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Savery's engine; the steam exerts no direct action upon the water, or on any part of the apparatus, it is merely employed as a means of forming a speedy vacuum under a piston attached to one end of a lever, the rod of a pump piston or plunger being affixed to the other extremity." Of this first atmospheric engine we shall subjoin, with Mr. Stuart's permission, his description:

"In this construction the power of the engine has no reference whatever to the strength or temperature of the steam, but depends upon the superficial dimension of the piston beneath which the vapour is introduced from the boiler, The steam cylinder, T, was now, for the first time, effectually detached from the water-pump,

"The steam generated in a boiler, b, was admittted through the cock, d, and pipe, q, intò a cylinder, a, under the steam piston, s, attached by the rod, r, to the lever or beam, i i, moving on the axis or fulcrum, o. The cylinder, a, was placed in another cylinder, forming a concentric space, zz, round it. This outer cylinder was connected by a pipe, f, to a reservoir, g, containing cold water. Another pipe proceeding from its lower end was inserted into the well or second reservoir of cold water.

"The piston being in the position shown in the Figure, and the cylinder, a, being filled with steam through the pipe, q, the cock, d, is turned, which shuts off the communication between the cylinder, a, and boiler, b. By opening cock f, cold water is now allowed to flow from the reservoir, g, through the pipe, f, into the outer cylinder, zz; this cools the cylinder, a, containing the steam, which condenses the included vapour, and forms a vacuum under the piston, s. The pressure of the atmosphere meeting with no resistance from the elasticity of the steam, forces the piston to the bottom of the cylinder.

"By this movement, the end of the lever, i, attached by the rod, r, to the piston, is depressed; and the other end of the lever to which the pump-rod is fixed, is raised, and draws up all the water above the plunger in the pump barrel along with it.

"Now, if we suppose the cold water which has been in contact with the steam cylinder to have condensed all the vapour, the atmosphere will press on the piston with a force equal to that

which would be produced by placing about 143lbs. weight on each inch of its surface. If the piston were 62 inches square, this would be about 915 pounds weight, operating to force it downwards; and, if there were no resistance from friction, it follows, that in the same time an equal weight placed at the other end of the lever beam, or a column of water, weighing 915 pounds, would be lifted as high as the steam piston had been depressed in the cylinder.

"When the piston has arrived at the bottom of the cylinder, the cock, d, is turned, which again opens a communication between the boiler, b, and the steam cylinder, a. In this engine, the steam being only equal to the pressure of the atmosphere, the piston, s, must be raised by other means to the top of its cylinder. This is effected by a counterpoise, m, fixed on k, which is so adjusted as to depress the pump rods, and thus to raise the steam piston into the required position. During this operation the cock, f, is shut, and the cock, e, is opened, and the water heated by the condensation of the steam in the condensing cylinder, z, escapes into the well or tank, o. A very small quantity of water being formed in the steam cylinder, a, by the condensation of the vapour, is allowed to fall through the pipe, p, into the same receptacle. The cylinder being a second time filled with steam, the cock, f, is opened, and cold water flows from the reservoir, g, into z; the steam under the piston is again condensed. The pressure of the atmosphere a second time having the preponderance, the piston is depressed, and the pump rod at the opposite end of the lever beam is elevated, lifting up the column of water in the pump barrel as before. By closing cock f, opening e and d, the counterpoise, m, again acts to raise the piston, s, and the operation may thus be indefinitely repeated.

"The fire-place under the boiler is shown at n, the ash-pit at w; xx are the smoke flues; N is a safety-valve; ca gauge pipe, as in Savery's engine; u a wall or post supporting the axis, v, of the lever beam, ii; tt a pipe connected with the pump barrel, in which the cold water rises to supply the reservoir, g; vl is the mouth of the well or mine which is to be drained; ha pipe proceeding from g, through which water flows on the top of the piston to keep it air-tight, a contrivance first used by Newcomen."

