action. Such is the close and widely-pervading connexion of all these great physical phenomena.

Almost the same may be said of Heat in its relation to chemical science. We can affirm it as probable, though not absolutely proved, that no molecular changes ever occur in bodies, whatever their nature or state, without some definite change of temperature. This is seen in the friction of simple solids; and still more strikingly in the recent experiment of Mr. Jowles, where, by strong and continuous mechanical agitation of the particles of water, the temperature of the fluid was raised to 160° Fahr. All chemical action in its essence is molecular change. And heat, like electricity, and ever in close relation with it, is an unceasing agent in decomposing or separating the material atoms of bodies, or in exalting the affinities which tend to unite them. Through chemical action also it may be said to be connected with, or even to create dynamical force, of which steam and other vapours furnish familiar examples; the more striking in the case of steam, from the exact proportion now determined between the degrees of temperature, and the amount of dynamical or motive power acquired. It is a case of simple arithmetic applied to weigh and measure the operations of these mysterious powers of nature.

From heat we pass to the kindred element of Light, as connected with chemical actions and affinities. Already we have seen how well this word kindred is justified, since even the beam which reaches us from the sun-seemingly simple, but concentrating within itself a world of wonderful phenomena-presents these powers in such relation to each other, that they are only dissevered by the different forms of matter on which they impinge. Until of late we knew the connexion of light with chemistry chiefly or solely by its evolution in effect of, and during chemical changes. Now we know it as itself an agent in these changes, and an agent so working as to put to shame all the ruder manipulations of human art. Just twenty years have elapsed since photography, the science and application of the chemical action of light, came into existence. To fix what we transiently see by reflection from a mirror, was the object sought for. Earlier efforts had been made to attain it, but ineffectually. Once attained, every subsequent year has added to the marvellous beauty and perfection of an art, which may especially be called the child of science. Nor can we affirm that it has yet attained its maturity. Whoever gazes on the splendid colours in the solar spectrum, interblended, as they are, with the chemical rays, as well as those of heat, cannot but be seduced into thinking that photographic chemistry may hereafter afford colouring to

our pictures, as well as the simple imagery of light and shade. Sir J. Herschel has already opened out this track of inquiry through the relation of complementary colours to that of the ground tint. In truth, we are still too scantily_acquainted with the wondrous and complex machinery of the solar beam, to warrant abandonment of the object; even were it not probable that the phenomena of colour are due in part to the atomical constitution of the bodies recipient of light, and to the organic structure of the eye itself, forming the last material link with the percipient being. This is, in fact, one of the cases, now frequent in science, where even failure may become a copious fountain of discovery. Negative conclusions have their value as well as positive ones; and we would fain hope that the argument we have pursued in this article, may be some index to the many collateral and connected results, which are sure to be derived from such inquiry. The recent photochemical researches of Bunsen and Roscoe are the best evidence both of what is yet to be learnt, and of those admirable refinements of modern experiment, and appliances of one science to another, through which this knowledge may be finally obtained.'

We cannot quit this curious topic of the relation of light to chemical actions, without briefly noticing other recent discoveries which connect this element with the internal conditions and changes of matter; not perhaps chemically, in the common use of the term, but in such way as to open a new avenue to those higher physical laws which may eventually bind together all these phenomena. We allude here to the researches of Becquerel on phosphorescent and fluorescent bodies; and, still more, to the experiments of St. Victor and Mr. Grove on the molecular impressions and changes produced by light and electricity. These researches, which might seem to give a sort of materiality to light, show it in its emission or exode from bodies

1 We have spoken above of the imperfection of our present knowledge of the physical properties of the solar light, as developed in the spectrum. Even since this article went to press, a Memoir has appeared by Sir J. Herschel, having immediate relation to the curious phenomenon of colourblindness; but in which, with his wonted sagacity, he revises generally the intimate mutual relations of the prismatic colours, bringing some things into doubt, which have been hitherto held as elementary facts in the science; such, for example, as the relations of the yellow and blue to the green, which has been always considered a resultant from them. The observations in this paper on the changes of colour produced by different degrees of illumination have great value; as also the distinction, pressed upon the notice of the experimentalist, between the study of pigments or negative colours, and prismatic or positive ones.

into which it has been previously received; and in the case of phosphorescence and fluorescence, with time of retention alone as the element of distinction. We have room to notice one only of St. Victor's original experiments, which, however, will show the direction and value of these researches. An engraving is kept for some time in perfect darknes. Then one-half of it is exposed to strong sunshine, the other half still sheltered from all light. So prepared, it is taken into a dark room, and the whole surface placed in close proximity to a sheet of very sensitive photographic paper. That half of the engraving which has been exposed to light is reproduced on the paper, while the half that has been screened produces no impression whatever. And further, an engraving exposed as before, and then placed in the dark upon white paper, conveys its impression to the latter, which again can transmit it to photographic paper. Any one accustomed to think on these matters will find in this single experiment a fertile germ of future discovery.

