be struck more softly, both card and shilling will fall.*

It would appear that transparency consists in a perfect facility in the progress of light, through the pores of any substance. Now, if we suppose that heat is material, we may conjecture that the calorific tension, when the vessel is highly heated, is sufficient to overcome the reluctant conducting power of water, and that it darts through the latter by virtue of the velocity acquired by such tension.

I do not wish to give this opinion the air of a theory, but there is one remark of M. Laurent's, quoted by Mr. Tomlinson, which I should much wish to see the basis of a series of experiments; he says that when he used coloured water instead of transparent, the water evaporated much more rapidly.

Now, to those readers of your excellent Journal who have facilities of making such experiments, I would beg to observe, that it would be very desirable to know whether a given quantity of water, at a given temperature, evaporates in a shorter time when coloured or black, than when transparent.

Black we know to result from the absorption of all light and colours from that of some of the rays. Now should we find that blackened water absorbs more heat, or absorbs heat more quickly, than transparent water, it would permit some evidence of the analogy between light and heat, and would also assist us in determining whether the rays of heat from an excessively heated vessel do really pass through the water too rapidly to impart much of their influence to it during the passage.

Professor Leslie made some very accurate experiments on the comparative absorptive power of different substances for heat, but I am not aware whether they extended to water differently coloured.

Whether metals and water become, at a high temperature, electrically excited with the same kind of electricity, and so repel each other, we cannot at present say; but should such turn out to be the

The acknowledgment of such a principle is con. veyed in the story of the Turkish headsman, who was so dexterous in the use of his scymetar, that he could cut through the neck of a culprit without disturbing the head, when he moved his seymetar with great swiftness,

fact, it might furnish us with a reason why the globules of water appear not to be in contact with the heated metal: but be this as it may, it would assist us in the solution of this interesting question, if we could determine whether the same apparent repulsion exists between the metal and coloured or blackened water, as between it and pure colourless water.

At all events, I can scarcely subscribe to Mr. Tomlinson's opinion, that a bed of steam exists between the water and the globule. Your obedient servant,

D.

LIST OF IRISH PATENTS, GRANTED IN THE MONTH OF FEBRUARY, 1837. Michael Linning, of Hill-street, Edinburgh, one of the Clerks of his Majesty's Signet in Scotland, for a certain improved method of operating for the purpose of converting peat-moss and peat-turf or bog into fuel, and ob'aining from it tar-gas and certain other substances or matter. Dated Feb. 25.

William Brindley, of Caroline-street, Birmingham, for improvements in the manufacturing of tea-trays and japanned ware, and in the board or material used therein. Feb, 25.

George Richard Elkington, of Birmingham, for improvement in preparing and applying Indiarubber, caoutchouc, to fabrics. Feb, 25.

NOTES AND NOTICES.

We are not disposed to question the date of the speculations sent us by "A Constant Reader," and admire their ingenuity; but their insertion would involve the Magazine in a controversy not very well suited to its pages.

Errata in our last Number.-Col. 1, p. 474, line 23, for Had matter and motion ever a beginning? If not," read Had matter and motion never a beginning or, if they had."

Col. 2, line 11 from the top, for far, far, "read "so far."

British and Foreign Patents taken out with economy and despatch; Specifications, Dis. claimers, and Amendments, prepared or revised; Caveats entered; and generally every Branch of Patent Business promptly transacted.

A complete list of Patents from the earliest period (15 Car. II. 1675,) to the present time may be examined. Fee 2s. 6dl.; Clients, gratis.

Patent Agency Office,

Peterborough-court, Fleet-street.

LONDON: Published by J. CUNNINGHAM, at the Mechanics' Magazine Office, No. 6, Peterborough-court, between 135 and 136, Fleet-street, Agent for the American Elition, Mr. O. RICH, 12, Red Lion-square. Sold by G. W. M. REYNOLDS, Proprietor of the French, English, and American Library, 55, Rue Neuve, Saint Angustin, Paris.