For the modern improvements in this valuable machine, we must refer our readers to Mr. Stuart's book, where they will find them amply and, we think, justly and impartially given. In reading them, it has occurred to us, that no subject is better calculated to teach a man of inventive genius a becoming humility, and to relieve less gifted persons from the awe they might be disposed to feel in his presence, than the contemplation of the gradual and successive steps, as they are here brought before us, by which science is enlarged or art improved. The utmost benefit which any individual has yet conferred on the human race only amounts to some slight extension of a principle, or some small improvement in art. We say slight and small in comparison with the mass of our knowledge, and with the widely extended field of human discoveries. It will be found, on examining the history of any remarkable invention or improvement, such as printing, the experimental mode of phi losophising recommended by Lord Bacon, the invention of fluxions, Mr. Watt's improvement in the steam-engine, and Sir Humphrey Davy's successful researches with galvanism, that they belong much more to the AGE than the individuals, who were, so to speak, but the foci, which collected and converged to one point the light and knowledge radiating from all around them. Successful efforts are in general eagerly 'seized on and recorded, while a thousand unsuccessful experiments, tending to the same object, and each of which seemed on the point of attaining it, though serving as guides to the successful inquirer, are suffered to sink into oblivion. Thus, after a short period, the successful man, the scaffolding by which his glory was built up being pulled down and put for ever out of sight, stands far above all surrounding objects, challenging, either in person or by reputation, the worship even of the men whose labours assisted in raising him to eminence.

The origin of the art of printing is lost in obscurity, but enough of it is known to satisfy us, that many minds, existing at periods remote from each other, concurred in perfecting the invention. Hooke and others had practised what Bacon taught, before he recommended it by his eloquent words. So prepared was the way for the invention of fluxions, that to this moment it is somewhat doubtful whether the priority belongs to Newton or Leibnitz. Jonathan Hull, 60 years prior to Mr. Watt, suggested that the "alternate rectilineal motion of a piston rod might be converted into a continuous rotatory one by the means of a crank;" and "this is now considered to be that invention which introduced the steam-engine as a first mover of every variety of machinery." We learn, also, from the present work, that Professor Robison " threw out to Mr. Watt the idea of applying the power of the steam-engine to the moving of wheel carriages, and to other purposes, but the scheme was not matured, and was soon abandoned on his going abroad." It is, further, quite evident, that but for the great number of chemical discoveries which were made about the period of Mr. Watt's great inventions, particularly relative to heat, and but for the skill which workmen at that period had begun to acquire in manufacturing iron, that his admirable and beautiful inventions could never have been thought of, and never carried into execution. After making, on these accounts, a rational deduction from the merit of Mr. Watt, enough still remains to challenge our admiration of him as the great improver of steam-engines; and were it more, we should rather regard him with awe, as some superior being, than as a man like ourselves, whose virtues we might imitate, and whose inventions we, living at a later period, may possibly surpass. In the same manner, if we look at the discoveries which are every day made in chemistry, and at the experiments which fail, there will

still remain enough in Sir Humphrey Davy's improvements to make us confer on him, with justice, the name of an illustrious man, but not to place him, as his partisans do, so far above all other men, standing the Buonaparte of science, admitting no equal, and suffering no rival. Each individual, however eminent, is directed in his exertions by public opinion, by times, and circumstances; and the most useful inventions are those called for by the wants of the moment, and to discover which a number of minds are directed. By slow and gradual improvements, sometimes chance-discovered, mingled, too, with many failures and rejections of what were afterwards found to be great advantages, has the steam-engine been brought to its present state. "It must be acknowledged," says the motto to Mr. Stuart's book, "that this is the most wonderful of all machines, and that nothing of the work of man approaches so near to animal life. Heat is the principle of its movements: there is in its tubes a circulation like that of the blood in the veins of animals, having valves which open and shut at proper periods; it feeds itself, evacuates such portions of its food as are useless, and draws, from its own labours, all which is necessary to its own subsistence." This wonderful machine, the perfecting of which is due to our own age and country, belongs as much by its origin as by its advantages, to the whole human race, the labour and the knowledge of many generations, existing in different parts of the world, having concurred in its creation.

HYDROCYANIC, OR PRUSSIC ACID.

