pass from yellow to rose, and from rose to violet, in which condition the power of exhibiting photographic images would be almost extinct. The plate thus prepared is ready for the operation which converts its surface into a picture. It has been hitherto scrupulously excluded from the light; it must now be placed in such a position that the image of the object it is required to copy shall be clearly defined upon its face, either by reflection from a concave mirror, or by refraction through the medium of a lens. This is accomplished by placing it in a small case while in the darkened room in which the preceding processes have been effected, so that it may be protected from the action of light, and thus removed to the camera or reflector. The case is so constructed that the side which covers the polished surface of the plate falls at the will of the operator, and exposes it to receive an impression from the outlines which are pencilled by the rays of light upon it. The period thus occupied, and consequently that during which the sitter is required to remain immoveable, varies somewhat with the state of the atmosphere and the time of day. About thirty to forty seconds may be considered a fair average sitting. The case is here now immediately closed; for not a glimpse of light may disturb the plate until it has undergone an operation for the purpose of rendering visible the effects of the previous one. Were we to examine it now, we should see nothing but the bright rose-coloured surface, slightly tinged in places with violet; and this, while we gazed, would soon become one mass of deep purple-not a trace of the beautiful picture, already in embryo, should we be able to discover. It is therefore transferred to the mercury-box, and subjected to the action of that metal upon its surface, in the form of vapour caused by the application of gentle heat, while it occupies an oblique position above the mercury, forming an angle of forty-five degrees with the horizon. This process occupies from fifteen to twenty-five minutes, and during this time it must be watched in order to ascertain the effects produced. Apertures containing yellow glass are provided in the box for this purpose, through which, by the light of a taper, a glance at the plate may be obtained. It may be remarked here that glass of this description, admitting only the yellow or orange ray, prevents injurious results to the picture, as the chemical action of light resides principally in the violet ray, and is scarcely perceptible in the former: but of this hereafter. The effect of this last operation has been to render visible the whole picture, and thus to show to the operator the result of his labours. It presents rather a cadaverous appearance, and is a faithful copy of the individual who sat to the magic painter. If he laughed, or knit his brows, which last is most usually the case, from the difficulty of supporting a steady gaze under any considerable degree of light, or if he has engaged an awkward position expressly for the occasion, an effect not at all unlikely to follow, from an extreme desire to be easy and look well, there it is fixed-made faithfully prominent in the miniature, which perchance may be designed for very tender and sentimental uses. Fidelity from the Daguerreotype we may expect, but not a spark of flattery. It now only remains to set the picture; that is, to protect it from the further action of light, which has been partially but not wholly arrested by its coating of mercury. An account of this process will more properly follow after the rationale of that already described has been furnished; and, in order to effect this, it will be necessary to pay some attention to the properties of light. The agency of light being evidently that which produces the process we have already detailed, it will be necessary that its composition and mode of action (although to some extent our acquaintance with these is but conjectural) should become subjects of examination. Let it be understood that it is not our province now to enter into the merits of any theories which may have been propounded respecting light, but simply to state its known properties in conventional language, which, although not perhaps strictly accordant with a true idea of its action, will convey more intelligible notions of the subject than any other. For example, when speaking of a ray of light, we shall understand by that term an emanation of luminous and other influences from the sun, regardless of an hypothesis which considers light to be a property of matter excited by means of some unknown solar influence, or of any other, excepting the popular one, which education and sensual impressions have led us to follow. In what follows, we therefore speak positively, not conjecturally. The first and most evident property of that which we term light, is luminous influence, or the cause of objects around us becoming cognizable to our visual organs, and which may be spoken of as luminosity. The second is also evident; its power of communicating or producing heat. The third, is its power of exciting chemical action in some bodies, and of disturbing their molecular arrangement; that is, causing a different arrangement in the minute and invisible atoms of which all substances in nature are composed. For instance, a sunbeam cannot pass over any form of matter, organic or inorganic, without leaving traces of its path. True, these may not be appreciable by the naked eye, but they nevertheless exist, and are to be made manifest by an application of the proper test. What a field for interesting inquiry does not the knowledge of such a fact present! What an idea does it afford of the magnitude and marvellous multiplicity of those influences which the action of light is daily exerting over the varied elements which compose our globe and its inhabitants! This chemical power of light has