rence only extended to the height of thirty-three feet. When Dr. Birkbeck told this story, there was a general laugh among the artisans present, which showed that they comprehended all the absurdity of the philosopher's answer. Thus, then, we see, and the sight is most cheering, that the workmen of the British metropolis are more correct in their knowledge, and possess profounder ideas than the Florentine Sage, even in his own favourite pursuit; and to Galileo cannot be denied the praise of having been one of the most profound men, and one of the greatest discoverers of his age.

NATIVE GOLD. MURIATIC AND SULPHURIC ACID IN A RIVER.

M. HUMBOLDT has informed the Academy of Sciences at Paris, that he has received information from Messrs. Boussingault and Rivero, two enterprising travellers in South America, of a large mass of native gold having been lately found near Antioguia, in the Republic of Colombia, weighing eight arrobas, or above 190lbs. The same gentlemen have detected sulphuric and muriatic acid in the waters of a little river, which falls from a volcano, called Puracé, near Popayan, and which is named by the inhabitants Vinegar River. They also say schools for instructing miners are about to be established in that, country; and already there are lithographic and other establishments, which show it to be in an improving state.

MAGNETIC INTENSITY. MR. G. HARVEY, M.G.S. &c., has found, by Coulombe's apparatus, that a box chronometer exhibited

singular proofs of strong and active magnetism. It contained a remarkable quantity of steel, and every part of it exhibited vigorous polarity. Every screw displayed its influence, and the frame alone contained ten large and several small screws; and the same intense and active magnetic power was exhibited by the chain, the axles of the wheels and pinions, the arbor of the fusee, and the balance of its springs. Mr. Cox, the agent for Arnold's chronometer at Plymouth, remarked, when he saw this chronometer, that it appeared nothing less than a magazine of magnets. Mr. Scoresby recommends platina, or an alloy of platina, for the balance of chronometers. Gold is said to be considered as well adapted for the balance-spring.

TO CORRESPONDENTS. W. L**y's communication has been received, and will appear in our next.

Gas will see that his suggestion had occurred to ourselves. There are so many persons about town who supply chemical instruments, that it would be invidious in us to recommend any one. We may say, however, that Mr. Gurney's blow-pipe is chiefly made by Mr. Banks, mathematical instrument-maker, Strand; that if Gas live east of Temple-bar, he may seek the shop of Messrs. R. and G. Knight, Foster-lane, Cheapside; if on the west side, he may emplay the scientific Mr. Newman, of Lisle-street, or Mr. Elliott, 21, Great Newport-street.

We defy the threats of Azot; and are not, as he will find, so easily stifled.

Some of our Correspondents complain of our being too learned. Is this our fault, or theirs? We will readily insert, if good, their unlearned Essays.

Cheap Drunkenness, which has come to hand, will appear in our next.

Anti-Stahl is unavoidably postponed.

a

ame of a lamp, at least, when those substances are operated on which it is capable of subliming. To remedy this obvious inconvenience in mineralogical research, and to enable the operator to preserve all the elements of any solid substance he wishes to examine in chemical analysis, when they are thus separated from their original state by this powerful agent, I have contrived, says Mr. Gurney, a simple apparatus, Fig. 1, and which I find from experiment to answer the purpose intended. is a solid slab of plaster of Paris or of metal, with its upper surface ground perfectly true, so that, when a ground glass is placed on it, it may remain air tight on the edges, similar to that on the table of an air-pump. In the centre of the surface of this plate is a little furnace, b, into which one of the jets belonging to the instrument is made to terminate, by perforating from the side through the solid part of the slab. Over the furnace is fitted a ground bell glass, or part of a large tube, with a cap and stop-cock affixed, c, and the whole made completely air tight. To the stop-cock is attached a bladder or silk bag, g, in the usual manner.

