of the beam is fastened to the upper wall of the vacuum chamber (v), and when the air tries to make the chamber collapse by pressing upon it with a given force, the weight at the long end of the beam is an exact counterpoise. If the air pressure exceeds this quantity, the weight is raised to a proportionate extent. If the air pressure is lessened, down goes the weight, just as the weight attached to the steelyard goes down, if after having balanced a leg of mutton we take off a slice.

Of course the motions are very small, but they are magnified by levers, as shown in the drawing, and cause an aluminium pointer (P) to traverse a considerable arc.

If it is desired to exhibit the minute oscillations of the vacuum chamber's walls on a very large scale, it becomes practically necessary that the hand which has to move over a wide arc should be extremely light, and in addition to the slender metallic pointer which will move over three or fourinches when the walls of the chamber have moved through

one hundredth of inch, Mr. Browning has provided us with another hand several feet long which weighs nothing at all. This hand he makes out of a beam of light! He did not invent these imponderable pointers, which were previously used in other instruments, but he has applied them to his new aneroids. If any reader who has a steelyard, puts a little bit of looking-glass on the beam near the fulcrum, and lets the sunlight fall upon the mirror, he may throw a line of light on the wall of the room, a yard or two off, and when his scale beam moves ever so little, up or down will go the light beam on the wall, marking the oscillations on a greatly enlarged scale. This is Mr. Browning's plan. A mirror, situated as described, receives rays from a lamp suitably placed, and when the beam moves a very little, the light pointer may traverse a scale of several feet. If self-registry is required, all that is necessary is to let the light make its own autograph on sensitive paper, as in the self-registering instruments at Kew.

Only one of the new aneroids has been finished and tried at the time we write, and it has performed too many peregrinations to scientific assemblies, to have permitted the exact value of its performance to be ascertained. A good pocket aneroid will beat an ordinary barometer in quickness of response to atmospheric changes; we may therefore infer that the new aneroid (which is much more delicately constructed) will at least equal, and probably exceed, the sensitiveness of first-class large barometers on the mercurial plan. That the new aneroid is very sensitive, is certain; but prolonged and careful trials under the same circumstances with the best mercurials is necessary, before an exact comparative statement can be made.

We hope to see the new aneroid in public institutions and in private houses. For the former it will be of great value, as if suitably placed and protected from the tremors incident to wooden floors, it can enable a whole room full of people to see ts indications without moving from their places. This advantage will be obvious to Lloyds, and other establishments where weather changes are anxiously watched.

We confidently expect the new aneroid will prove to be the best instrument yet constructed, for making noticeable, and registering, the oscillations of the atmosphere during storms, and enabling us to see the rate at which the form of the air waves traverse a given place. When such wave forms pass over us, we are at one time under their crests, and at another under their troughs, and the rate at which such changes take place it would be very interesting and useful to know.

KUHLMANN ON CRYSTALLIZATIONS.

THE principal facts and statements contained in M. Fred. Kuhlmann's paper recently read before the Academy of Sciences, are as follows: The author commences by observing that at the moment of the formation of certain bodies, through the agency of chemical action, they are particularly disposed to assume a crystalline structure, and especially so when their origin has been due to a current of gas. Oxydes of antimony give beautiful needles when acted upon by sulphide of hydrogen at a high temperature, and oligist iron, under the same circumstances, affords a sulphide of iron having the natural lustre and aspect. Even oxyde of zinc can be transformed at a sufficient temperature into a white sulphide, crystallized in broad shining plates. Chloride and carbonate of thallium, under the influence of a sulphide of hydrogen current, yielded at first pseudo-morphic crystals, which, on being sublimed at a higher temperature, afforded the true forms.

Substituting fluoric acid gas for sulphide and chloride of hydrogen, he obtained fluorides of several metals; but he did not get the fluoride of iron in a crystalline state. Generally speaking, when crystalline minerals are formed by the reaction of gases, their forms are pseudo-morphic; but at higher temperatures many of the crystals modify their form, and this modification, observes M. Kuhlmann, "proceeds from a natural attractive force which gives to the bodies new forms similar to those which they have in nature."

In studying the crystallization of pastes composed of amorphous silicon, M. Kuhlmann met with numerous instances in which the molecules of bodies already solidified, had a tendency to further movements and the assumption of the crystalline form. The presence of water, heat, or mere vibrations facilitated this change.

