unchanged, the spaces near the equator will have very nearly their true figure, and the distortion near the south pole will be diminished by one-half. This semicircle can be formed into a cone by bringing together the bounding radii (considering the semicircle as a sector), and the connection between different parts of the globe can thus be indicated as clearly as in Fig. 1. But a still better representation of the globe may be formed by taking two equal sectors of about three right angles, dividing each into thirty-six equal angles, describing parallels up to the equator, and drawing in the continent-outlines of the northern hemisphere in one sector, and of the southern hemisphere in the other. If the two sectors be formed into cones,* and these joined along the equator, an equigraphic representation of the globe on a double cone will be formed, the vertices being the poles. The construction of these figures, or of others formed in a similar manner from sectors of varying angles (or constructed on other projections), would form an interesting and instructive employment for the young geographer.

A work has lately been published, by a person writing under the pseudonym of "Parallax," in which it is gravely maintained that the earth is plane, and of a figure somewhat resembling Fig. 1, the parallels, however, being equidistant. The sun travels at a distance of about 4000 miles from the earth, in a circle of varying radius about the north pole as centre, being vertical over the equator at the equinoxes, and over the tropics of Cancer and Capricorn, in summer and winter respectively. The tides are caused by the motion of the earth, which floats (!) upon a bottomless ocean, and carries fire in its "hold" for the eventual destruction of its inhabitants. The moon has one luminous hemisphere, which, being presented in different proportions to the earth, produces the lunar phases, lunar eclipses being caused by an invisible semi-transparent satellite! The new system is supported, as is fit, by new principles for instance, by a new law of perspective, "the law taught in our schools of art being contrary to everything seen in nature;" and a new law of projectiles, which, it appears, ascend diagonally in a straight line, and descend vertically. The author is described in the Athenæum as an enthusiast who goes about the country enforcing his views in lectures, and a hope is expressed that "he may make some converts," so that a Wesleyan agitation may lead astronomers to defend the established" system more clearly than has yet been done!

66

*The vertical angle of either cone will be a right angle, if the angle of either sector is 2541. In this case the axis of the double cone will be equal in length to the diameter of the equator, and the distortion will not be very great in any part of the projection. In the case first considered (of a semicircle) the vertical angle of the cone would be 60'.

Such a work is, of course, beneath criticism in these pages, but one remark on books of this kind may be useful to the young student. It will generally be found that for one page of the author's writing there are three of quotations, and that these are so ingeniously garbled, that their meaning is altogether perverted. Hence, if the student should feel perplexed by confident appeals to well-known authorities, it will be well for him to suspend his judgment till he has read the passages in the original. As instances of the necessity for this caution, an extract or two taken at random from the book mentioned will suffice. To prove that the parallel of Cape Horn is longer than the equator," Parallax"* quotes Sir J. C. Ross as saying, "From near Cape Horn to Port Philip (Melbourne) the distance is 9000 miles;" therefore, these two places being 143° apart, the length of the parallel of Cape Horn is 22,657 nautical miles. It happens, however, that Captain Ross is quoting the words of a landsman, so that the miles are presumably statute miles; and that reference is made to the course of a bottle, which travelled in an easterly direction, and therefore passed over 217° instead of 143°, Ross himself adding that the bottle must have passed Kerguelen Island. Again, the author, wishing to prove that the Pole Star can be seen as far south as the tropic of Capricorn, quotes the Times of May 13, 1862, in which, under "Naval and Military Intelligence," it is stated that Captain Wilkins, of the troop-ship " Earl Grey," saw the Southern Cross and the Pole Star at midnight, April 19th, in 23°53′ lat., and 35° 46′ long. Here again the evidence would be good, were it not that Captain Wilkins was north of the tropic of Cancer on the 19th of April,† having left St. Helena on March 18th (homeward bound from the Cape), and sighted the "Lizard" on May 10th; therefore (as usual when no doubt can arise) the denomination of the latitude, as of the longitude, was omitted in the report.

An examination of this sort affords a ready means of determining whether the author of such a work is simply ignorant, or whether (in addition) he is practising on the assumed ignorance and credulity of his readers.

• From Tapaλáσow, "to alter a little, esp. for the worse."-Liddell and Scott. + The Southern Cross had passed the meridian about an hour and a half before the time of observation, and the whole of the constellation was still several degrees above the horizon.

SCHULTZE'S ARTIFICIAL DIATOMS.

BY HENRY SLACK, F.G.S.,

Member of the Council of the Microscopical Society of London.

THE Quarterly Journal of Microscopical Science for April, 1863, contained an abstract of a paper by Professor Max Schultze on "the structure of the valves of diatoms, as compared with siliceous pellicles produced by the decomposition of fluosilicic acid in moist air." I do not propose to enter upon any discussion of the markings of diatom valves, but to suggest to amateur microscopists, who do not appear to have taken much notice of the paper in question, some very curious and interesting experiments.

The numerous readers of the INTELLECTUAL OBSERVER who are well acquainted with chemistry will excuse my beginning with a description of fluoride of silicon, and the mode of obtaining it. Fluorine is put down in the chemist's list of simple substances that is, of substances which have not been decomposed. It is not known in a free state, but from its behaviour in compounds it is usually grouped with chlorine, iodine, and bromine. "These," says Dr. Apjohn, speaking of the group, "have numerous points of resemblance, being all electro-negative in a high degree, bearing strong affinities for the other metalloids and the metals, and forming with them chemical compounds of great interest and importance, analogous in composition, and generally possessed of very similar properties."

