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come to mix with air in the atmosphere. We cannot, however, doubt that to such fluids is owing the multitude of phenomena still unexplained intelligibly, both in the atmosphere itself, and in its connexions with vegetation and the animal economy, when we consider what progress has been made in this knowledge, by only attending to the chemical affinities of light and fire, and by a beginning of discovery on those of electric fluid, the existence of which on bodies would be unknown to us, were it not for the motions produced in visible bodies by the disturbance of its equilibrium : this is one of its characteristic properties, and our test of the degrees of its intensity in different cases; as the thermometer is for free fire, and vision for light.

These first steps in the knowledge of causes, which are themselves imperceptible, must render experimental philosophers more and more attentive to all the circumstances that may lead to the discovery of new tests of the actual state of the air, in consequence of other impalpable fluids mixed with it; and also to the electric phenomena that may appear in chemical processes, since the electric fluid is always present on the bodies which enter into chemical combinations, as it is present on all bodies: in this diffused state, it produces no known chemical effect; but all the phenomena before pointed out undoubtedly prove, that its compositions and decompositions have the greatest influence in the terrestrial phenomena.

The general character of the system above, extracted from my works already published, by supposing a multitude of still unknown substances, will undoubtedly encounter the disapprobation of those philosophers who consider simplicity as the characteristic of the operations of nature : but if this word, has any sense, it must signify enough and nothing more ; therefore, the first condition is enough, and when, in certain phenomena we find a deficiency of known agents, the chasm is not to be filled up by arbitrary hypotheses, which are nothing; analogy is our only sure guide in the investigation of hidden causes, as being a thread offered to us by nature itself.

This is one of the precepts of the father of true philosophy, the immortal BACON, who taught us, in particular, not to dread the multitude of substances when they are wanting for the production of phenomena accurately defined. Among his remarks on this subject is the following, under the 98th head of his Silva Silvarum ; which remark I have the more admired the longer I have studied the phenomena of our globe :-“Cognitio humana determinata hactenus fuit speculatione et visu ; ita ut, quicquid oculos fugeret, sive propter tenuitatem corporis, aut partes exiles, aut subtilitatem motus, parum sit exploratum. Hæc tamen naturam maxime regunt, illisque posthabitis, vera analysis institui nequit, aut indica rinaturæ processus. Spiritus aut pneumatica (expansible fluids) quæ omnibus tangibilibus insunt, vix cognoscuntur...... Spiritus enim nihil sunt præter corpora naturalia, proportionaliter rarefacta, tangibilibus

corporum partis inclusa velut tegumento : neque minus inter se differunt, quam densa et tangibiles partes, omnibusque tangibilibus corporibus insunt plus minusve, et plerumque nunquam cessant. Ab his, eorumque motibus, præcipue procedunt arafactio, colliquatio, concoctio, maturatio, putrefactio, vivificatio, et præcipua naturæ effecta."'*

Not to admit the existence of such substances, because they escape our sight or touch, would be returning back to occult properties, essential qualities, which, in the infancy of natural philosophy gratified the imagination under the shape of causes. However, these conceptions were a beginning of knowledge, as under that form were gathered a certain number of important phenomena, successively observed ; but of these the agents were still to be sought for. However, it has been only at the birth of pneumatic physics, and when its progress has occasioned the investigation of the chemical affinities of light and fire, that many mysteries in nature have been unfolded ; and what a field of new researches has been opened by the attention given to a third imponderable substance, the electric fluid ! Now these very great steps teach us that no progress, marked by such memorable epochs, and followed by so many important consequences, can be expected, but by farther discoveries in the same class of substances, some of which may happen to manifest themselves also by characteristic effects, either known but mistaken, or yet unnoticed ; and in these cases they might in some degree be submitted to analysis, by the changes they operate in certain phenomena, already known, but not sufficiently determined.

It is not to be expected, that, by groping in a desultory manner among the objects of nature, any main road of investigation can be opened for the discovery of new causes ; as their effects are so much intermixed in perceptible phenomena, that we cannot ascend to them with certainty in a retrograde manner. Many more discoveries concerning them may be expected from researches carried on by connected steps along the roads already opened in the maze of imponderable substances, the greatest agents in the phenomena of nature.

