Pagina-afbeeldingen
PDF
ePub

a

probably already known to many experimenters, still have not hitherto been made public, as far as I know.

I put forward no claims to originality; my object in now publishing an account of the experiments, is to furnish the lecturer, the student, and the amateur, with a few striking illustrations of some of the beautiful phenomena of the wonderful subject of crystallization.

It has often been remarked to me by some of the most eminent chemists, that for want of a record of the experiments of the lecturetable, they are either wholly lost or only verbally known, and therefore it is my intention occasionally to describe in this journal, some of the most striking experiments suitable for class-illustrations.

Experiment I. Place a smooth glass rod, and a stick of the same size, in a hot saturated solution of alum ; upon examination the next day, the stick will be found covered with crystals, whilst the glass rod is perfectly free from them.

If I may be allowed the expression, the crystals appear to have a preference for the fibrous surface of the wood, it affords them good hold; they cling to it in quantity; they have none for the smooth surface of the glass rod.

When solutions are suffered to crystallize in a tall glass vessel, it very rarely happens that any crystals adhere to its sides, but as fast as they form at the surface of the solution, they fall to the bottom of the vessel ; in a tall wooden vessel the case is different, for the whole of its sides and bottom become studded with crystals.

Experiment II. With a file roughen the surface of a glass rod at certain intervals, and then place it as a nucleus in a hot saturated solution of alum; all the crystals will adhere to the rough surfaces, leaving the smooth surfaces perfectly bright and clear.

Experiment III. Tie a few threads of lamp-cotton at certain intervals around a clean and polished glass rod, and employ it as a nucleus in a similar solution of alum; the threads will be covered with crystals, whilst the polished parts of the glass rod are perfectly free from them; and thus it is not difficult to obtain six or eight distinct bunches of crystals.

Experiment IV. Tie some threads of lamp-cotton, here and there, around a copper wire, (or a glass rod,) and then place it in a hot saturated solution of sulphate of copper (blue vitriol), the threads will be covered with crystals.

Coke is an excellent nucleus for alum-crystals on account of its very porous nature, affording them plenty of secure hold; but gas-coke very often has a smooth, shining, and almost metallic surface, and if a piece of it be placed in a solution of alum, it will be found that the crystals avoid the smooth surface, and form only on the most irregular and porous parts. Is it not possible that some action of this sort

may

be the reason why, in crystalline minerals, we so often find single, welldetermined crystals, adhering to certain spots of foreign matters?

In making crystals of alum on coke nuclei, I have found it best to employ a boiling saturated solution of alum, and to bore a hole through the coke, so as to pass a string through, by means of which it may

be suspended in the solution; it will of course float, and therefore the

string should be left so slack, that when the coke becomes saturated with the solution, and loaded with crystals, it may sink to about the middle of the solution ; this is better than loading it with lead, and the finest crystals will always be found on the lower surface, because they have formed quietly and undisturbed by the fall of smaller crystals from the upper part of the solution.

If powdered turmeric be added to the hot saturated solution, the resulting crystals will be of a bright yellow ; if litmus be used instead, they will be of a bright red; logwood will yield them of a purple, and common writing-ink of a black tint; and the more muddy the solution, the finer will be the crystals, hence no filtration is necessary.

But in all cases of coloured alum-crystals, they are much more brittle than pure alum, and the colours are to some extent fugitive; the best way of preserving them is under a glass shade containing a saucer of water; this keeps the atmosphere constantly saturated with moisture, the crystals never get too dry, and their texture and colour undergo but little change. The same plan may be adopted with many other crystals, especially those of sulphate of copper. Those beautiful blue “crystal baskets,” now so common in the bazaars and toy-shops, are made with sulphate of copper, and if one of them be kept for a day or two on the mantel-shelf, it loses its beauty, becoming pale, dry, and brittle ; but keep it under a glass shade with water, as just directed, and it retains its original beauty unimpaired.

Wire is a bad substance for a nucleus, for two reasons; first, if it is very smooth, crystals will adhere to it with great difficulty, or not at all; and secondly, supposing it has attracted crystals, they are apt to split off, on account of the expansions and contractions of the wire by alternations of temperature.

