above solution of aceto-nitrate of silver, float a sheet of the iodized paper upon the surface of this sensitive bath, leaving it there for about ten minutes. During this interval, having placed the glass or slate of your slider quite level, dip a sheet of thick clean white printing (unsized) paper in water, and lay it on the glass or slate as a wet lining to receive the sensitive sheet. An expert manipulator may then, removing the sensitive sheet from the bath, extend it (sensitive side uppermost) on this wet paper lining, without allowing any air globules to intervene. But it is difficult, and a very simple and most effectual mode of avoiding air globules, particularly in handling very large sheets, is as follows: Pour a thin layer of water (just sufficient not to flow over the sides) upon the lining paper, after you have extended it on your glass or slate, and then lay down your sensitive paper gently and by degrees, and floating as it were on this layer of water; and when extended, taking the glass and papers between the finger and thumb, by an upper corner, to prevent their slipping, tilt it gently to allow the interposed water to flow off by the bottom, which will leave the two sheets of paper adhering perfectly and closely, without the slightest chance of air-bubbles; it may then be left for a minute or two, standing upright in the same position, to allow every drop of water to escape; so that when laid flat again or placed in the slider none may return back and stain the paper. Of course, the sensitive side of the sheet is thus left exposed to the uninterrupted action of the lens, no protecting plate of glass being interposed, and even in this dry and warm climate I find the humidity and the attendant sensitiveness fully preserved for a couple of hours.

To develope views thus taken, the ordinary saturated solution of gallic acid is employed, never requiring the addition of nitrate of silver; thus preserving the perfect purity and varied modulation of the tints. The fixing is accomplished as usual with hyposulphite of soda, and the negative finally waxed.

Dry Process.-In preparing sheets for use when dry, for traveling, &c., I have discarded the use of previously waxed paper,-thus getting rid of a troublesome operation, and proceed as follows: Taking a sheet of my iodized paper, in place of floating it (as for the wet process) on the sensitive bath, I plunge it fairly into the bath, where it is left to soak for five or six minutes-then removing it, wash it for about twenty minutes, in a bath, or even two, of distilled water to remove the excess of nitrate of silver, and then hang it up to dry (in lieu of drying it with blotting paper). Paper thus prepared possesses a greater degree of sensitiveness than waxed paper, and preserves its sensitiveness not so long as waxed paper, but sufficiently long for all practical purposes, say thirty hours, and even more. The English manufactured paper is far superior for this purpose to the French. To develope these views, a few drops of the solution of nitrate of silver are required in the gallic acid bath. They are then finally fixed and waxed as usual.

These processes appear to me to be reduced to nearly as great a degree of simplicity as possible. I am never troubled with stains or spots, and there is a regularity and certainty in the results that are very satisfactory. You will have observed, too, how perfectly the aerial perspective and gradation of tints are preserved-as also how well the deepest shadows are penetrated and developed-speaking, in fact, as they do to the eye itself

in nature. In exposing for landscape, I throw aside all consideration of the bright lights, and limit the time with reference entirely to the dark and feebly-lighted parts of the view; with a 34-inch lens; the time of exposure has thus varied from ten minutes to an hour and a half, and the action appears to me never to have ceased.

The influence of the air-pump in this appears to me very sensible, and deserving of further examination and extension. I purpose not only iodizing, but rendering the paper sensitive with the action of the air-pump, by perhaps suspending the sheet after immersion in the nitrate bath under the receiver of the air-pump for a few minutes, before exposure in the camera, or by some other manœuvre having the same object in view.

I should add, that I have chiefly employed Canson's French paper in iodizing with the aid of the pump. Few of the English manufactured papers are sufficiently tenacious in their sizing to resist the action of the pump, but they may easily be made so; and were, in short, the English paper so far superior in quality to the French, only better sized, that is with glue less easily soluble, even though more impure, there is scarcely any limit to the beauty of the views that might be produced.

There are more minor details that might be given; but I fear repeating many a "twice-told tale," acquainted so little as I am with what is doing; the preceding, however, may have some interest, and whatever is of value is entirely due to our friend M. Regnault, ever so generously ready as well as able to aid and encourage one's efforts.

Ever yours,

JOHN STEWART.

Pau, Pyrenees.

Translated for the Journal of the Franklin Institute.

Note on Two Modifications of Bunsen's Battery; of which the one Augments the Internal Conductibility, and the other the Tension. By MM. LIAIS and FLEURY.