The tendency of molecules of the same nature to combine when their mobility is augmented by solution or liquefaction, he considers to explain the magnificent crystals of sulphate of lime that often occur in plastic clays, or of different silicates in glass kept for some time in a state of fusion; but when minute crystals imbibe water, and transform themselves into large crystals of great hardness, he thinks it necessary to ascribe to the solid particles a tendency to approach each other in certain directions. He found this phenomenon. strikingly exhibited in the deposits of sulphate of barytes near Philippville, in Belgium, and in carbonate of lime in the grotto of Adelsberg in Illyria. In the latter case, the microscopic crystals impregnated with water formed, in the first

T

instance, concentric crystalline layers; but the concentric lines, marking a succession of deposits, soon disappeared, and left crystalline transparent masses whose cleavage was not affected by the concentric layers: or rhombohedral figures seemed to spring from the surface.

If the solid molecules are not maintained in a condition of moisture, the crystallogenic force is counteracted, and bodies of little cohesion and no characteristic form is the result.

In calcareous or silicious pastes, moistened with water, diversely coloured by metallic oxydes or bituminous matters, certain separations may occur, and the mass may be traversed by crystalline veins of uncoloured calc or silicious spar; hence arise agates, jaspers, etc. In general the veinage of stones may be considered as likely to have resulted from internal movements, and not necessarily to have been occasioned by accidental ruptures and subsequent infiltrations.

After referring to other cases of an analogous nature to those just mentioned, M. Kuhlmann inquires whether the tendency of microscopic crystals in a sufficiently moist state to solder themselves together in larger crystals, may not throw a light on the formation of glaciers. The great masses of ice have, for their point of departure from the uncrystalline state, microscopic crystals, which a certain degree of moisture permits to unite when they descend from the regions of perpetual frost.

AIDS TO MICROSCOPIC INQUIRY.

CONSIDERATIONS FROM PHYSICS.

NOTWITHSTANDING the many excellent works on the microscope and its management, and on the various branches of natural history which that instrument is specially calculated to elucidate, we find that young students and families still require further aid, and we therefore propose to publish a series of articles, written for the express purpose of removing difficulties which numerous communications from all parts of the country prove to exist, and in many cases to oppose a very serious obstacle to a delightful and instructive pursuit. In following out this plan, we must of necessity deal with the elements of many sciences with which a great body of our readers will be familiar; but while we thus address a portion of our pages to beginners, we shall not fail to provide our more erudite subscribers as we have hitherto done-with a succession of papers embodying the most recent discoveries, and presenting science in its highest forms.

目

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Without intending to supply a treatise on physics, we shall commence by adverting to a few considerations, founded on physical laws, which are necessary to be entertained before the various phenomena exhibited under the microscope, by organic bodies, can be rightly understood.

All the creatures inhabiting our globe are, in the first place, under the dominion of gravitation. Every particle of which they are composed gravitates toward the centre of the globe, with a force proportioned to the quantity of matter it contains. The pound of feathers, the still bulkier pound of air; the pound of flesh, the still denser pound of bone, etc., tend downwards with precisely the same force. Each substance has its own specific gravity; that is to say, a square inch, or any other given mass of it, weighs a certain and constant weight. If put into water it displaces its own bulk of that fluid, and the quantity of water thus displaced by a given weight of any material will depend upon the density of the substance employed. If, for example, we forced an ounce of air into a cask of wine, and allowed an equal bulk of wine to run out, the quantity of the latter would be considerable; while an ounce of gold thrown into the cask would only displace an insignificant drop. "A cubic inch of water weighs 814-75 times as much as a cubic inch of air; both being at the temperature of 60°, and under a pressure measured by a column of 30 inches of quicksilver whose temperature is 32°."*

It is obvious that the higher the specific gravity of an organism, the greater will be the exertion required to move any given bulk of it. The muscular power which enables a man to move his own body in walking or running would be quite inadequate to his locomotion if his solids and fluids. weighed on the average as much as platina or gold. Thus the muscular power of any creature intended to walk, to crawl, or to swim, must be proportioned to the specific gravity of its components, and to the resistance afforded by the medium in which it is to live. A human body will float in water so long as the chest is distended with air, and drowning usually occurs because the sufferers have not learned how to balance themselves in the fluid so as to keep their heads above the liquid level. If a man were made as big as a whale and extended over seventy feet of earth, he would be perfectly helpless, because his strength would be disproportioned to his bulk. His two legs could make nothing of such a mass; but give him the whale's ability to live in water, and provide for respiration by coming to the surface at short intervals, and his condition

Apjohn Manual of the Metalloids, p. 192.