IN 1782 Scheele first taught the chemical world that the substance called Prussian blue, which had been then known somewhat more than seventy years, consisted of iron combined with a peculiar acid, which was afterwards named prus

sic by Guyton Morveau. The elder Berthollet and Fourcroy afterwards showed that this acid consisted of carbon, hydrogen, and nitrogen. Porrett attempted to assign the proportion of each,* and inferred the existence of another acid, which he called the ferro-chyazic; but it is to Gay Lussac that we are indebted for the knowledge that the prussic vapour, deprived of its hydrogen, forms a gaseous substance which is the basis of the prussic acid; and to which, for that reason, he gave the name of cyanogen. The acid formed by the union of hydrogen with this substance he termed hydrocyanic acid, and that resulting from the union of equal portions of chlorine and cyanogen the chlorocyanic acid.†

The prussic acid is extremely volatile; it boils at 790.7 Fahr., under a pressure of 29.9 inches, and at 50° Fahr. it sustains a volume of mercury of 14.95; its congelation, however, takes place at 5o, but the cold it produces when allowed to evaporate in the open air, is sufficient to congeal it at the temperature of 68° Fahr. It is little soluble in water, but is easily dissolved by alcohol. It is readily decomposed, even when kept in close vessels and secluded from the light. There are two principal modes in which this acid is prepared; the one as used by Gay Lussac, and the other known by the name of Scheele's method. The latter is the one employed in this country, and the former on the Continent. We shall first give Gay Lussac's, and afterwards Scheele's method.

Hydrocyanic acid is obtained by digesting the crystallized deutocyanuret of mercury in two-thirds of its weight of liquid and slightly bulated retort, which communifuming hydrochloric acid, in a tucates with a receiver containing fragments of chloruret of calcium and chalk, and which itself com

*Carbon 24.8; hydrogen 34.5; nitrogen 40.7. † Edinburgh Medical Journal, July 1824.

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municates with a much smaller receiver, destined to collect the product. These receivers must be surrounded by a mixture of ice and salt. After the deuto-cyanuret of mercury and the acid have been successively put into the retort, a slight heat is to be applied; a little ebullition soon succeeds, arising in part from the evaporation of the prussic acid which is formed, and is condensed in the first receiver with a little hydrochloric acid and water. When the quantity of water becomes very sensible, the operation must be suspended, in order that the product already obtained may be purified; this is performed by isolating the first receiver from the retort, taking away the ice which surrounds it, and replacing the ice by water at 89o. 6 or 91o. 4 Fahr. Under these circumstances the hydrocyanic acid passes alone into the small receiver, for the water and hydrochloric acid which were at first volatilized with it, are now retained in the first receiver; the water by the chloruret of lime, and the hydrochloric acid by the lime.* Scheele's process, as given by Mr. Thomson,† is as follows:Mix two ounces of Prussian blue with six ounces of red precipitate of mercury, and add six ounces of water; boil the mixture for some minutes, constantly agitating it, when the blue colour will disap pear, and the mass assume a yellowish-grey hue. Pour the whole on a filter, and wash the residuum with a little hot water, which is to be added to the filtered liquor. Pour this upon an ounce and half of clean iron filings, and add three drachms of strong sulphuric acid. Shake this mixture well, and after the powder subsides, pour the fluid into a retort, and distil one fourth part of it over a well luted receiver. This is the hydrocyanic acid, containing an admixture of a little sulphuric acid, which is rea

*Majendie's Formulary for the Preparation of new Remedies, translated by Thomas Haden, Esq., 2d edit. corrected by Dr. Dunglison.

+ Edinburgh Dispensatory, p.23.

dily separated by means of barytic water. La Planche recommends 1-6th only to be distilled over, and this to be rectified by means of a gentle fire, over 1-200th of carbonate of lime, distilling off afterwards, by means of a gentle fire, 3-4ths only of the whole. The acid is obtained of a uniform strength by this method, which is the one always used in this country, where it is given from four to fifteen drops in a dose. M. Majendie says, that the medical properties of prussic acid, prepared according to Scheele's method, are not sufficiently determinate, on account of the arbitrary nature of the process. It is better to use M. Gay Lussac's acid, taking care that it be properly diluted by adding six times its volume, or 8.5 times its weight of distilled water.