A very striking experiment showing how the colour of a crystal very often depends upon water of crystallization, consists in carefully drying a crystal of sulphate of copper in a crucible, until it becomes perfectly white; then drop it into water, and it instantly becomes of its original blue colour by absorbing water.

If a crystal of the ferrocyanuret of potassium (prussiate of potassa,) be similarly dried, its yellow colour vanishes, but reappears directly upon being dropped into water.

I have commenced a set of experiments on the instantaneous crystallization of concentrated solutions of Glauber's salts, in vessels secured from the access of air; and if they tend in any way to elucidate this mysterious subject, I shall make them known through the medium of this Journal.

a

Chemical School, St. Bartholomew's Hospital,

October 13, 1836.

302

AN ELEMENTARY COURSE

OF

DESCRIPTIVE GEOMETRY.

I.

HISTORICAL NOTICE. PROBABLY there are few of our readers who have not heard of the existence of a curious branch of mathematical science, called, by Monge and the modern French writers, Géométrie Descriptive ; but those who have confined their reading to English authors, have little chance of knowing much of its nature and objects. It is true that several years ago, a considerable extract was made from Monge's work in the Architectural Dictionary of Mr. Peter Nicholson; but the high price and scarcity of that work preclude most readers from the opportunity of consulting it. Besides, the work of Monge, elegant as it is, is not exactly adapted for abridgment, and still less for unconnected extracts*. Mr. Nicholson afterwards commenced a work in numbers, bearing the title of Descriptive Geometry; but the commercial casualties of the period (1825) put a stop to the undertaking before he had entered upon the essential part of his subject; so that we have no means of judging of the method in which he intended to develop it. Nothing further on this branch of science has appeared in England.

In America, however, two separate works, on the more elementary parts of descriptive geometry, have been published for the use of the United States' Military Academy, at West Point. The first was by M. Crozet, the professor of engineering, and the other by Mr. Charles Davies, professor of mathematics in that institution. M. Crozet's work is a very indifferently selected series of extracts from Monge and Hachette; but Mr. Davies's is a work of a far higher character in its mathematical composition, though still much too confined in its scope and applications. The former was published in 1821; and was superseded by the latter in 1827; and this, again, was reprinted in 1832.

These are all the attempts which have been made to supply a treatise in our own language; and the American treatises are so little known, that we have never seen more than the single copy before us of either of them. They were all intended, too, to be subservient to mililary engineering, and hence must, of necessity, be very deficient in their applications to the wants of engineers and architects, as well as to those of the cultivators of physical science. These desiderata we hope to supply in the course of this series of

papers:

and we shall especially have regard to these objects, seldom taking our examples of the application of the principles of the science from other than civil or scientific pursuits, avoiding except in rare cases, the military examples of the French, Italian, German, and American writers.

Notwithstanding the importance of Geometry in the arts, as well as in philosophy, how few there are amongst those devoted to either pursuit,

* Those extracts, we believe, were made by Mr. Webster, F.G.S., the very able geologist, who discovered the tertiary formations in the Isle of Wight, and the author of several valuable papers on different geological subjects.

а

:

who have a competent knowledge of its nature, and how few, indeed, are those whose acquirements are of sufficient extent, to be of any real use to them! It is, indeed, a source of constant complaint, especially amongst practical men, that there is no work from which they can obtain the kind of geometrical knowledge, which is adapted to their daily wants. It is true that a sufficient number of works, under various appellations, and of various degrees of merit, have been written on the geometry of rule and compass, the construction of plane curves, and such subjects; and also that we have excellent treatises on theoretical or speculative Geometry, besides that of Euclid: yet, on the construction of problems relating to space, except of a purely theoretical kind on one hand, or a set of artificial rules adapted to single cases on the other, we are, to the present hour, unsupplied with one single work,--a work in which theory and practice go hand in hand, and mutually subserve each other. The consequence is that the small number of persons who attend to the theory, see in it only a collection of abstract propositions, connected with each other, and mentally beautiful in that co exion it is true, though still having no practical bearing on the arts and necessities of highly-civilized life: whilst the practical, who are the many that more especially require it, see only a series of isolated operations, unconnected by any common principle, and view its didactic rules as being merely so many happy contrivances discovered by accident, and resting on no other evidence than their own experience that the precepts answer their special purpose. The speculatist is satisfied with the contemplation of the truths unfolded: the practical man is satisfied so long as he finds no want beyond what his rules will help him through. Rather, we should say, this was the case, than is :