been recently termed actinism; and actino-chemistry distinguishes that class of chemical phenomena which is excited by its influence. The working of this principle we shall observe in in the process under our notice. From exposure of the polished surface of the silver-plate to the vapour of iodine, a thin film of the ioduret of silver is formed upon it. This is slightly changed by its subsequent exposure to the solution of bromine, as previously described, becoming now the bromo-iodide of silver. This substance is changed by the action of light, and rendered capable of receiving and retaining vapour in proportion to the intensity of the rays which have fallen upon it. The precise manner in which this change is effected is still a matter of doubt. But we may venture thus far to speculate: The bromo-iodide of silver upon the plate's surface has been formed, it will be recollected, in the dark: no light has as yet fallen upon it. Were a ray to be admitted through yellow glass, or any transparent yellow medium, still there would be no change. There would be abundance of luminosity in this instance, but no chemical action proving to us that these two influences, luminosity and actinic power, although both dwelling in the same ray, are separable. Now let us suppose an admission of pure light: The prepared surface is rapidly changing; it may be, assuming a chrystalline structure; at all events, it is so altered in character that, although but the moment previously it had no capacity for the retention of vapour, it now possesses it in an extensive degree. Now once more suppose the light to have been transmitted through a purple or violet medium, the change would still have taken place, although in this instance the condition of luminosity could not co-exist, proving that the change was caused by the actinic power, which resides principally in the violet ray, and entirely independent of luminosity. The other property of light, viz. the power of communicating heat, also forms no part of the agency in effecting this change. This it is equally easy to separate from its adjunct properties; but no detail of experiments can be necessary in order to prove that heat alone, any more than mere luminous influence, is not the agent in producing pictures by the photographic process. We shall now suppose the plate removed to the mercury-box. It was stated that its surface had been rendered capable of retaining vapour, in proportion to the intensity of the rays which have acted upon it. In the case of a portrait, for example, we may imagine the sitter's dress to have exhibited the extremes of white and black, while the flesh-colour of the face and hands furnished middle tints. His image, reflected or refracted, as the case may be, was composed of various rays, proceeding from his person, of different powers. The white sends forth a full strong ray, which rapidly acted upon the sensitive coating; while the black, absorbing every ray, and reflecting none, produced no effect upon the corresponding portion of the plate; and every intermediate tint was proportionably marked accordingly as it partook more of light or shadow. During its exposure to the mercurial vapour, those parts unaffected by the light, for instance the black coat or hair, remain in their original state; that is, in a condition in which vapour will not adhere, and consequently present to the eye the lustrous black surface of the naked silver: while those parts which received the rays of light, becoming readily coated by the minute globules of mercury, which are in this form white, present light tints answering to those furnished by the person of the sitter. This mode of action of course holds good whether the denomination of picture be landscape or architectural, and applies to the practice of either equally well. An The remaining manipulations which the picture undergoes have for their object the removal of superfluous iodine, or of any impurities which may have attached to it during the operation, and consist chiefly in washing it with distilled water and in a solution of the hyposulphite of soda. additional beauty is given by heating the plate over a spirit-lamp, its surface being at the same time covered with a liquid preparation of the chloride of gold, which imparts a warm tinge to the whole, resulting from a combination of the gold with the mercurial deposit. This process accomplishes all that is necessary to the complete finish of the picture. Yet, astonishing as it must appear, it may still undergo another, which converts it into an engraved copper-plate, adequate to the printing of a number of impressions upon paper, and this by the aid, among other things, of electrical action. What an illustration is this of the truth of an allusion made at the outset to the extent of man's dominion over the great powers which nature employs in her operations ! That our very pictures, and these too of the best and most truthful character, should be thus underwritten when correctly described, "Painted by Light," " Engraved by Electricity." There are few subjects which have of late yielded so rich a harvest of discoveries to the inquirer as those connected with the operations of light; and in the course of time, as the result of further research, more confident opinions may be offered respecting the nature of the laws which regulate such phenomena. Every approach to a solution of the wonders which nature presents to us discovers that beautiful simplicity which regulates every one of her operations. Complication and intricacy are no terms of her vocabulary; they are essentially expressions of human ignorance. Composed of few elements which invariably act under certain infrangible laws, the whole universe will present more of truth and beauty to our view, in proportion as we become more acquainted with the principles on which she acts. The knowledge of this fact should