The action of hydrocyanic acid on the animal economy has not escaped observation, and within the last eight years its medicinal yirtues have been carefully examined; but the means of detecting the presence of this acid in persons who have been poisoned by it have not been the subject of much investigation. Orfila, who has described with accuracy, the action of this poisonous substance on the animal economy, the disorders which it occasions when taken in too large quantities, has made no researches whatever, for the purpose of enabling one to discover traces of its presence on the dead subject. This is the more surprising on the part of Orfila, as his labours on the subject of poisons in general are unequalled, and the means of detecting their presence (more particularly, however, the mineral poisons) are laid down with great care and correctness.{ Hydrocyanic acid is one of the most powerful poisons in nature, the

quantity sufficient to cause instantaneous death being very small; and the number of instances on record of its producing this calamity, form so many reasons why we should be acquainted with every test capable of detecting its

presence. Its smell, which is very strong, and similar to that of peach blossoms or bitter almonds, may lead ore to suspect the presence of this acid, and in some cases we know has been considered conclusive, without any other evidence being adduced in support of its existence. But a paper has been lately read before the Royal Academy of Medicine at Paris, by M. Itard, in which some instances of the spontaneous development of hydrocyanic acid are mentioned. One person in whom this phenomenon was observed, laboured under inflammation of the bowels, and another under inflammation of the liver. In both these cases the alvine evacuations smelt very strongly of bitter almonds. These facts are important, and show that this peculiar smell alone ought not to be received as positive proof of hydrocyanic acid having been taken. We know that some doubts have been raised whether the acid did or did not exist in the motions, although they smelt so strongly of it; and whether the smell might not have been produced by the presence of some other substance. M. Itard is open to this objection, for not having analyzed the fæces; but our position will not be invalidated even if the fæces, although they smelt so strongly of it, contained none of the acid. On the one hand, if there was any hydrocyanic acid in the fæces, that acid (the patients having taken none of it during their illness) was spontaneously developed on the other hand, if there was none of this acid in the fæces, and they,nevertheless, smelt strong ly of it, it only shows that a smell similar to that of hydrocyanic acid may exist in cases where there is none of the acid present, and also confirms what we have before advanced, that the smell alone of hydrocyanic acid ought not to be received as proof of its having been taken. Having made these few preliminary remarks, we shall now consider the action of this acid on the animal economy, the antidotes which have been recommended for

counteracting its poisonous effects, and the tests that have been used for discovering its presence after death. (To be continued.)

DICTIONARY OF CHEMISTRY,

CARBON MINERAL. Charcoal united with earth and iron without bitumen; it is of a greyish-black colour.

CARBONATES. Salts composed of carbonic acid and some base. Thus chalk and marble are carbonates of lime, consisting of lime and carbonic acid.

CARBONATE OF BARYTES, witherite, aerated heavy spar, aerated baroselenite, aerated heavy earth, bu rolite. A mineral, being a compound of barytes and carbonic acid.

- OF LIME, calcareous spar, marble, chalk, limestone. Carbonic acid and lime.

spar, strontian.

OF STRONTIAN, heavy

CARBON, CHLORIDES OF. An interesting class of compounds, con sisting of carbon and chlorine, and for a knowledge of which we are indebted to Mr. Faraday.

In

CARBONIC ACID, fixed air, aerial acid, mephitic acid, calcareous acid, choke damp; a compound of carbon and oxygen, in the propor tion of 28 carbon, 72 oxygen. its uncombined state it exists in the form of a gas, is generally found in the atmosphere, and is a very general product of combustion. It is much denser than common air, and hence occupies the lower part of caverns and mines whereever it is produced, and renders them destructive of life. It abounds in a fixed state in nature, and forms a large part of lime-stone, marble, and calcareous spar.It is emitted in large quantities during vinous fermentation, and gives a sparkling appearance as well as a sharp pleasant taste to many fermented liquors. To the discovery of this acid and of its compound nature, much of the progress of chemistry during the last century was owing; and the

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