The method of using this appendage in connexion with the blow-pipe is this: the substance to be examined is placed into the little furnace; the jet which perforates the slab is screwed to the safety apparatus of the instrument; and after the pressure has been on the press-board, either by weight or the hand, the gas is to be inflamed at the jet by a taper, and the glass instantly inverted over the furnace; the intense heat of the blow-pipe will now fall on the mineral, and the whole of the volatile or gaseous parts will rise, and either be condensed on the inside of the glass, or pass into the bladder through the upper stopcock in the gaseous form; thus the whole of the elements will be retained, and may be examined by the proper tests after the action of the instrument has been discon

tinued. The glass may be removed by placing the slab under water, either with the safety cylinder and flexible tube attached, or by previously unscrewing it from the tablet, without any possible loss of the contents, and may be decanted into smaller vessels for more accurate examination. Any solid substance, whether a mineral or chemical body, may be analysed in the same way, and the most satisfactory results obtained.

Should the water, formed by the combustion of the oxygen or hydrogen gases, be an objection to the immediate object under analysis, a mixture of chlorine and hydrogen in the proportions to form muriatic acid may be used to produce the flame from the instrument.

Fig. 2 in the drawing is illustrative of some curious phenomena concerning flame, which Mr. Gurney observed during his experiments with the blow-pipe He noticed in his attempts to distinguish flame by pressure, that its colour changed to all the tints of the rainbow; and by giving certain degrees of pressure, he was able to produce any colour at pleasure, the tints following each other in the same order in which they are observed in the prismatic spectrum. On pursuing the inquiry which this suggested, he found that each of the rays had a different heating power, as in the prismatic spectrum; and that, as in it, there was a point beyond the rays of colour, where the heat was greater than in any part of them. On applying sufficient pressure to extinguish the flame, the wire, with which he was testing its power, instead of becoming, as he expected, cold, became more intensely heated, and fused into globules. At this time there was not a vestige of flame. Mr. Gurney afterwards found, by holding a bar of platina in a flame produced by the blow-pipe, it increased in its temperature as it approached nearer the jet. From this, he says, he is convinced that flame is hollow, and consists of a thin film, or coat of ignited matter.

This coat is composed of several layers or coats, lying one on the other: the outer coat is white, then comes red, then orange, yellow, green, blue, violet; and within the violet is an invisible coat where the combustible matter is condensed, where the chemical change between the elements takes place, and where alone the heat produced by the combustion is evolved or formed. This is fanciful enough: however, it is ingenious; and Fig. 2 is a copy of Mr. Gurney's diagram for illustrating it. a is the combustible gases mixed before condensation; b the line or invisible coat of actual union, or where the combustible matter is condensed; c the first effect of heat in producing colour, or the violet layer; d the green; e the yellow; f the orange; g the red; h the full combination of rays, or white, gradually losing themselves in the atmosphere. We shall not follow Mr. Gurney further in his speculations; but to us they appear more imaginative than correct.

BREAD.
(Continued from p. 102.)

IT is the remark of a very intelligent chemist, that no set of experiments, with which he is acquainted, have been instituted and carried into effect for ascertaining what happens in the preparation of bread. As the change which the flour undergoes is, in all respects, a chemical change, this fact, which is correctly stated, is very surprising. Notwithstanding this, we remember, that the early editions of a very celebrated system of chemistry contained a detailed, if not a correct description of the whole process. We observe, that it has been omitted in later editions, which is a tacit proof that it was unworthy of public notice, and of the author's reputation. The principal experiments on bread with which we are acquainted, were made in France, a circumstance not surprising; for though other countries are called the land of cakes, that is the land of bread. Though these experiments, having

been instituted as a matter of police, to ascertain what quantity of weight bread lost in the baking, are, in some respects, very correct, they are essentially incomplete as to the changes which the flour undergoes before it becomes bread. We are obliged, therefore, to begin this part of our Article by acknowledging that, on this subject, our information is neither so complete nor so accurate as we could desire it.

