Vifion-Military Works.-Inftruments.

89

preffure is greateft; and that, contrary to the obfervable motion in a wheel, the parts nearest the axis in the prefent cafe move the swiftest *.

XI. On vifion.-The inverfion of images formed in the darkened chamber by rays admitted through a small hole in a plate of metal are explained by a diagram, and applied to the phenomena of vifion. It is alfo remarked, that the apparent magnitude of remote objects may be enlarged by rendering the pencils of light more convergent. This is fhewn by a figure, but the means are not perfpicuously explained.

XII. On military architecture.-Under this title the author gives directions for constructing military works; and confiders their relative properties with regard to attack and defence. He exhibits, as his commentator remarks, a fketch nearly complete of the art of fortification, and the attack and defence of places, at the commencement of the fixteenth century. We obferve platforms, buttreffes or counterforts, ravelins, the glacis furrounding the ditch, trenches, advantages of the ricochet, mines and countermines, and in fome defigns of de Vinci there are even counterguards. It must however be remarked, that Leonardo was much fuperior to the engineers of his time; in proof of which his obfervations in this fection may be compared with what Machiavel delivers on the fame subject in his Art of War, and what Albert Durer has written upon fortification †.

It is pretended that the baftions of Verona ‡ were the first which were constructed with right-lined faces, flanked in the modern fashion; and the merit of this invention has been afcribed to San Michel. The bastions of Verona were however conftructed after the death of Leonardo, who had explained the neceffity of that difpofition, and has marked embrasures in each flank of the baftion in one of his figures.

Geufs, in his Théorie de l'Art du Mineur ||, reproaches Vallière for having robbed Peter Navarro of the glory of the invention of mines, to attribute it without foundation to Francis Georgi§. Vallière may be defended by the teftimony of Biringuccio, who was the countryman and contemporary of Georgi. He exprefsly fays ¶," that it was this engineer who, being confulted by Navarro refpecting the means of taking the caftle of Naples, proposed and executed the mines, and that the glory remained with the commander General Navarro, because renown disposes itself on the fide of great men." But, in fact, the invention of mines with gun-powder is more ancient than either of these perfonages. The Genoefe executed them in 1487 **. The manufcript N° 7239 of the French National Library was written by an Italian who lived at the commencement of the 15th century. The author explains very clearly (fol. 32) the manner of taking a caftle fituated on a mountain, by the operation of mines t.

XIII. Concerning certain inftruments.-1. The proportional compaffes. Its centre is moveable; and, as the author remarks, it may be used for the proportions of incommenfurable quantities, and alfo to form an oval which shall have a given ratio to a given

* Profeffor Venturi has treated this fubject in his Recherches Expérimentales, &c. p. 58, for an account of which work fee our Journal, I. 525.

+ De urbibus arcibus caftellis, fol. Paris, 1535.

In 8vo. Maftricht 1778. § 21.

Pyrotechnia, liv. 10. cap. 4.

Maffei Istoria di Verona, part. 3, cap. 4.
Folard, tom. iii.

** Guicciardini Iftoria, lib. vi.

++ Mr. Venturi gives a more particular account of this manufcript and its contents in the notices at the end

of the prefent treatise.

VOL. II.-MAY 1798.

N

circle.

Greek Fire.Extraction and Purification of Oil.

circle. M. Venturi informs us, that from a drawing in the manuscript it does not appear to have been different from the inftrument now in ufe. 2. An inftrument to fhew the conftitution and denfity of the air, and when the weather inclines to rain. From the figure it appears to have been a balance loaded at each end, and provided with a graduated circle to fhew its changes of inclination. M. Venturi thinks that one of the weights in equilibrio was a body proper to become charged with the humidity of the atmosphere. This does not very well agree with the confideration of denfity, though it must be confeffed that the equal magnitudes of the two weights in the drawing do not give much countenance to the notion of its having been a statical baroscope. 3. A leathern cafe or clothing for a diver in the pearl fishery. 4. A piece (bâton) which locks alternately in the teeth of a crown wheel to operate like the verge of the balance in clocks; but for what purpose is not faid.

--

XIV. Two chemical proceffes.—1. A flaming ball compofed in the following manner :Take the charcoal of willow, nitre, brandy, refin, fulphur, pitch, and camphor. Mix the whole together over the fire. Plunge a woollen cord in the mixture, and form it into balls, which may afterwards be provided with spikes. Thefe balls being fet on fire, are thrown into the enemy's veffels. It is called the Greek fire, and is a fingular compofition, for it burns even upon the water. Callinicus the architect taught this compofition to the Romans (of Constantinople), who derived great advantage from it, particularly under the emperor Leo, when the Orientals attacked Conftantinople. A great number of their veffels were burned by means of this compofition.