excite us to devote some attention to scientific inquiries; and however limited may be our means of doing so, the result will be to some extent an increase of mental power and vision. But let us, while endeavouring to cultivate an intelligent acquaintance with natural phenomena and their causes, never forget the great Author and Ruler of all. To little purpose shall we have augmented our stores of information, unless our conceptions of his power, wisdom, and benevolence are proportionably enlarged. Of these we have abundant proof in all the works of his hands; and whether we regard animate or inanimate creation, shall find sufficient to lead us to admire and adore. Thus, apart from the information to be obtained from a study of these, we may, if prosecuting it aright, learn those moral lessons they were intended to convey; and, in the spirit of language familiar to us all, "Find tongues in trees, books in the running brook, H. T. THE PHILOSOPHY OF HUMAN HEALTH: BEING A VIEW OF THE MEANS OF CULTIVATING AND PRESERVING HEALTH, PROLONGING LIFE, AND PROMOTING TERRESTRIAL HAPPINESS. BY FELIX WEEKES RICHARDSON. "To know That which before us lies in daily life THE object of this paper is not to make our readers their own physicians, but, what is far more preferable, to prevent men from being their own destroyers, by offering such simple, intelligible, and scientific suggestions on the important subject of health as to enable them to perceive why certain circumstances are injurious or beneficial-to understand that disease is not an accident, but has fixed laws, all of which, in the great majority of cases, they can avert; and thus enable them to guard against disease and deformities, retard the premature approach of decay and death, bring to maturity the bodily and mental faculties, render the personal charms and general beauty of the human figure more attractive and permanent, enjoy cheerfulness and real terrestrial happiness, and, in fine, preserve for themselves what all mankind universally and justly esteem the most valuable of sublunary blessings. Health is a jewel so inestimable that too much can not be given to purchase it-the best of riches being a sound and healthful constitution of body. It enables the rich to enjoy the advantages of wealth, and the poor to do much work in a short time, and, by means of industry, to become affluent. But, alas! how rarely do we find mankind bestow the least care, or a single thought, on the preservation of this invaluable blessing, until it is too late to reap any benefit from advice. Without health no one can be happy or useful, either to his family or to his country; and as for the peasant, he must starve, for he can earn nothing, or at least little, very little, and that little must be by slow exhausting toil; and the high and mighty, although living in magnificent dwellings, reposing on downy couches, and in the midst of profusion, yet, when labouring under the miseries of disease, racked by pain, and approaching an early dissolution, must loathe the grandeur, luxuries, and superabundance which surround them-yea, at that time, they would sacrifice pride and all factitious distinctions, part with their wealth, and submit to any surgical operation or other remedies, to again enjoy the pleasure and procure the inexhaustible riches of health, or perchance to add a few more months, weeks, days, or even hours to their existence. Let it not then be said that information of this description, which ought most surely to be considered the first of all human acquirements, and become an essential branch of education-knowledge which has ameliorated and will improve the condition of the human family—is trivial or unnecessary to the non-medical community; for society groans under the load of disease and misery unequivocally induced by causes susceptible of removal, but left in operation in consequence of ignorance and wilful error of, and indifference to, the laws by which we "live, move, and have our being." But once let the people be acquainted with those conditions which will enable them, by a proper attention to what is beneficial, and a careful avoidance of what is hurtful, to take due care of their health, and a great amount of disease, suffering, want, and consequent mental depression will be removed, and there will be a corresponding increase in strength, beauty, cheerfulness, and happiness. It is a pleasing and encouraging sign of the times that medical and other philanthropic individuals are now bringing prominently before the public notice the frightful evils with which the human race is inflicted, and pointing out how they admit of a safe and effectual remedy. In The Report of the Health of Towns Commission, it is stated "that the annual slaughter in England and Wales from preventible causes of typhus, which attacks persons in the vigour of life, appears to be double the amount of what was suffered by the allied armies in the battle of Waterloo." This is the devastation produced but by one disease. The deaths of infants is another great and preventible evil. According to Dr. Combe, "The average mortality of infants among the rich and poor in this country, and with little variation throughout Europe, is about one in every four and a half before the end of the first year of existence." These premature deaths, and the predisposition to disease in children, result from errors in diet, the neglect and mismanagement of those who have the care of them, the injurious practice of giving them laudanum, Godfrey's Cordial, and Elixirs to produce quietness and sleep, and allowing them to have fermented liquors, which, if they do not poison the children, will lay the foundation for future intemperance. About a century since, when the pauper children were received into the London hospitals, twenty-three out of every twenty-four died before they were a year old; so that, out of 2,800 |