When the diaphragm of a Bunsen's battery is suppressed, the carbon being porous and impregnated with nitric acid, the internal conductibility of the pile is increased five-fold; which, according to the laws of electric currents, amounts to a similar increase of surface without increase of expense. We have shown this fact by the following experiment: An element thus modified caused an electro-magnet to support 58 killogrammes. To support the same weight by increasing the surface of the old pile, it was necessary to connect five Bunsen elements by their similar poles, so as to form one element of five times the surface.

To keep the porous carbon impregnated with nitric acid, we employed the following arrangement: A glass cylinder surrounds the carbon so as to keep an annular cavity between them, which is filled with nitric acid. Clay or cement fastens the two cylinders together at their lower ends. When the carbon is inside the pile, a cavity may be drilled in it. By introducing in the preceding battery of carbon impregnated with nitric acid, a diaphragm, and charging on the carbon side with concentrated sulphuric acid, and on the zinc side with dilute acid as usual, the conductibility of the battery is almost the same as in the Bunsen battery, but the tension is nearly doubled.

If, instead of causing, by means of a single diaphragm, the concentrated sulphuric acid to act directly on the acid at 12°, several diaphragms are interposed so as to enable the concentrated acid to act on an acid less concentrated, and so on by degrees to the acid of 12° in which the zinc is plunged, it will be found that there is a considerable increase of tension, but we have not as yet measured it exactly.

One element of this latter battery behaves like a Bunsen battery of several elements, and costs much less.-Comptes Rendus de l'Academie des Sciences de Paris, 29th Nov., 1852, p. 802.

For the Journal of the Franklin Institute.

The Caloric Engine and the "Mechanic."

In the Mechanic (New York) of March 5th (No. 21, vol. I,) is the following article, which has just come to my notice :

"Erroneous Estimates, in the Journal of the Franklin Institute, upon the Caloric Engine."

"We have noticed in the February number of the Franklin Journal, a comparative estimate upon the caloric and steam engine, which figures out a saving of 51 per cent. for the caloric engine, while in reality it can be shown, even by the same figures, that the saving does not amount to 25 per cent.; and further, if the proper elements were introduced, the results would turn in favor of steam.

"It is very much to be regretted that such an important error should have been allowed a place in the only scientific journal which we now have in this country, and the only one which is considered reliable authority.

"We give below the section of the article referred to."

Then, after quoting from the Journal of the Franklin Institute, the calculations referred to, which are on page 126, he proceeds to make the following remarks:

"In this estimate, Articles 1st and 2d are correct. Article 3d should read-16 lbs. cut off at one-third gives 114 lbs. mean effective pressure; which, added to 124, gives 23.75 lbs. effective pressure.

Eight pounds pressure of air cut-off two-thirds, gives 7.5 lbs. mean effective pressure.

23.75

Then, -=3·16, which gives 2×3×3·16-19 nearly. Then,

7.5

Cutting-off one-third gives

194×2
3

128X60X10.5= 80640.

80640 853

=

94× 62.5 =

=

5875
7.5

783; the caloric being 550. But it is stated on good authority that, had the time been correctly given, the consumption would have been 800 lbs. per hour."

Before the editor of the Mechanic endeavored to criticise statements made in a journal respecting which he holds such flattering language, he should have been careful to assure himself that he understood the principles on which they were based.

The mode of ascertaining the average pressure of steam in a cylinder with a given cut-off is not as he supposes it to be in his "corrected calculation," viz: applying the co-efficient to the initial pressure above the atmosphere, and then adding the vacuum to the result; but to apply the coefficient to the total initial pressure, and deduct from the result the back pressure constantly opposed to the piston by the vapor in the condenser. Thus, 16 lbs. per sq.in. being the initial pressure (above the atmosphere),

and 14.7 that of the atmosphere in round numbers, a vacuum of 12.5 lbs. by the gauge gives a constant back pressure of 14.7 12.5 2.2 lbs. Then, the coefficient for cutting off at one-third being 7, the mean pressure exerted during the whole stroke is (16-+14-7) ×·7—2·2=30·7×·7 -2.2-21.49-2.2-19.29 lbs. per sq. inch.

The pressure given in my article was 19.25 lbs. per sq. in., which was considered sufficiently near the truth for the purpose.

As regards the assumption of 8 lbs. instead of 7.56 for the effective pressure of the caloric engine, the note at the foot of page 126 will show that the difference was fully understood; but the larger number was taken because that difference was slight, and indeed within the limits of imperfect observation, or conflicting reports of the real initial pressure. Finally, whether the consumption of fuel was 800 instead of 550 lbs. per hour, is quite unimportant; the smaller number was too great for economical application, as was shown in my former article.

