it follows, that if an opaque screen be perforated with circular holes sufficiently minute and near to each other, it would produce a succession of coloured rings. When a beam of light passes through a lock of cotton, wool, or raw silk, inflection and deflection will take place every possible direction, producing a blending of all the colours into white light in the centre, and a succession of coloured rings in receding from the centre. As in the feather there is a regular arrangement of the difracting fibres, there is a corresponding arrangement of the coloured spectra. The explanation of the coloured rings produced by transmitting a beam of light through a lock of cotton, &c., applies to those produced by transmitting a beam through a plate of glass covered with fine particles.

Having satisfied myself with regard to the structure of the vane of the feather, and the mode in which it operates in producing coloured spectra, I concluded, that if that structure could be imitated by any artificial contrivance, the same effects might be produced as by the feather. I shall not detain the reader by detailing all the expedients which I resorted to in the subsequent course of my experiments, but will endeavour to indicate some modes of imitating the structure and effects of the vane of the feather more perfectly than can be done by any means which are at my command. Let it suffice to say, in the mean time, that silk cloth of a close and delicate texture, a dense gauze of fine wire, and similar contrivances, answer as clumsy substitutes for the vane of the feather.

The difficulty of obtaining the necessary materials, and of commanding the requisite mechanical skill, has prevented me from executing the most desirable plans, that have presented themselves to my mind. A convenient mode of arranging parallel fibres is, to bend a steel wire thus, and wind a fine silk thread or delicate wire across its parallel sides. With a contrivance of this kind I was able to produce a row of spectra or fringes in a line perpendicular to the parallel fibres. I made use of a fine silk thread, but it is manifest, that fibres more minute, skilfully arranged, would greatly increase the brilliancy of the phenomena. In this case, it will be seen that the rays undergo two difractions in the same plane. The second set of fibres would increase or diminish the effect of the first set, according as its difracting influence coincides with, or counteracts that of the first; and it is probable, that both of these effects are produced upon different rays. A preferable construction would be to take a rectangular metallic frame, and wind the finest platinum wire across two of its parallel sides, so close as just to admit the passage of light between the parallel turns of the wire. The wire may be fastened by metallic bars screwed down upon it, where it crosses the exterior sides of the frame, and then one set of the parallel turns of the wire may be cut away, so as to leave only one set to act upon a transmitted beam of light. Two of these contrivances might be placed together and turned upon each other, so that the parallel wires in one could be made to cross those of the other at

any convenient angle, and thus the phenomena of the feather would be imitated. The crossing of the wires might be secured by winding the same frame in opposite directions, fastening the wires and cutting them away on one side in the manner above described. The first method, however, is preferable, as it admits the change at pleasure of the angles at which the two sets of parallel wires cross each other. This apparatus would probably be rendered more perfect by using fine hair, or wool, or better still, a single thread of the silk worm, instead of a platinum wire. As difraction takes place at the edges of transparent as well as opaque bodies, probably an apparatus of the kind above described, made of very fine spun glass, would exceed all others in delicacy and power, as refraction in this would co-operate with difraction. Since writing the above, Prof. Sturtevant has suggested the mode of fastening parallel fibres into a wooden frame by gluing pieces of wood upon its exterior sides. I have acted upon this suggestion and constructed an instrument with fine silk thread, which, though immensely inferior to the vane of the feather, produces phenomena similar in kind.

As my frontier location deprives me of the means of attaining the desirable perfection in the constructions which I have described, it is hoped that others more favourably situated, will be able to realise what I have hinted at above. It remains to be determined, whether art, in the construction of a difracting instrument, will ever attain to that perfection which is presented to us by the hand of nature in the vane of the feather. Even with the latter we are able to render the chromatic effects of difraction and interference as conspicuous to a class of students as those of refraction. Prof. Sturtevant lectured a few days ago upon the phenomena of the feather for the first time, to the great satisfaction of his audience.

During the progress of the above investigations, several inquiries have arisen, which Prof. Sturtevant and myself are now pursuing, and one or both of us may be expected to be heard from again upon this subject.

Illinois College, April 16, 1841.

Researches on the Cause of the Movements which Camphor presents when placed on the Surface of Water, and on the Cause of the Circulation amongst Chara. By M. DUTROCHET.*

(First Part.)

1. THE progress which is daily being made in different branches of physical science, tends in general to unite them in a single group, and to show the unity which exists in the universe, Physiology alone holds herself apart, considering the vital forces as entirely

• Comptes Rendús, January 1st, 1841.

different to those forces which are obeyed by the inorganic world. For my own part, I have always thought that if the vital phenomena are not explicable at the present day by the means of physical phenomena, it is because the latter are not all known. Thus, for example, the forces under whose empire the liquids contained in the vegetable world are moved, ought, in my opinion, to be found in the physcial world. Already has endosmose revealed the existence and shown the mechanism of one of these moving forces, but endosmose will not account for all the movements which the liquids contained in vegetables present. The movement of circulation which is observed in the central tube of each merithalle of chara and in the interior of the cellules of many plants, cannot at present be explained by any physical force known. This rotating movement in closed cavities, and the movement of the later which takes place in the anastomosé vessels, and which has received from M. Schulk the name of cyclose, certainly both depend on the same moving force, but this force is altogether unknown, both in its nature and in the mode of its action. When studied amongst the chara, this force has been considered by certain physiologists as being of an electric nature; but there is nothing to justify this allegation, which is purely hypothetical. The researches which have been made in common, by M. Becquerel and myself, by putting together the force which presides over the circulation of chara and that of voltaic electricity, have even tended to remove to a greater distance the idea that this circulation was due to electricity. The latter force, besides, from what we know of it, is altogether different to the singular properties which have presented themselves to me in the force which presides in the circulation of chara. I have shown, in fact,* that the circulation on being arrested by the influence of certain mechanical causes, is re-established spontaneously, without the continued influence of the cause which arrested it; there is then here a phenomenon of habitual movement. The force which presides over the circulation, at first overcome or suppressed, has the property of reacting spontaneously to establish the arrested circulation, and that after a suspension of a greater or less duration. Now, the electrical forces, such as they are known to us, do not offer anything similar to this property of reaction and habit.