Bread, as our readers know, is made from flour, and more gene-rally from wheaten flour than from any other. This is found to consist, chemically, of a small portion of mucilaginous saccharine matter, soluble in cold water, from which it may be separated by evaporation; of a great quantity of starch, which is scarcely soluble in cold water, but capable of combining with that fluid by means of heat; and of an adhesive grey substance, called gluten, insoluble in water, ardent spirit, oil, or ether, and which, in many of its proper. ties, resembles an animal substance. The problem is, to ascer tain what changes these substances. undergo. When the flour is kneaded with water, it forms a tough indigestible paste, in which all these constituent substances may still be found. Heat produces a considerable change in the glutinous part of this compound, and renders it easier to chew and digest. The cake is, however, in this state heavy and tough, and is only made light by the addition of leaven, or some substance having corresponding properties, or capable of producing similar effects. If flour be mixed with a small quantity of water, and kept in a warm place, it undergoes a species of fermentation; it swells, becomes spongy, and at length acquires a sour taste, and gives out a sour smell. This is leaven. If this is left to itself, it at length begins to putrefy; mingled with other dough, it makes it rise readily, and gives it a greater degree of tenacity; the dough or paste undergoes spontaneous decomposition, the saccharine part is

converted into an ardent spirit, the mucilage tends to acidity and mouldiness, and the gluten verges towards putridity. During this -process a considerable quantity of gas is produced, which is supposed to be carbonic acid gas; the dough becomes porous and increases its bulk. If this process be suffered to go on by itself, the outside and the inside are not equally affected; and it is found that some parts become mouldy before the others have acquired the proper degree of fermentation. It is found by experience, that the addition of a small quantity of leaven, and the employment of a gentle heat, accelerate the process of fermentation, which kneading makes equal throughout; and a sufficient quantity of carbonic acid gas is generated from the saccharine matter, to make the bread light before the gluten has begun to putrefy. The tenacity of the gluten prevents the escape of the gas, and the bread becomes light and porous. A certain quantity of moisture is necessary for the continuation of this - process; and baking the bread, whatever other effect it may have, for this point is not exactly ascertained, evaporates the moisture so as to check the fermentation, though, unless carried to a very considerable excess, it does not wholly stop it. The very grateful smell of new baked bread, and the constant alteration which takes place in it as it grows stale, losing this smell, changing its taste, and becoming hard and dry till it moulds, are all proofs of a chemical change constantly going on in bread, though of what nature has not been distinctly ascertained. Very great care is necessary in making bread of leaven; and in general such bread does not get properly fermented, or it acquires an acid taste, from having too much feaven mingled with it, or from the fermentation being allowed to proceed too far; and it having been found that yeast answers all the purposes of leaven, it is very generally employed in most parts of Europe. In some

places, however, such as on board ship, and in warm climates, where no beer is made, leaven is still geperally in use. Yeast is found to make the dough rise more rapidly than leaven, and therefore makes the bread lighter.

It is obvious, that making bread is a most delicate operation, requiring definite proportions of water, flour, and yeast, or leaven; the dough, also, must be kept at a certain temperature, or, if too hot, the fermentation goes on too rapidly, and if too cold, will not go on at all: the oven, too, must be of one certain temperature, or it would not properly bake the bread. It is also to be observed, that different kinds of flour require different quantities of water, and do not all ferment alike. To regulate all these nice points, the baker has no instruments, and nothing but his experience to guide him. He trusts to his sensations: throws a little flour in his oven, and observes if it blackens or burns; plunges his hand into the water, or feels the dough; and so nice does his tact become, that what no philosopher could, perhaps, tell by the most accurate of his instruments, the journeyman baker, or the housewife, decides at once; and, perhaps, of all the batches of bread baked in this metropolis, not one out of ten thousand is spoiled. It must not, therefore, be supposed, that instruments are of no use: thermometers, to measure the heat of the bakehouse, and pyrometers, to measure the heat of ovens, have been employed with advantage. No instruments, however, can supply the want of skill and practice; and, where these are, instruments such as we have mentioned may be dispensed with.

The method of making household bread is said to be this :-To a peck of flour they add a handful of salt, a pint of yeast, and three quarts of water. The whole being kneaded in a bowl or trough, will rise in about an hour; it is afterwards moulded into loaves, and put into the oven. To make French bread, ten eggs, a pound and a