solution of hyposulphite of soda, that part of the silver coating which had not been decomposed by the light, was removed. Had it not been removed, the paper would have been uniformly blackened by the farther action of daylight, and the picture of course would have become obliterated. It was, therefore, by means of the hyposulphite of soda that the picture was fixed. The first pictures Talbot obtained were very imperfect; a longcontinued action of the light being indispensable for their production, none but thoroughly immovable objects could be portrayed. An improvement in Talbot's mode of proceeding was brought about in a most strange way by experiments of Daguerre. Daguerre exposed his silvered plates to the action of iodine vapours, and in this way coated them with an extremely fine film of iodide of silver; but on these plates no picture was produced in the camera obscura. His experiments carried on for months and varied in manifold ways gave no result. Chance, however, in its most proper sense assisted him. A number of plates he had previously experimented upon in the camera obscura, had been put aside into an old cupboard, and they had remained there for weeks without being further noticed. But one day, in removing one of the plates, Daguerre to his greatest astonishment found on it an image of the most complete distinctness, the smallest details being depicted with perfect fidelity. He had no idea how the picture had come, but he felt sure there must be something in the cupboard which had produced it. The cupboard contained all sorts of things: tools and apparatuses, chemical re-agents, and amongst others a basin filled with metallic mercury. Daguerre now removed one thing after the other from the cupboard, with the exception of the mercury, and still he regularly obtained pictures, if the plates, which had previously been submitted to the action of images in the camera obscura, were allowed to remain for several hours in the cupboard. For a long time the mercury escaped his notice, and it almost appeared to him as if the old cupboard were bewitched. But at last it occurred to him that it must be the mercury to whose influence the pictures were owing. For as a drawing made with a pointed piece of wood on a clean pane of glass, remains invisible even to the most acute sight, but comes to light at once when breathed upon, owing to the condensation of the watery vapour deposited in small drops, taking place in a different manner on the parts touched with the wooden point and those left untouched: just so originated Daguerre's pictures.

Mercury being a volatile substance, the cupboard had become filled with its vapour, and this had deposited itself on the plates in the form of most minute globules in such a way that the parts most illumined were covered most and the shadowed parts less, the result being that the outlines and shades of all objects became distinctly visible. I will not here enter upon the improvements made in regard to the optical apparatus, nor do I wish to detail how one came to fix and make unalterable Daguerre's perishable pictures by depositing on them a thin film of gold; but, returning to the pictures on paper, I still wish to say a few words about

the influence Daguerre's discoveries had in improving the mode of proceeding as adopted by Talbot.

Daguerre had found, that if his prepared plates were exposed to the light, even only for a few seconds, a picture could be obtained by afterwards exposing them to the action of mercurial vapour. As Talbot used for the preparation of his paper the same materials Daguerre had on his plates, he concluded that the exposure of the paper for a few seconds to the action of the light in the camera-obscura must have produced an impression. Talbot was convinced there must be a picture on the paper, although he could not see the least trace of one. This conviction urged him on to seek something that would make it visible, for he had no doubt such a thing was really to be found.

How, now, came Talbot to use for this purpose a solution of gallic acid? The solution of this problem most people might be inclined to attribute to chance, as was the case with Daguerre's pictures; but the selection of gallic acid was no chance. Daguerre had not put the basin with mercury into the cupboard for the sake of his experiments; his pictures were obtained without his doing anything for the purpose. Talbot, on the contrary, searched after the means suited to his special purpose, and from among many thousands of substances his imagination instinctively excluded all those that stood in no relation to it, and directed him to those which acted in a similar way to light.

The salts of silver are blackened by warmed gallic acid as well as by light; the action of both is identical in kind, but gallic acid is by far the most powerful. The solar rays had, as he thought, produced an action on the prepared paper in the camera, but so slight that it was not visible; perhaps, so he concluded, this action might be continued and increased by means of gallic acid. The experiment succeeded, and the justness of the induction was thereby proved.

These examples may suffice to render generally intelligible the nature of induction. It will be seen that for Talbot's and Daguerre's purpose it was quite irrelevant to inquire how light and gallic acid act upon silver salts, and why silver salts dissolve in hyposulphite of soda.

Persons who are not familiar with the combinations of ideas produced by imagination, will of course not believe in them, and are generally inclined to ascribe every discovery to mere chance, should it even have been arrived at by most ingenious reasoning. Chance has its great share in discoveries, no doubt, just as even the understanding frequently derives the elements for its conclusions from so-called accidental circumstances. But from the circumstance that experimenting must be learned, that it is an art having its rules, and that, to practise it successfully, it is necessary to be acquainted with a great number of facts and phenomena, it becomes evident that it is founded on a peculiar working of the mind in which the understanding participates as spectator, frequently too as adviser and helpmate, but without conducting it, or without that working being dependent on it.

In science as well as in every-day life the operations of the mind are not conducted according to the rules of logic, but we generally take things to be true, and adopt a certain view about an occurrence, or the cause of a phenomenon before proving the correctness of our opinion. One does not arrive at the conclusion by means of syllogism, but the conclusion precedes, and the premises are only afterwards sought out for demonstration. In a conversation about the share which imagination takes in scientific working, one of the most renowned French mathematicians advanced the opinion, that by far the great majority of mathematical truths had not been arrived at by deduction, but by help of the imagination, or empirically, and he maintained this even in regard to the properties of triangles, of the ellipsis, &c. This is as much as to say, that without artistic genius the mathematician can achieve as little as the natural philosopher.