CUNNINGHAM and SALMON, Printers,
Fleet-street.

Mr. Pritchard, whose valuable labours in the field of microscopic science we have on various occasions noticed in our Magazine, has lately favoured the world with the further results of his researches in a work entitled "Micrographia," which will be found extremely useful to all engaged in this interesting branch of study. To those who already possess Mr. Pritchard's "Microscopic Illustrations" and "Microscopic Cabinet," this will supply all the practical information microscopists can require which is not afforded by those works.

Among the numerous exhibitions of the present day, few have been witnessed with more pleasure, or have conveyed more profitable amusement, than those of the Oxy-hydrogen Gas Microscopeand we have in the " Micrographia," for the first time, an ample description of the instrument, and of the method of managing it, together with figures of the various apparatus for obtaining and holding the gases. From this part of the work we make the following extracts:

"Illumination for Gas Microscopes.

"It has been assumed (which, however, is not strictly true, although sufficiently so for practical uses), that the rays of light emitted from the sun are parallel to each other, and that it belongs therefore to the illuminating portion of the solar microscope merely to divert them from their parallel course, and make them converge toward the object intended to be illuminated. In the case now under consideration, the rays emanating from an artificial light placed at a short distance from the condenser are divergent, and all except the central ones fall obliquely upon the surface of the lens; hence a double operation must be performed upon them before they can be made, as in the former instance, to converge upon the object interposed for illumination; that is to say, it is necessary first to bring them parallel, and then, as in the instance of the solar, to converge them toward the object they are intended to illuminate. This, however, may be effected, as we shall presently see, with a single reflector also. In both cases the main object to be attained is to collect the greatest possible

*Micrographia; containing Practical Essays on Reflecting, Solar, Oxy-hydrogen Gas Microscopes, Micrometers, Eye Pieces, &c. By C. R. Goring, M. D., and A. Pritchard, Esq., M. R. I., &c. Whittaker and Co. pp. 231.

number of rays that can be taken up; to accomplish which with a lens, the surface next the light should be concave, or at least a plano, otherwise the rays nearest its margin will, owing to their great obliquity, be reflected from and not refracted through it. From the numerous experiments I have made in constructing gas-microscopes, I find the best arrangement for the illuminating part, when lenses are employed, to be similar to that shown in fig. 1 (see front page), where a plano-convex lens D is first placed with its flat surface next the light G, and at such a distance from it as to bring the divergent rays nearly parallel; and then, in close connexion with it, a double convex lens 'D, to condense them upon the object B. In some instances I have used three plano-convex lenses, but I do not think the advantage obtained by thus dividing the refractions at the surfaces compensates for the loss sustained by the introduction of an extra lens.+ The contrivance given at fig. 1 answers so completely, that it leaves little room for any im. provement to be effected by a combination of lenses, an angle of light of between 50 and 60 degrees being taken up."

In front of the object B is then placed the different magnifiers for projecting its magnified image upon a screen placed at a distance from it, for the details of which we must refer the reader to the work itself.

The apparatus by which the light is furnished is shown at fig. 3 (front page), which " represents a side view of the apparatus. It consists of a square frame of wood, running on castors, and furnished with horizontal partitions or shelves upon which the bags or bladders, O and H, containing the gases, are to be placed; the upper shelf being usually preferred for the oxygen gas, in order that the stopcock at o may be the more readily adjusted, and the lower shelf for the hydrogen. Small pipes, with stop-cocks, as shown at o, a, i, and h, are annexed to the different bladders, and made to communicate with the jet J. On the upper shelf stands a purifier, bereafter to be described, for freeing the gases of their impurities, so that, without passing through any other intermediate vessels, they may, as soon as generated, be conveyed at once into the bladders, Ó or H, to be used when required. WW are the weights, or sand-bags, pressing, by means of inclined boards, upon the distended blad

+ In cases where the diameters of the condensng lenses are great, and the focus short, it might be an improvement to use fluid lenses; or even, in some cases, Sir D. Brewster's polygonal lenses might be serviceable.

