projected upon live coals, it burns like the oils: on boiling it with the red, yellow, and white oxides of lead, it only dissolves the yellow one: when distilled several times with the nitric acid, it produces in it the formation of the oxalic acid; when distilled in a retort in the open fire, a part ascends in distillation, as observed by Scheele: on increasing the fire, it gives empyreumatic oil as a result, acetic acid, carbonic acid, carbonated hydrogen gas, and a slight spongy charcoal, which does not contain any oxide of lead.

From what I have described, it is strongly to be presumed that oil, when combined with the white oxide of lead, is no longer in the same state as it was before this combination.

In order to separate it from this oxide I made use of the acetic acid, because the solubility of the acetate of lead afforded me an easy method of separating it from the oil, the properties of which I was about to examine.

This oil has the consistence of fat, having also the same rancid taste: it is insoluble in water, and soluble in alcohol, and is precipitated by water in the same way as the volatile oils, and like these last is volatilized in part with the oil in distillation *.

The slightest ebullition is sufficient for combining it perfectly with the white oxide of lead, and gives it a strong emplastic consistence, which does not take place with litharge and massicot.

The yellow and white oxides of lead cannot be combined with the common oils; I ascertained this fact by an ebullition much stronger than if I had employed litharge.

It results, therefore, from these experiments, that when we treat the fat oils with litharge, the oxygen of the latter carries off their carbon, and previously their hydrogen, in order to form water and carbonic acid.

That this subtraction, rendering oxygen more abundant in the oils, gives rise to that saccharine substance which Scheele calls the sweet volatile principle of oils.

That this sweet principle differs from the mucoso-saccha-.

*All the fat oils are dissolved in alcohol; those, however, which have been treated with litharge are much more strongly characterized with this property,

rine by the property it possesses of dissolving the yellow oxide of lead; that its saccharine taste is independent of the presence of the oxide; that it differs from sugar by its volatility, and by the impossibility to ferment it.

That oil deprived of the principles which have given rise to the sweet principle, and of the quantity of hydrogen and carbon which constitutes it a fixed oil, acquires several of the properties of the volatile oils.

Finally, that this last state of oil is the only one which can be combined with the white oxide of lead.

From the knowledge I have acquired of the theory of this combination of the oils, I did not think it right to neglect to ascertain to what extent the opinion of several chemists is founded, who consider the plasters as true metallic soaps. The analogy between the plasters and the soaps can only be verified by observing in their respective combinations a resemblance in the phænomena, or at least in the results.

I mixed some pure soap-makers' ley with olive oil; I exposed this mixture to the air under a bell glass. Eight days afterwards there was only a slight absorption; the soap had still a strong alkaline taste, and the oil of this soap was not entirely dissolved in alcohol: but at the end of six weeks the absorption of the oxygen was complete; the soap was very white, of a good consistence; the alkaline taste was but feeble; diluted sulphuric acid liberated carbonic acid from it; the oil proceeding from this decomposition had the same consistency with that from the plasters, was dissolved cold in alcohol with the greatest facility, and was precipitated from it by water.

I made soap in the same way as the soap-makers; I examined with the greatest care the liquor remaining after the operation was finished, but I could discover no trace of sweet principle.

As the absence of this principle in the alkaline soap-making probably depends only upon a greater or less subtraction of carbon or hydrogen, and the action of oxygen upon oil, and the state of the oil, are absolutely the same in the making of plasters as in soap-making, I think the plas-,

ters

ters should be considered relatively to the soaps as the insoluble metallic salts are relative to the alkaline salts.

I am convinced that the defect in the consistency of the soaps of potash by no means depends upon the state of the oil, but rather upon the kind of combination; for I never obtained any thing but a soft soap on treating by potash some oil proceeding from a very dry soap of soda.

XI. Description of the Mountain Barometer, invented by Sir HENRY C. ENGLEFIELD, Bart. F.R.S. and made by Mr. THOMAS JONES, of Mount Street, Berkley Square.

SIR,

To Mr. Tilloch.