It is a matter of course that for deductive as well as inductive investigations, if they are to be successful, a certain range of knowledge is required; for deductive a thorough knowledge of the laws already known, which can be acquired by the help of books and lectures; for inductive investigations, an extensive acquaintance with the natural phenomena, to be obtained in chemical, physical, and physiological laboratories. schools, laboratories are, as is well known, of modern origin, and their influence on the development of all departments in any way connected with natural sciences, cannot fail to be noticed by an attentive observer.

As

The inductive inquirer, for the solution of his problems, must combine acquaintance with natural phenomena; that is, knowledge of the nature and properties of things, with recollection of impressions on the senses, i.e., memory of sight, taste, and smell, and with a certain amount of ability and skill. The more extensive and comprehensive his knowledge of facts and phenomena, the greater, as we express it, his experience, the easier his work becomes to him. One who has experience has to make much fewer experiments than one who has none, and who has first to make himself acquainted with many phenomena which the other is already familiar with. For many purposes, therefore, experiments are unnecessary to the former, the combinations of processes or facts being already known to him. Both the deductive and the inductive inquirer commence the solution of problems in the same way. The one, like the other, starts from a complex idea, pertaining either to the understanding or imagination, of which generally only a part is true, whilst the rest is founded on erroneous conclusions or combinations. The deductive philosopher tests and experiments with ideas to find the truth, just as the inductive inquirer uses impressions on the senses to find the thing of which he is in search: both, by testing and improving, cast off, during their work, their erroneous views, and gradually find what was missing for the completion of that idea with which they commenced their inquiry. Often the idea from which they started is quite wrong, the right one being only developed during the investigation. This is the origin of the opinion of many of the greatest

observers, that working does everything, and that whatever theory stimulates working leads to discoveries.

In deductive inquiries, it is the conviction of the correctness of a (conclusional) idea, which urges on the inquirer's understanding to exercise its proper function; with the experimentalist, the conviction of the existence of a thing is the first and most powerful motive for setting his imagination to work. The discovery of a new fact or reaction, which may be brought in connection with the idea of something hitherto unknown, of something useful and important for industrial purposes or daily life, is sufficient to raise in many individuals the conviction of its existence; and it happens frequently enough that it really is discovered by several contemporaneously.

Reason and fantasy are equally necessary for science; to each of them belongs a certain defined portion of all problems occurring in natural philosophy and chemistry, in medicine and political economy, history and philology, and each occupies a certain space in these respective domains. The portion over which fancy presides is wider and more extensive in the very ratio that the positive knowledge encompassed by the understanding is undefined and vague. What characterizes progress is, that, with the increase of knowledge, those ideas vanish which had their origin in the imagination; and whereas, during the first period of science, fantasy has full sway, it afterwards subordinates itself to the understanding, and becomes its useful and willing servant.

Induction under guidance of the imagination is intuitive and creative, but undefined and boundless; deduction under guidance of the understanding analyzes and limits, and is defined and measured.

What principally characterizes deductive examination in natural sciences is "measure," and the final aim of all its endeavours is to find an unalterable numerical expression for properties of things, processes, and phenomena. Imagination compares and discriminates, but does not measure; for to measure one must have a standard to measure by, and this is a product of the understanding.

When an art is developed into science, the advantage, scarcely enough to be appreciated, is, that the art, as such, with its individual character, is destroyed, being shaped into rules which can be acquired and taught, and through the knowledge of which even the ungifted acquires the power of the most gifted, most skilful, and most experienced practitioner, obtaining his object in the shortest, surest, and most economical way. This is the case in regard to agriculture and medicine, and the different branches of industry. What at first belonged to an individual then becomes the common property of all.

JUSTUS VON LIEBIG.

VOL. XII.-No. 69.

15.

My Persecutors.

Ir is a prevalent idea that in free and happy England the days have long since passed away in which a man was subjected to persecution on account of his opinions; and I can only say, happy are those whose experience justifies them in entertaining a belief in the truth of this idea. -mine does not. Whether it is that my case is an exceptional one, and I have been a "" martyr to circumstances," or, whether the old spirit of persecution is not so thoroughly eradicated from the human heart as is generally supposed, I am not able to determine; all that I know with that absolute certainty which alone justifies a positive and unqualified assertion is, that for some considerable time past I have been persecuted in a manner almost worthy of the good old times, for refusing to be "convinced against my will" that my own opinions upon a subject to which I shall presently refer are utterly erroneous, and those of sundry of my acquaintances infallibly correct. I am well aware that a man with a grievance is a bore and a social nuisance; and even apart from this restraining knowledge, I would not think for a moment of attempting to "ventilate a mere grievance; for I scorn the idea of crying out about any of those petty annoyances of every-day life which are exaggerated by grumblers until they assume the proportions of a grievance. I am no grievance-monger. I never wrote to the Times on the subject of "The Hotel Nuisance," although I once had to pay a tavern bill for bed and breakfast which in point of extortion surpassed any transaction of the "sixty per-centers" of which I have ever heard or read, and which induced a sporting gentleman who had been charged a like amount for the same accommodation, to tell the proprietor of the tavern, that although he (the sporting man) did not know his (the proprietor's) exact pedigree, he was confident he was full brother to a robber. So far, indeed, from being a grievance-monger, I may say for myself that I am a particularly longsuffering individual, and have borne, with fortitude or indifference, annoy ances that would have driven any person of a less philosophical turn of mind than myself to despair and the police-courts. I have been importuned and abused by garotter-like mendicants, to whom I have given alms instead of handing them over to the police. I have been threatened and derided by the coarsest cabmen, with whose demands (generally about double their legal fare) I have complied, when others would have taken their number and "made an example of them." I have even been taken before a bench of magistrates upon suspicion of being a burglar; this last decidedly unpleasant event occurring through the stupidity of a policeman, who stopped me as I was leaving my work late one winter night, carrying the implements of my trade, (which certainly have a strong