The modifications of the sun's rays to produce heat, as followed by M. de Saussure and Dr. Herschel, and I may say by myself ;

• “Human knowledge has hitherto been guided by viewing and beholding; so that whatever escapes our eyes, either from the smallness of the body itself, the tenuity of its particles, or the subtilty of its motions, is but little explored. By these, however, nature is chiefly governed; and if they be neglected, a just analysis cannot be made, or the process of nature disclosed. The expansible fluids that exist in all tangible substances, are scarcely known. These fluids are nothing but natural bodies, proportionally rarified, included in the parts of tangible substances as in a case: nor do they differ less from each other, than the dense and tangible parts, they inhere more or less in all tangible bodies, and for the most part are never still. To these, and their motions, are owing in particular rarefaction, dissolution, concoction, maturation, putrefaction, vivification, and the principal effects of nature.”—C.

as well as the first observations made by Dr. Priestley on the chemical effects of light, have opened one of these roads, which requires to be pursued in all its ramifications. Much is to be done also concerning the nature of fire, i.e. the cause of heat, or of that expansion of bodies of which the thermometer is the measure ; a road which has been much obstructed by the obscure idea of caloric, introduced in the modern theory of chemistry, at the time when several experimental philosophers were engaged in researches concerning the nature, modifications, and combinations of the expansible fluid long known under the name of fire. Much more remains to be done in the study of the electric fluid, its production and decomposition throughout so many phenomena. Lastly, almost every thing remains to be done to acquire some knowledge of a fluid, the existence of which is manifested by some characteristic effects, but which is itself totally unknown; though it cannot be without some, and it may be a great influence, in terrestrial phenomena : I mean the magnetic fluid, on which I shall say here only a few words.

Being now informed that the motions of bodies occasioned by amber when it has undergone friction, of which the cause was unknown to the ancients, are the effects of a fluid which has much greater functions in nature by its compositions and decompositions, when we come to consider the analogous, though much more limited effects produced by steel bars which have undergone proper frictions, we are led to conclude, not only that these particular motions are also the effects of a particular fluid, but that its functions in nature are not confined to those of attracting or repulsing iron according to c'rcumstances, and producing in a moveable needle the property of keeping more or less parallel to the meridian of the place, with a determined end pointing towards the north : though the latter, by its use in navigation, is become of great importance.

With respect to this astonishing phenomenon, Prof. Van Swinden, of Franeker, has much advanced what Bacon calls the history in every class of phenomena, by an indefatigable perseverance in observing the variations of the magnetic needle compared with various circumstances. This, for every phenomenon, is the first step towards the discovery of causes; for the nature of those that may be devised must answer to all the modifications of the phenomena carefully observed, before confidence can be granted to any hypothesis.

In magnetism, the main point which must direct the natural philosopher in search of a cause is the same which directs the navigator, namely the direction of the magnetic needle ; for this must belong to a cause, which in some manner influences the whole earth. This consideration has suggested to Prof. Prevost, of Geneva, an idea, which, though not completed, deserves notice. After all the discoveries already made in meteorology and chemistry, it cannot be doubted, that light has, in various ways, a great share in the formation of many atmospheric fluids, and thus probably of the magnetic: but there must be some cause of the formation of a greater quantity of it on the northern than the southern hemisphere of the earth, since the magnetic needle has a tendency to turn that way. I shall not enter into particulars on M. Prevost's hypothesis, and shall only mention its ground, in order to show, that this object may not be unattainable ; it is the circumstance, that the sun remains about eight days longer on the northern side of the ecliptic, than on the southern.

With respect to phenomena which may indicate a formation of this fluid, M. de Saussure has invented a very important instrument, which he has called a magnetometer; showing variations in the intensity of attraction of a magnet in different parts of the day, and also in different days and seasons, as the aërial electroscope shows variations in the electric state of the air in the same circumstances. These two kinds of variations, therefore, deserve to be followed, comparatively with each other, and in their connexion with other atmospheric phenomena, as these observations may forward our knowledge respecting the magnetic fluid, which probably, as well as the electric fluid, by its composition, decomposition, and combinations with other substances, has an influence in terrestrial phenomena.