THE various advantages which are likely to be derived from taking altitudes of every description, in a short time, with very little trouble, and at a small expense, gives me every reason to suppose the curious and enlightened mind will be pleased with a description of a new portable mountain barometer, contrived, and most peculiarly adapted, for that useful purpose, by a gentleman well known in the philosophical world. Its great simplicity in use, as well as portability, renders it superior to any barometer yet made.

The celebrated experiment devised by Pascal (says the French National Institute, in their Transactions of 1805,) and which proved, that a column of mercury decreased in proportion as the barometer was carried to a greater height, after having proved the gravity of the air, must have made the mercury be considered as a scale capable of measuring the height to which it is carried. But this scale being very small, in comparison of the heights which it ought to measure, it was soon perceived, that it would be necessary to improve the construction of the barometer, so far as to render sensible and appreciable the smallest changes in the height of the mercury. The necessity of avoiding or of calculating the continued variations which the barometer experiences, even without changing its place, presented another obstacle, much more formidable, and which seemed

to

to take away all hope of approaching the truth, or coming near it these difficulties, however, philosophers have been able to surmount; so that barometric measures, properly employed, may vie in exactness with the trigonometrical measures, to which they are superior, on account of their facility and generality of the method.

On this subject it is not necessary that I should add anything further, the inventor of the instrument, of which a drawing is herewith sent, having satisfactorily and correctly detailed every thing connected with it, in the paper, of which, by his desire, I send you a copy subjoined.

The paper alluded to appeared in a respectable periodical work nearly two years ago, but it has since been revised and received some considerable improvements from the author, which renders its republication desirable.

I flatter myself, that the section which I have likewise sent, of the principal and most essential improvement,, will not be unacceptable to the gentleman, nor the person who may wish to make such an instrument. a, a, a, a, (Plate I) represents the cistern, made of box-wood ;-c, c, c, the cover, made of brass, which screws on, and is prevented from being unscrewed (by idle curiosity,) by four small screws, t.--e e represents the stem of the cistern, into which the glass tube, n n, is firmly glued ;-R R represents the mahogany tube, in which is inserted the stem of the cistern, where it is secured by the screws, t t, passing into it.

I am, sir, your very humble servant,
THOMAS JONES.

An expeditious Method of determining Altitudes, of every Denomination, with a new portable Mountain Barometer; with a Description of the Instrument..

The mensuration of heights by the barometer has been, by the labours of M. De Luc, sir George Shuckburgh, general Roy, and several other scientific men, brought to such perfection, and affords so much an easier mode of ascertaining the clevations of the different parts of the surface

* Nicholson's Journal.

of

of the earth, to a considerable precision, than any other known process, that it might have been supposed that, in the course of thirty years, which have elapsed since this branch of science has been perfected, a very great number of observations would have been made, and the heights of almost the whole surface of our own country ascertained by the numerous travellers who continually traverse it. The contrary is however the case; and the small number of observations of this kind may be attributed to several causes.

The instruments are of considerable expense, and, from their complicated construction, easily liable to be out of order in the course of a long journey.

The observations themselves, though each not taking up any very long time, yet, when multiplied on every hill and valley, as they ought to be, for the purpose of obtaining a just idea of the face of the country surveyed, in the aggregate consume much of the traveller's time; and the constant unpacking and re-packing the instrument, becomes a greater labour than our natural indolence easily submits to.

It has moreover been generally supposed, that two instruments and two observers making simultaneous observations at the upper and lower stations of the height to be measured, are indispensably necessary. This, of course, would put it out of the power of a solitary traveller to make any observations at all.

Whether from these, or other causes, the fact is, that whoever reads the numerous Tours, Surveys, and Reports of the different parts of our island, published within these last twenty years, and many of them professedly with a view to science, either of agriculture, mineralogy, or geology, will be perpetually disappointed, by meeting with mere guesses at the clevations of the tracts of country described; though a knowledge of those elevations is almost indispensable to the geologist, mineralogist, and military surveyor;. highly useful to the scientific agriculturist, and very interesting to every one, who, from mere motives of enlarged and cnlightened curiosity, reads books of travels, or cmploys his own leisure in traversing the countries described by other voyagers.

I cannot,