24

ders, and thus, in conjunction with the stop-cocks, acting as regulators for the steady and proportionate supply of the gases. It is scarcely worth mentioning that, during the time the bladders are being filled, the weights and boards must be of course removed.*

"The addition of a few words more on the practical method of generating and purifying the requisite gases will, I think, be in strict conformity with the design of the present tract, and suffice to render this part of the subject tolerably intelligible to the generality of readers. It has been already noticed, that a bottle, termed a purifier, is placed, for convenience sake only, on the stand at P; and, on the other side of it, not discernable at a side view, is stowed a leaden bottle also, adapted for the purpose of generating the hydrogen gas. Fig. 2 represents these two bottles, taken from their respective situations, and now supposed to be employed in the order they are exhibited. Some water and granulated zinc are first put into the leaden generator G (about a pint of water and a pound of zinc will be sufficient), and the purifier P is about two-thirds filled with water. The bottles are then securely corked, and a communication is established between them, by tubes perforating the corks, and brought into connexion with each other by an union-joint at u. In like manner the tube at p is made to communicate successively with the pipe, 1 or 2, of the several bladders intended for use. If a small quantity of sulphuric acid (about half a wine-lassful) be now poured into the funnel at c, a portion of the water will be decomposed, and the hydrogen gas speedily evolved, and pass through the purifier into the bladder or bags H; when the evolution becomes languid, fresh acid may from time to time be added, until the needful supply of gas is obtained.†

"The mode of generating and purifying the oxygen gas is extremely simple, and may be very summarily explained.

The method by which the pressure on the gases is obtained in the apparatus. fig. 3, has been objected to, as it varies with the inclination of the boards to which the weights are attached to remedy this inconvenience, the gas-holders might be constructed similar to the receivers of organ bel. lows. ee Edinburgh Encyclopædia, vol. xv. p. 676, plate 447, figs, 5 and 7.

+ The apparatus, fig. 2, would be found useful for many chemical experiments, and especially for exhibiting the combustion of metals by the mixed gases.

If an iron retort, partly filled with lump manganese, be inserted into a strong fire, as soon as the manganese attains to a red heat it will part freely with the oxygen it contains; and if a communication be made between the retort and the purifier P, by means of a long tube, the oxygen will pass over into the purifier, through 2 and a, into the bladders at O, in the manner before described for the hydrogen. Thus the same purifier will serve for both gases; too much care, however, cannot be taken to retain the gases quite separate from each other, and not to risk the possibility of an explosion. I would caution also against the purifier being more than two-thirds filled with water, lest any of the water should be conveyed to the bladders and destroy them. The quantity of oxygen that can be contained in the bladders suited for the apparatus I have now described, will sustain a light for one hour; that of the hydrogen for about half that time; but since the latter can be very readily procured, little, if any, delay will be thereby occasioned."

The manner in which the apparatus for the combustion of the gases is constructed is shown at fig. 4, "where o and h represent the feeding-pipes of the jet, which are to be attached to the corresponding tubes of the vessels O and H (see fig. 3), designated by the same letters. At S are situated two of Hemming's safety-tubes, containing bundles of fine copper wire, gauze, or asbestos, to cool the gases, and prevent explosion, should any accident occasion the ignited gas to return towards the vessels. When speaking of the return of the gases in the direction of the vessels, and thus forming in the tubes, or vessels themselves, an highly explosive mixture, there is one point to which I have not yet alluded, but which I conceive to be of the greatest possible importance; and it is this-viz that the areas of the bases of the vessels H and O should bear exactly the same proportion to one another, as that in which the hydrogen and oxygen gases, in reference to volume, are required to be used for consumption. The necessity of this will be sufficiently manifest, since more than twice the quantity of hydrogen to that of oxygen is expended in producing the requisite light. Whilst the gases are being consumed, their under surfaces, in the vessels H and O, by the proposed arrangement will be continually kept at