2. Oil fit for painting. Choose the most perfect nuts, take off the shell, soak them in a vcffel of water to separate the skin; then leave the kernels in clear water, which is to be. changed as often as it becomes turbid, which may be fix or eight times. After a certain time, the nuts on being stirred will be found to mix with the water, and form a folution refembling milk. Expofe this in plates to the open air, and the oil will rife to the surface. In order to obtain this oil very clean and pure, provide cotton wicks, one end of which is to be dipped in the oil, while the other part paffes over the border of the veffel, and defcends into the neck of a glass phial two inches below the furface of the liquor in the plate. The oil will by degrees filtrate along the wick, and pafs very fine into the phial, all the impurities remaining in the plate. All the oils of grain and of nuts are equally clear. It is the manner of extracting them which renders them foul.

The compofition of the Greek fire here given by Vinci, is found nearly in the fame words in Baptifta Portat, whence it appears that both authors derived their information from the fame fource. The modern discoveries in chemistry refpecting combustion have disclosed the whole fecret of compofitions which burn without accefs to the atmosphere, but by means of oxygene afforded from nitre. The balls defcribed by Frezier ‡, which fhine on the furface of water, are nothing else but the Greek fire.

XV. Concerning method.-It is always of advantage to the understanding to acquire The interesting commentary or note on this laft paffage has already been given, for the most part, at page 51 of the prefent number.

+Mag. Natur. lib. xii. cap. 2.

Des feux d'artifices, edit. de 1747, p. 355. See also p. 332.

knowledge

Remarkable Changes of Affinity by Cold.

91

knowledge whatever may be its nature; we may afterwards choose the good, and reject the ufelefs. (Qu.?)

Theory is the General; Practice is the Army.

Experiment is the interpreter of the contrivances of nature. This interpreter never deceives us. Our judgment fometimes mifleads itfelf in expectation of confequences which experiment refuses to verify.-We must recur to experiment, and vary the circumstances until we have deduced general rules, for it is experience which affords the true rules.--But you may afk, What is the advantage of these rules? I reply, that they direct our researches into nature, and the operations of art. They prevent us from deceiving ourfelves or others by the promife of refults which cannot be obtained.

There is no certainty in fciences to which fome part of the mathematics cannot be applied, or which do not depend in fome manner upon mathematical knowledge.

In the ftudy of fciences which depend on the mathematics, thofe who do not confult nature, but authors, are not the children of nature. I would call them her grand-children. Nature, in fact, is the fole leader of true genius. But fuch is the folly of men. He who chooses rather to learn from the original fource, instead of applying to authors, becomes an object of derision for his fingularity *.

IX.

Obfervations and Experiments on the Formation of Sulphate of Soda, or Glauber's Salt, in Salt Waters, at a Temperature beneath the Freezing Point of Water, and upon an eafy Method of difengaging all the deliquefcent Salts. By M. GREN ↑.

THE influence of temperature upon the established laws of mutual decompofition of

the neutral and middle falts, is a circumftance very effential to be obferved. Many falts are not decompofed above the temperature of freezing water, though very well beneath that temperature. Such, for example, are the fulphate of alumine and the muriate of foda; the fulphate of magnesia and the fame muriate.

The formation of Glauber's falt during congelation in falt water, wherein previous analyfis did not difcover it, had long fince engaged the attention of the author. He concluded that the fulphate of foda owed its origin to the mutual decompofition of fulphate of lime and muriate of foda, at a temperature below zero. A more accurate analysis of these waters after the fevere cold of 1794 and 1795, proved that he was mistaken, and that the fulphate of foda had been formed by the double decompofition of the muriate of foda and the fulphate of magnesia. The exceffive cold of 1785 had already exhibited this decompofition to Scheele, and he had shown that fulphate of foda was produced from a mixture of two parts of the solution of fulphate of magnesia, and one part of the muriate of foda, exposed to the temperature of ice.

* Professor Venturi intends to publish as speedily as circumstances will permit, in three complete treatifes, the whole works of De Vinci un mechanics, hydraulics, and optics. N.

+ The memoir of M. Gren was addreffed by himself in manufcript to the Council of Mines in France, who published it in the xxviith Number of the Journal des Mines. Citizen Defcotils abridged the same in the xxivth volume of the Annales de Chimie, page 121. The above paper is a tranflation from his abridgment. N. N 2 M. Gren

92

Easy Methods of rendering common Salt very pure.

M. Gren was convinced by experiment, that the fulphate of magnesia in falt waters forms at a fufficient degree of cold all the fulphate of foda which can then be afforded, and of which they did not before contain the smallest particle. He endeavoured, but constantly without fuccefs, to reproduce the muriate of foda, and the fulphate of magnesia, by an elevation of temperature. So that falt waters which contain fulphate of magnefia change their nature; and their compofition is no longer the fame when they have been fubjected to fome degrees of cold below the freezing point; and a fubfequent increase of heat will not restore their former state.