The loadstone with respect to magnetism, and the tourmalin to electricity, are bodies which produce these phenomena from their own nature; but there is a method in our power to produce them by other bodies, namely friction : it is therefore very important in either case, to discover in what manner friction acts to produce these effects. We have yet no hold in this pursuit with respect to magnetic phenomena, but some light may be reflected upon them by a determination of the manner in which friction produces electric phenomena. I have studied this subject with much attention, and I propose to relate, in another paper, some experiments of this kind, leading to the analysis of the electric machine, and demonstrating the error of the idea of two kinds of electricities, or of two fluids acting in the electric phenomena. Ashfield, near Honiton,

1st October, 1810.

Preparation of Perchloric Acid. By M. NativeLLE.

(Extract from the Journal de Pharmacie.) M. NativELLE first takes a general view of the usual method of preparing perchloric acid, and remarks, that “if the operation be closely observed, it will be found that the water added to the sulphuric acid is useless, or nearly so, and that the quantity of sulphuric acid employed is insufficient to accomplish the decomposition of the whole of the perchlorate of potash : because in this case, contrary to the ordinary law of substitution, the influence of mass, or an union of forces, if I may be allowed the expression, is required to overpower the pre-existing affinity; the perchloric acid, under certain

circumstances, being capable to displace the sulphuric. Having arrived at this inference, it only remained to ascertain the proportion of sulphuric acid necessary to decompose the whole of the perchlorate of potash. This proportion I will now state, which, although at first sight it may appear arbitrary, is nevertheless indispensable for obtaining a maximum of perchloric acid.

Place in a glass retort 500 parts of perchlorate of potash reduced to powder, and as free as possible from the chlorate. Add to this 1,000 parts of sulphuric acid at 66°, free from nitric acid : afterwards add 100 parts of distilled water. This small quantity of water is indispensable, otherwise the perchloric acid would be produced in a solid crystalized form. Fit to the retort a long adapter, terminating in a tubulated receiver, surrounded by cold water. Avoid luting the apparatus with paper, or other organic substance ; for, when heated in contact with the vapour of perchloric acid, these substances would cause its decomposition, attended by feeble detonations. If the apparatus be well adjusted, lutes may be dispensed with ; but if found necessary, asbestos will be most suitable.

By a careful application of heat, the whole of the perchlorate will soon dissolve; but attention will be necessary in regulating the heat, to prevent sulphuric acid from distilling over to the receiver, which is accomplished by keeping the contents of the retort below the boiling point. Perchloric acid rises in vapour at 316° Fah., a temperature much below that at which sulphuric acid distils. The end of the distillation may be known by the residue in the retort being transparent and colourless ; or still better by the distilled drops falling slowly, even when the temperature is nearly sufficient to distil the sulphuric acid.

The quantity of perchloric acid obtained will vary accordingly with the manner of carrying on the operation, which will also regulate the quantity of sulphuric acid that is carried over. When the above described process is conducted with care, the 500 parts of perchlorate of potash will yield 300 parts of impure acid of about 45° density. If the distillation be pressed on too rapidly, the density of the impure acid will be nearly that of sulphuric acid, and the quantity equal to that of the perchlorate employed.

It is a very easy matter to purify this acid, by removing the sulphuric acid and the small quantity of chlorine which it holds in solution. To accomplish this separation, the impure acid is first to be agitated with a slight excess of saturated solution of sulphate of silver: the chlorine will be precipitated with the silver. This chloride of silver is to be separated by a filter and the acid placed in a large capsule, ready to recceive artificially prepared hydrated carbonate of baryta, which, being well washed, is to be added to the liquor, until the whole of the sulphuric acid has been precipitated, and a small quantity of perchlorate of baryta formed. This precipitate being also separated by the filter the liquor will now contain nothing but perchloric acid, combined with a small quantity

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