2. Nothing is less philosophic, in the researches for the causes of phenomena, than always to endeavour to refer the effects observed to causes which are known to us, as if our feeble knowledge embraced all the causes of natural phenomena. This disposition of the mind is, in fact, the very circumstance which removes us farther out of the way of making discoveries. I have thought it my duty, in many circumstances, to guard especially against this, and more particularly in the study of the phenomena which are in action here. The cause of the circulatory movement of chara escapes all direct

• Observations on the circulation of fluids amongst the chara fragilis; in the Annales des Sciences Naturelles; January, 1838.

research; the moving forces which are known to us do not explain it. I have sought to discover if there was not in physics some phenomena of movement, the cause of which was equally unknown, and which appeared to offer some analogy to the movement which is observed amongst chara. In this comparative examination, a physical phenomenon, very commonly known, presented itself to my mind. I allude to the movement which parcels of camphor present when placed on the surface of water; these parcels, by the very force which moves them on the water, gives a movement to the liquid if they are fixed in such a manner as to remain immoveable. May it not be possible that the moving force which animated them, is equally the same as that which animates the green globules fixed on the sides or interior partitions of the central tube of the chara, from which green globules evidently emanates the moving power to which is due the circulation of the liquid which touches them, and likewise the inert corpuscles which this liquid carries? I resolved to verify this suspicion by research, by leaving completely aside all the explications which had been given touching the movements of camphor on water, of which explications I felt the entire insufficiency. This latter phenomenon was to me, like that to which I compared it, an inexplicable phenomenon, and in order to know if the two phenomena had really any analogy between them, I resolved to submit the movements of camphor on water to proofs analogous to those to which I had submitted the circulatory movement of water, that I might be enabled to see whether the results would be the same. This is what I have executed, descending thus from physiology to physics; and taking from the first of these sciences the principles of experimenting, which I transported into the second, I have had the satisfaction of seeing my expectations confirmed. I shall not follow, in the exposition of my experiments, the order in which they have been made. The new facts which were successively revealed to me, and that often with much difficulty, by these numerous experiments were not those which ought to occupy the commencement of the series in which they ought to be naturally placed: it will then be this natural series of facts to which I shall confine myself. I shall commence by studying the movement of camphor on water, in order to acquire the knowledge of the true cause of it, afterwards, taking successively the different experiments which I have made on the circulatory movement of chara, I shall attempt to apply them by imitation, to the analogous experiments made on the movement of camphor. If in these comparative experiments I obtain similar results, I shall have a foundation to conclude therefrom that the physiological force which produces the circulatory movement of chara, and the physical force which produces the movement of camphor on the surface of water, are identical.

3. The discovery of the movements of camphor on water belongs to Romieu,* who attributed these movements to electricity, by esta

Mémoires de l' Academie des Sciences, 1756.

blishing himself in support of this assertion on erroneous observations. He pretended that if the water on which was floating the parcels of camphor is contained in a metallic vase, we should not perceive any movement in them, whilst the movement would be manifested if the vase is of glass, of sulphur, or of rosin; this is not true. Camphor moves on water placed in vases of any kind of material whatever.

4. Benédict Prévost* rejecting the idea of explaining this phenomenon by electricity, attributed the movements of camphor on water, to an atmosphere of the fluid, odorous and elastic, emanating from the camphor. This effluvia meeting with resistance from the air around it, and likewise from the water, reacted mechanically on the parcel of camphor, and thus impressed it with motion. B. Prévost did not limit his observations to camphor; he extended them to all odorous substances, and he found that parcels of cork soaked in these liquid substances, moved on water precisely in the same manner as camphor. At a later period he announced that camphor, when placed on the surface of mercury, presented the same movements as when placed on the surface of water.

5. Fourcroy, the author of the extract from the memoir of B. Prévost, published in the Annales de Chemie, adds, at the end of this extract, that he thinks the movements of camphor on water may be referred to the attraction of the odorous matter for the air and water, and to the dissolution which is produced in one or the other, or in the two at the same time.

6. Almost at the same time, Venturi+ published his researches on the same phenomenon; he admits that camphor, when placed on water, emits an oily vapour, which unites itself with the superficial parts of this liquid, and that the movements of the pieces of camphor is only the mechanical effect of the reaction of this oil in spreading on the water, exercised against the camphor itself. He saw that sawdust soaked in oil turned on water in the same manner as camphor. B. Prévost observed that camphor evaporated 30 or 40 times quicker when placed on water, than when simply in contact with the air. Venturi confirmed this fact by the following experiment. Having cut pieces of camphor into columns of diminitive size, he plunged them vertically into water, leaving their upper parts above the surface of the water. These columns of camphor underwent an evaporation far more rapid at the parts where they left the water than in the remainder of their extent, which was situated in the air, in such a manner that they were not long in being cut asunder in that place.

7. Carrodorit reviewed the experiments of B. Prévost and of Venturi; he admits with the latter, that camphor owes its movements to the expansion of an oil which emanates from this substance,

• Annales de Chemie, T. 21, N 40.

† Annales de Chemie, T. 21.

Memoires de la Societè Itallienne, T. 11, 12, and 15,-Annales de Chemie, T. 37 and 48.