The quantity of muriate of magnesia in falt waters which contain fulphate of magnesia, is increased by a temperature beneath zero. It is therefore productive of a real lofs to fuffer the falt water to remain in the bafons during the winter, because they will afterwards afford a muriate of foda of a bad quality, and the fulphate of foda which is obtained will not compensate for this disadvantage. It appears likewise that the hope of concentrating fea water by froft is chimerical.

It was an important question to be refolved in the art of making falt, what might be the means of feparating the deliquefcent falts even before the evaporation. In this respect M. Gren has fucceeded. His proceffes are founded on the decompofition, well known to chemifts, of muriate of lime by fulphate of foda, and muriate of magnesia by cauftic lime. If the water contain only muriate of lime with the muriate of foda, the fulphate of foda is fufficient: if there remain muriate of magnefia and fulphate of foda, lime will fuffice.. In this cafe muriate of lime is formed, which afterwards decompofes the fulphate of foda by double affinity.

If the water contain the muriate and fulphate of magnesia without fulphate of foda, it will be necessary to employ lime and fulphate of foda at the same time.

The fame substances must also be employed, if the water contain the muriates of lime and magnesia together; which then entirely exclude the fulphates of foda and of magnesia. Lastly, if the fulphate and the muriate of magnesia be prefent with the fulphate of foda, lime only need be employed, unless the quantity of the fulphate of foda be found infufficient. In this cafe a portion of the falt must be added.

These proceffes introduce no foreign falt into the waters, because they are already faturated with fulphate of lime, which does not render the muriate of foda more impure, because it separates in the state of fchlot. They have the advantage to clear it of the fulphates of foda and magnesia, which alter the purity of the muriate of foda, and render it bitter. Befides which, in the practice, no mother water remains. The whole of the fluid may be evaporated to the last drop, and all the falt extracted, which will conftantly be pure marine falt. The directors of falt works will eafily perceive how much profit will thus be obtained with regard to time and fuel confumed in the evaporation and drying of the falts. But this is merely a fecondary profit, greatly inferior to the principal advantage of obtaining a pure falt not deliquescent, but capable of resisting the impreffion of the air, and of being preserved and transported without loss.

The strongest objection against the use of these proceffes may arise from the necessity of purchafing fulphate of foda; and this objection would be very well founded, if the falt waters were not for the most part of such a nature as to afford it themselves at a small expence; that is to fay, by their maceration at a temperature below zero. It would be fuffi

New Bark for Dyeing, called Paraguatan.

93

cient to leave a certain quantity in the bafons expofed to the cold during the winter. It would not indeed be proper to extract the common falt from these waters, after having feparated the fulphate of foda, because they would then contain too great a quantity of muriate of magnesia. It would be more advantageous to add fulphate of iron to prepare the fulphates of foda and of magnesia.

With regard to fuch waters as do not contain any fulphate of magnefia, which is very rare, it would still be of advantage to purify them by these means; because the water must be very bad, and highly charged with deliquefcent falts, to require more than fifty or seventy-five pounds of fulphate of foda for thirty quintals of falt obtained by evaporation.

In order to make ufe of this method, it is proper to be well acquainted with the prin 'ciples which enter into the compofition of the water intended to be purified. It then will be known whether lime alone, or fulphate of foda, or both fubftances together, ought to be used. Immediate experiment must afterwards determine what quantity ought to be used to insure the most perfect fuccefs.

X.

A Report made to the Council of Trade and Mines in Spain, on a new Kind of Wood for Dyeing, named Paraguatan. By D. DOMINIQUE GARCIA FERNANDEZ, Inspector of Coinage *.

IN compliance with the order of the Supreme Council of Commerce and Mines I have

undertaken a chemical examination of the wood known at Guiana by the name of Paraguatan; which I have carried as far as appeared neceffary to ascertain the nature of the wood, and the advantages to be derived from it. In the first place I obferved that the bark, the wood properly fo called, and the leaves of paraguatan afford different colours. The leaves do not deferve notice, because they produce only a false and disagreeable colour. My researches were chiefly directed to the bark, which is the part most deserving of attention. But my remarks upon the bark may alfo be extended to the wood; for, though this affords a different colour, it presents nearly the fame phenomena as the bark itself.

When the bark is boiled in water it affords a coloured extract, which, when expofed to the action of the fulphuric, muriatic or nitric acid, refifts their agency for a longer time than brazil or logwood does. The colour may be revived by means of alkalis, after it has been deftroyed by combination with acids.

Vinegar, lemon juice and tartar render this colour more brilliant. They give it a fine rofe colour; whereas these acids entirely destroy the colours of brazil and logwood.

The fecula of the bark of paraguatan fixes and attaches itfelf to wool, cotton, and filk. The colour is brighter on filk than on wool, and brighter on wool than on cotton. The fame fecula dried is afterwards foluble in alcohol. It communicates to it a tinge fimilar to that afforded by cochenille.

By mixing alum with a very concentrated decoction of the fame fubftance, a kind of

Tranflated from the Spanish by Cit. Venturi, Annales de Chimie, XXIII. 320.

lac