i as most men of science now turn their attention more or less to the utility of their inquiries; and the recent rapid strides made in every branch of the arts, shows the unavoidable and unconquerable difficulties which stand in the way of practical men, whose knowledge is not based upon theory as well as upon personal experience and traditional dogmas. There are few architects and engineers,— few even of carpenters and masons,—who have been called into an active share in executing the great undertakings that have been entered on during the last twenty years, without having felt considerably embarrassed by their want of familiarity with the higher branches of Geometry, and some degree of physical science. It is impossible to look on the great amount of labour lost, the expenses incurred, and the vexation and disappointment which have ensued,--all from the want of proper mathematical acquirement in those to whom they were intrusted, --without feeling deeply anxious to prevent, as far as possible, the recurrence of such lamentable events. We believe, indeed, that there is much truth in the statement, that “Many an excellent design has been changed to suit the workman's rule for execution, instead of the rule being extended to suit the design." Skill and taste obliged to bend to the ignorance of the carpenter and the mason! It is, on the contrary, equally true that taste is too liable to outrun the laws of nature, and to violate the rules of geometrical construction; and that many designs which are beautiful on paper, are inconsistent with the principles of equilibrium, and involve impossible or incompatible geometrical conditions: and the only way by which the architect or engineer can secure himself from the disappointment and chagrin of total or partial failure, is to store his mind well with the principles upon which stability is founded, and the methods of investigating those problems by which his constructions can be executed in detail. Nature is true to her own laws,-Geometry is unvarying in her principles; and the remotest consequences of the one can be followed out by means of the other.

Every man, whose profession leads him to design a structure, of whatever kind it be, is false to his own reputation, as well as false to those who repose confidence in him, if he do not, for himself, ascertain, without the experiment of success or failure, whether his design be compatible with the physical and geometrical principles that pervade alike all materials and all their thousand forms. Descriptive Geometry is one of his most essential requisites in some shape or other: and the doctrine of equilibrium and of motion, is perhaps the only one which exceeds it,-if, indeed, it do exceed it*.

The gorgeous structures of Greece rather astonish us by their magnitude, and delight us by their exquisite and inimitable beauty, than by the display of mechanical and geometrical skill: whilst those of Egypt and India, from the difference of the beau idéal in our minds and theirs, have little to recommend them to our attention besides their antiquity and their unequalled magnitude. The “ Gothic," on the contrary, is full of the most ingenious contrivance, and manifests an intimate acquaintance with the principles of strength, which was totally lost with the dispersion of that singular fraternity, under whose superintendence such structures were erected; which even the most accomplished architects of the present time have been unable to develop anew; and which the most scientific amongst us dare not venture to imitate in any structure of his own. Much as we gained by the Reformation, we also lost much! It is true we have a relic of that fraternity, in the convivial band of free-masons; and we have doubtless other relics, in the traditionary practice and rules for the simpler operations of building: but the soul is gone from that body; and can only be recalled by the assiduity of those who seek in a better spirit than has been displayed till very recently, to restore to us the knowledge which died with it.

The earliest treatise with which we are acquainted, in which any attempt beyond the most obvious geometrical operations, was made to give a body of practical instruction on subjects connected with building, was by Philibert De l'Orme, almoner to Henry the Second of France, in a work on cutting of stones, under the title of Secrets de l'Architecture, published in a folio volume, in 1642. Seven years later, the Jesuit Derande, and the architect Desargues, both of Lyons, published a more extensive work on the same subject; and in 1728, La Rue published a

:

* There is a multitude of excellent | had prepared a paper on oblique arches works on the first principles of statics; with this view, but are obliged to defer it yet the number in which these principles till next month, when we shall examine, are well applied to the wants of the en- with some detail, the geometrical and stagineer and architect are exceedingly few. tical conditions of that kind of structure. We purpose to give a series of articles For the present, all we can say to those on different, and the most important, who are projecting such arches, is,-hesiof the applications of the doctrines of tate! beware! equilibrium in our future numbers. We

« VorigeDoorgaan »