At the time those remarks were made, we knew less about the caloric engine than at present, and it was supposed by many engineers that success was at least possible, although it might be the work of years to develope it. It was therefore the aim of the writer to show that the results at that time attained, were not such as to warrant any hope of its future success on the ocean, or indeed for locomotive power of any description. Further experience has destroyed all probability of its success in any capacity.

M.

On the Composition of the Substances employed by the Chinese in the Decoration of Porcelain. By MM. EBELMEN and SALVETAT.*

(Continued from page 242.)

The special results of these investigations show

1. That the Chinese have, only a small number of the colors known as muffle colors (colors which fuse a low temperature in comparison with that at which the procelain is baked).

2. That the palette of the Chinese is not composed of true colors, but rather of enamels; these enamels are glasses, that is, compounds of oxide of lead and alkalies, variously colored by a few parts per cent. of coloring oxides dissolved in them.

3. The composition of this glass is but little variable; it is always weakly colored; and it is even this weakness of tone and liveliness of tint which furnish the means by which the Chinese produce harmony and variety of color upon their porcelain.

4. These enamels are colored by oxide of cobalt, oxide of copper and gold; therefore by substances which are all readily soluble in the fluxes, the preparation of which is very simple.

In addition to these, antimony is employd for yellow, tin and arsenic acid for opake white. The Chinese mix these substances with the enamels, and these latter with one another, to produce very various shades of color. Their compound colors however are always referable to five coloring elements,-blue, oxide of cobalt; blue or green, oxide of copper; rose, gold; yellow, oxide of antimony.

If we add to these the very impure oxide of cobalt, which always gives

From the London Chemical Gazette, No. 242.

a blue under the glazing; the same oxide, which is mixed with white lead to be burnt upon the glazing, and then furnishes a black; oxide of iron mixed with white lead or flux, to produce various shades of red; lastly, gold, which is rendered fit to be melted upon the glazing of the porcelain by being mixed with a tenth part of white lead,-we have all the means at the command of the Chinese decorator of porcelain.

The enamels are laid on with water, or sometimes with solution of gum or glue.

The authors attribute the circumstance, 'hat glassy materials, such as these enamels, can be laid upon Chinese porcelain, without removing the glazing, to the peculiar composition of the latter. The composition of this was communicated in their previous memoir. A pure felspar glaze does not suit enamel. The authors attribute the peculiar appearance which distinguishes the Chinese and Japanese porcelain from European entirely to the composition of the glaze.

There is yet another kind of decoration employed for Chinese stone ware or porcelain, in which the objects are entirely colored, either with the pencil or by dipping. This mode of coloring will be considered by the authors in a third memoir.

The processes employed in Europe in the decoration of porcelain are shortly as follows: Sometimes variously-colored pastes are employed, sometimes the coloring matter is introduced into the glaze, and sometimes the colors are laid on the surface of the white porcelain. The two first methods require the application of as high a temperature as that necessary for the baking of the porcelain itself. The colors so employed are therefore called strong fire colors (couleurs de grand feu). But when on the other hand, the colors are laid on the suface of the porcelain, they require for their vitrification a much less elevated temperature than the preceding. These colors are called muffle colors (couleurs de moufle).

They are the only colors which have hitherto presented, for painting on porcelain, resources comparable to those furnished by painting in oil; and it is with these colors, such as they have become by gradual improvement in the course of the last fifty years, that several works of the great masters have been reproduced on porcelain with the greatest exactitude.

The colors employed by the Chinese may also be divided into two similar classes of strong fire and muffle colors.

Some of the Chinese strong fire colors have not yet been reproduced on the European porcelain. They are extremely delicate, and often so beautiful that it is a matter of the greatest interest to be able to imitate them. The authors have nearly terminated their analyses of these colors, but in the present memoir they devote their attention exclusively to muffle colors.

The materials examined by the authors are derived partly from the collection formed in China by Father Ly, partly from the collection of the Ecole des Mines, and partly from that of the Musée Céramique. These colors are particularly employed at King-te-ching. The authors also made use of the colors employed at Canton and its neighborhood, which were partly collected by M. Itier, and partly by Mr. Rutherford Alcock, the English Consul at Shanghai.

M. Itier took his colors from the table of a Chinese painter who was engaged at the time in the decoration of porcelain. They offer a remark