, that they are coutained in every elementary work on electricity, as facts long and sufficiently established.

PROPOSITION I.

Bodies, dissimilarly electrified, attract each other.

PROPOSITION II.

Bodies, similarly electrified, repel each other.

PROPOSITION III.

Positive electricity can only be neutralized by the addition of negative electricity, and conversely, negative by positive.

PROPOSITION IV.

The formation of vapour is invariably accompanied by the deveJopment of electricity.

"If a hot plate of metal be placed upon a gold-leaf electroscope, and water be dropped upon the plate, the moment the vapour rises the leaves of the electroscope diverge with negative electricity.

"The general fact was noticed by Laplace, Lavoisier, and Volta,

Isham Baggs, Esq., the discoverer of the method of printing by electricity, models of whose apparatus may be seen at the Royal Polytechnic Institution.