The well-known and beautiful mineral fluor spar is a compound of fluorine and calcium, the metallic basis of lime. If we take some fluor spar, or fluoride of calcium, and pour sulphuric acid upon it, hydrofluoric acid, a compound of hydrogen and fluorine escapes. This substance is a rapid solvent of glass, and is used in etching on that material. If we add to the powdered fluor spar powdered glass, or clean white sand chiefly composed of silex, part of the silex is taken up by the fluoric acid at the moment of its formation, and the result is a gas, which makes white cloudy fumes on coming into contact with air containing moisture. Silica, found pure in clear quartz, and nearly so in flints, is a compound of silicon and oxygen, and is also called silicic acid. Hence the compound of fluoric acid and silicic acid is called fluoride of silicon, or in more recent nomenclature, "silicic fluoride."

When silicic fluoride comes into contact with water, it is decomposed, and deposits one-third of its silica, the other

Το

two-thirds remaining in combination with the fluorine and hydrogen. The silica in this decomposition will be deposited differently according to the circumstances of the case. make Schultze's artificial diatoms the process should go on slowly and gradually. For this purpose I have found the following arrangement very convenient. Take two six-ounce and moderately wide-mouthed bottles, and put into one a teaspoonful of powdered fluor spar, and about the same quantity of clean silver sand; pour over it to the depth of an inch sulphuric acid, and stir up the mixture with a glass rod. Then take a tuft of cotton wool pulled out loosely, wet it, and put it as a loose stopper in the mouth of the bottle. Fill the second bottle with plain water; take some lamp cotton as thick as a pencil; drop one end into the water, leaving it long enough to go to the bottom of the bottle. Place the two bottles side by side where they will not be disturbed, and put the other end of the lamp cotton on to the top of the cotton stopper in the bottle containing the chemical ingredients. Fluoride of silicon, or silicic fluoride, will slowly rise from the mixture, and on coming into contact with the wet cotton, will deposit a portion of its silica in vesicular form. The lamp cotton will, by capillary action, carry water from the bottle in which one end is immersed to the cotton stopper, and keep it constantly

moist.

The whole arrangement should be left undisturbed for a day, when the cotton stopper may be cautiously removed, and the white deposit upon its threads delicately swept on to a piece of blotting-paper. When it is quite dry it is ready for examination. The cotton may be replaced, and left for two or three days or more, when a second crop can be removed.

Care must be taken, in examining this deposit, not to let the moisture it contains come into contact with object-glasses, or any part of the microscope, as it corrodes glass with great rapidity. When quite dry it may be viewed under a low power, as an opaque object, and a general notion of the mode of deposition will be obtained. Some of it should then be gently crushed on a slide, under a thin covering glass, which may be fastened down by a little cement at the edges. This should be viewed with a quarter or higher powers, when some of the broken vesicles will look amazingly like fragments of diatom valves. In the number of the Quarterly Journal of Microscopical Science, to which allusion has been made, several diagrams are given of the patterns Professor Schultze obtained, and all of them, and many more will be easily found by following the directions given.

I will describe a few in one slide as viewed under a magnification of 1000 linear. Some, formed most quickly, are

irregular, while the better specimens are quite regular, though diversified. Here I find one fragment exhibiting round bosses, appearing, under the magnification specified, one-tenth of an inch in diameter, and regularly grouped, the interspaces being larger than the bosses. In another fragment little nodules or bosses, about one fifteen-thousandth of an inch in diameter, are widely scattered, and in another similar little bosses, about one twenty-thousandth of an inch in diameter, are closely and symmetrically arranged. In some specimens a number of the larger bosses are grouped in symmetrical patterns, and each one is surrounded by much smaller protuberances, equally symmetrical. These are very elegant objects. The slide also contains tubular fragments of vesicles, like sausage skins, with here a twist, and there a constriction. Some of these are quite plain, while others are elegantly marked. Many of the markings on the portions of vesicles are so delicate as to look very fine and small even under the magnification of one thousand linear.

In some cases large hemispherical bosses, closely grouped together at the base, exhibit, when in focus, a hexagonal appearance at the lines of contact, and a concentric structure is visible in many under appropriate illumination. By employing an achromatic condenser and a central stop, some-at a certain focus-have the aspect of groups of eyes. First comes a luminous white circle, then a dark one, like an iris, and in the centre a bright spot like a pupil.

Close examination indicates that many of the little bosses which look like hemispheres are in reality composite, and it is common to find little mammillæ, or rounded projections, on the top of the bosses. In some cases the form of the bosses is irregular, and many will be seen composed of two or more individuals stuck together.

In the paper of Max Schultze that suggested these experiments, it is said that these patterns are not due to a crystalline structure, and several reasons are adduced for this belief. It is, however, certain that crystalline deposits in the form of needles may be found in the bottles in which the hydrofluosilicic acid is prepared, and miniature quartz crystals resulted from the following variation of the experiment:

Some fluor spar sand and sulphuric acid was placed in a Florence flask, and through a cork inserted in its mouth a bent tube was inserted. The free end of this tube was immersed in a little mercury contained in a small evaporating dish, and as the gas escaped through the liquid metal, it was allowed to blow little bubbles, or vesicles, of white of egg, a layer of which was placed over the mercury. These were permitted to dry and harden, and when the crushed fragments were