equal altitudes; and since the water ascends to these under surfaces, and the pressure there exerted is dependent upon These altitudes, as before mentioned, the pressure in both vessels will remain uniformly the same. By admitting, therefore, twice as much hydrogen as oxygen to pass through the stop cocks of the feeding-pipes of the jet, the two gases, in their due proportion, will arrive at the mixing-chamber C, pressed forward by equal forces; nor will there be any disposition in the one to overcome the force of the other, and thus to repel the mixed gas from the chamber C, back into either of the vessels, but it will proceed steadily on to the mouth of the jet, and so much of the gases only will be admitted to mix as can be contained in the small chamber at C. R represents the rod upon which the lime, in the form either of a sphere or cylinder, is sustained-(the latter form for ordinary uses is generally preferred.) In either shape the lime, by means of clock-work, or the hand, should be made to revolve, and thus present a new surface to the ignited gas, otherwise it will be liable to burn away unequally, and to burst. The lime cylinder is sometimes placed horizontally, and the flame brought to play upon its base; but this arrangement does not afford so steady a light, and a cavity being soon produced by the combustion, a strong shadow will be thrown upon the screen."

We cannot conclude this notice of Mr. Pritchard's work without extracting descriptions of two other contrivances of his, which we consider as useful as they are novel and simple:

"New and Simple Method of Obtaining a Delicate Adjustment of the Foci of Microscopes.

"In Chapter XV. of the "Microscopic Cabinet," I have described the various methods by which the adjustment of the focus of microscopes is effected; such as the rackand-pinion, the screw, the bent lever, and the excentric,* with their various advantages and defects. I have also minutely described a plan of mine, by which a coarse and fine movement may be readily obtained. This is the primary object to be accomplished. Now although that method is unobjectionable for the microscope, where single lenses or doublets are used, yet it is not so when applied to an engiscope (that is, compound micro

See Treatise on Optical Instruments, p. 36,

fig. 36.

scope), the size and weight of the body to be moved rendering it unsteady. To remedy this defect, various ingenious contrivances have been devised, retaining the rack-andpinion movement, for coarse adjustment, and applying the finer one to the stage which carries the object. All these methods, however, where the desired effect has been obtained, have been so complex, that it is not likely they will come into general use. Under these circumstances, I was led to construct the following, which it is not probable will be surpassed in simplicity or effectiveness, and, what is of great importance, cannot be easily deranged. Fig. 5 is a perspective view of it.

"It consists of a plate attached to the stage of the instrument: on this plate is fixed a socket for holding a fine steel screw with a conical point, which latter acting against a block carrying the object held in a safety slider-holder, is forced upwards by the conical end of the screw acting as an inclined plane, while a spring is so arranged as to keep it down to its bearing. The milledhead of the screw, if required, might be divided, by which means the elevation or depression would become a measured quantity. Improved Mode of Supporting a Candle or Lamp for Microscopes.

"Observers with the microscope are fully aware of the importance of the proper illumination necessary to make that instrument produce the best effect: much depends upon the direction of the illuminating source, and when once properly arranged, any accidental alteration destroys the effect, and the whole must be commenced afresh. From these considerations, I have been induced to attach the holder for carrying the candle or lamp to the stand of the instrument, as shown in the sketch below, by which the not unfrequent movement of the instrument, especially when more than one person is observing, will not affect the direction of the illumination, as all will move together. I am convinced those who have much employed a microscope will appreciate this trifling improvement, and approve of its introduction in this work.

"A, fig. 6, is the stem of the stand of a microscope; on this the socket slides to the required elevation, and the arms allow of the light being placed in any direction. Bisa shade, to take off the direct light from the observer."

ELECTRICAL THEORY OF THE UNIVERSE MR. MACKINTOSH'S REPLY TO KINCLAVEN AND A COUNTRY TEACHER.

Sir, I have read with some astonishment the letter of Kinclaven, inserted in your 710th number, which he seems to