From the molecular changes of chemistry we pass by an easy step to that striking and beautiful symmetry of atoms, which crystallography brings before us;-a new science it may be termed, but already rich in discovery, and closely linked with every other branch of physical knowledge. Frequent and familiar though it is in all we see around us, symmetry, as a physical principle, has hitherto not been duly regarded. When we find that out of some 380 mineral substances, nearly 300 assume crystalline forms; and find, further, the same symmetrical tendency in the innumerable organic compounds of vegetable and animal life, it is clear that a great law lies at the bottom of the whole. The geometry of crystals we owe to Romé de Lisle, Haüy, Wollaston, Weiss, &c., being the first step in the science. Their chemical relations form a vast volume of facts, but all connected and embodied under common laws; while the special influences of light, heat, and electricity on the crystalline structure confirm what we have said of the correlation of these elemental forces, and of their action on the ultimate atoms of the bodies they penetrate and pervade. The numberless and beautiful phenomena connecting light with crystallography, dating from Huyghens, but suddenly and largely extended by the discovery of Malus in 1810, form in themselves a science, and one mathematical in its proofs. The connexion of heat with crystallization is shown not solely in the influence of temperature on the act of crystallization, but also by the later researches of Mitscherlich and others in the alteration of actual crystals by heat;-in some, as regards the measure of their angles; in others, still more wonderfully attested by changes of internal molecular arrange

ment, without any alteration of external form. The relations of electricity, under its magnetic conditions, to crystallography, are not less striking; and they acquire further interest from being connected with the optic axes and with the various phenomena of light transmitted through crystals; as well as with that remarkable distinction between paramagnetic and diamagnetic bodies, which may still, perhaps, be counted among the unsolved problems of this science. The Memoir of Faraday on the Magnetization of Light was the first inroad upon this great field of inquiry, in which he has so eminently since laboured in conjunction with Plücker, Tyndall, and other philosophers. The controversies which have partially arisen out of this difficult research are, in effect, the best pledges for its future progress and ultimate success.

Examples such as those we have given to illustrate the connexion of the physical sciences, crowd upon us from every side. But our space does not allow us to notice more than one other, already adverted to in part, and to which we now briefly recur. This is the physiology of animal and vegetable life;—a subject which, including even Man within its scope, assumes the highest place among the sciences, yet is so closely interwoven with all others, that we cannot stir a single step without reference to those phenomena of mutual action of matter and force which have just been under our consideration. The chemistry of

organization and life is in itself a copious science, enlarged every day by new and more subtle research; and no longer limited to analysis, as in the earlier period of organic chemistry, but carried by synthetical methods to the creation of various artificial compounds, identical. with the products of animal and vegetable life. We have spoken before of the question, whether there be any, and what other powers, than those familiar to us in the inorganic world, concerned in the fabric and peculiar functions of living bodies; and to this question we are now, as before, forced to give an equivocal answer. We cannot detach or define any such independent power, or even conceive of it as a single force, seeing the many and incongruous actions it is required to fulfil. But neither can we, upon our present knowledge, assign these actions to any other power of which we have more certain experience. Electricity, heat, and light are all essential, in one degree or other, in the economy of life, or even to its very existence. Whether operating through chemical changes in the solids and fluids, or through the more mysterious functions of the nervous system, equally is their influence seen in the growth and decay of animal life, in the healthy and diseased states of the living organization. But still we cannot

affirm that any one of these forces is more than a minister or appendage to other powers of a different and more occult kind. Electricity, by the admirable experiments of Du Bois Raymond, Matteucci, and Helmholz, has been, more than any other of these forms of force, assimilated to that power which acts through the nerves and muscular organs. But we have no experimentum crucis on which to found an absolute conclusion of identity. Research in these secret places of science is occupied with effects so momentary, and quantities so inconceivably minute, that it requires subtlety both of hand and head, to obtain clear results, and to derive right inductions from them.

The latter remark applies equally to all that regards the action of Light on living organisms. Recent inquiry shows this influence to be far more extensive and various, both in animal and vegetable life, than had before been supposed. As respects the latter, indeed, every day's familiar observation is an index of the fact ;-simply but strikingly shown in that physical instinct (chiefly connected, if experiment says truly, with the indigo rays) by which plants bend themselves forcibly towards a stronger light, as if seeking to imbibe more of an element essential to their vigour and beauty. The observations of Decandolle, Liebig, Boussinghault, Hunt, &c., have carried this knowledge much further; associating with the action of light many of the most important chemical changes belonging to vegetable life, and notably those which pertain to the various colouring of plants and flowers. Even the different elements of the solar ray have been shown to possess different or unequal power over the vegetable processes. The actinic force, or that of the violet end of the spectrum, quickens germination much more than the luminous. Flowering and fruiting depend especially on the heating rays, as distinct from the luminous and actinic.

The influence of light on animal organization is less easily scrutinized. It appears most strongly marked in the lower parts of the animal kingdom; but ever subject to a certain ambiguity from the conjunction of heat with the luminous element of the solar ray. In man, it has been the general presumption that light finds ingress by the eye alone, and with the sole effect of giving direct and instant vision of things without. But recent inquiry calls upon us to recognise, even in the eye itself, the retention, and probably reflex actions of light, within the precincts of the organ; affording, through what we may venture to describe as photographic impressions on the retina, the only plausible explanation of what have been termed its subjective functions; and of other phenomena, little heeded from their familiarity, but presenting problems of the highest interest to