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determine the rate of relaxation of that state of strain which the light indicates.

If the motion of the spatula in its own plane, instead of being in the plane of polarization, is inclined 45° to it, no effect is observed, showing that the axes of strain are inclined 45° to the plane of shearing, as indicated by the theory.

I am not aware that this method of rendering visible the state of strain of a viscous fluid has been hitherto employed; but it appears capable of furnishing important information as to the nature of viscosity in different substances.

Among transparent solids there is considerable diversity in their action on polarized light. If a small portion is cut from a piece of unannealed glass at a place where the strain is uniform, the effect on polarized light vanishes as soon as the glass is relieved from the stress caused by the unequal contraction of the parts surrounding it. But if a plate of gelatine is allowed to dry under longitudinal tension, a small piece cut out of it exhibits the same effect on light as it did before, showing that a state of strain can exist without the action of stress. A film of gutta percha which has been stretched in one direction has a similar action on light. If a circular piece is cut out of such a stretched film and warmed, it contracts in the direction in which the stretching took place.

The body of a sea-nettle has all the appearance of a transparent jelly; and at one time I thought that the spontaneous contractions of the living animal might be rendered visible by means of polarized light transmitted through its body. But I found that even a very considerable pressure applied to the sides of the sea-nettle produced no effect on polarized light, and I thus found, what I might have learned by dissection, that the sea-nettle is not a true jelly, but consists of cells filled with fluid.

On the other hand, the crystalline lens of the eye, as Brewster observed, has a strong action on polarized light when strained either by external pressure or by the unequal contraction of its parts as it becomes dry.

I have enumerated these instances of the application of polarized light to the study of the structure of solid bodies as suggestions with respect to the application of the same method to liquids so as to determine whether a given liquid differs from a solid in having a very small "rigidity," or in having a small "time of relaxation"*, or in both ways. Those which, like Canada balsam, act strongly on polarized light, have probably a small "rigidity," but a sensible "time of relaxation." Those which do not show this action are probably much more "rigid," and owe their fluidity to the smallness of their "time of relaxation."

*The "time of relaxation" of a substance strained in a given manner is the time required for the complete relaxation of the strain, supposing the rate of relaxation to remain the same as at the beginning of this time.

XLIX. Intelligence and Miscellaneous Articles.

CALORIFIC EFFECTS OF THE MAGNETISM IN AN ELECTROMAGNET WITH SEVERAL POLES. BY A. CAZIN.

I

HAVE observed the calorific effects which accompany the disappearance of magnetism in the core of a rectilinear electromagnet presenting several consequent points, and have arrived at a very simple law:

When a rectilinear core of iron is magnetized by a series of identical coils through which the current passes in alternately opposite directions, if the coils determine equal concamerations, the quantities of heat produced in the core by the disappearance of magnetism are inversely proportional to the squares of the numbers of concamerations.

The apparatus I used is a sort of differential air-thermometer, the reservoirs of which are formed by iron cylinders 42 centims. in length, 5 centims. in diameter, and about 2 millims. in thickness. The bases of these cylinders are closed by plates of copper. A glass tube having an internal diameter of 2 millims., bent in the shape of a U and containing water, unites the two cylinders: it serves as a manometer to measure the difference of pressure, H, which establishes itself between the two reservoirs when one of them is heated; and if it is only a question of relative thermic effects, we may content ourselves with observing this difference of pressure; it is proportional to the difference of temperature of the two reservoirs.

In order that these reservoirs may be in the same conditions in regard to their surroundings, each of them is enveloped in a coating of cotton and a tube of pasteboard; and then two are placed in the axis of two identical bobbins of wood without contact between the wood and the pasteboard. The copper wire which receives the current is coiled round one of the series of bobbins, and can magnetize the corresponding core. These precautions are necessary in order that the voltaic heat of the wire may not be transmitted to the core. I ascertained that this condition was satisfied: the current remaining closed for a long time, the water manometer indicated only an insignificant change.

To make an experiment, a discontinuous current is thrown into the magnetizing coils; a tell-tale registers the number n of the interruptions; at each minute the difference of the levels in the manometer is noted; and the noting is continued when the current is suppressed, until the initial levels are restored. A graphic sketch of these data makes known, on the one hand, the total effect H produced during the action of the discontinuous current, and, on the other, the correction h due to the cooling action of the surrounding bodies. Dividing H+h by the number n of the interruptions, we have the effect of the disappearance of magnetism at each breaking of the voltaic circuit. This quantity serves as a relative measure for the heat generated by the magnetism.

The following is an example of this class of experiments. The core is magnetized by four coils of 18 centims. external diameter and 8 centims. depth; the iron core extends 5 centims. beyond the extremities of the coils; the copper wire is 2 millims. in diameter.

The pile consists of eight rectangular Bunsen elements; and the intensity of the continuous current is 0.0218, the unit being that which disengages in a voltameter 1 milligramme of hydrogen per second.

In the following Table, by the side of the values of n is set down the corresponding duration in minutes, and the values H and h are millimetres of water.

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The experimental law of which the above is an exposition is conformable to the law of magnetic energy which I communicated to the Academy on Nov. 18, 1872. Let us designate by m the quantity of magnetism received by the core when the current traverses the four coils in the same direction without forming consequent points, by the interval between the two poles; I have shown that the quantity of heat generated by the disappearance of this magnetism is measured by m2l.

Supposing that the current changes direction once, so that there is one consequent point in the middle of the core, this will behave like the combination of two independent cores of which each pos

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The differential thermomagnetic apparatus above described is susceptible of an improvement by which the core is completely screened from the voltaic heat of the magnetizing coil. This purpose is accomplished by surrounding the compensating cylinder of iron with a wire like that of the magnetizing coil, with merely the difference that the current traverses each layer of wire in alternately opposite directions. In this way the compensating cylinder is submitted to the same perturbing influence as the magnetized cylinder, but no magnetism is developed in it; the manometer indicates only the effect of the heat, of magnetic origin, which is generated in the magnetized core.

I have thus constructed a very accurate apparatus, by means of which I can deduce the value in calories of the thermic effect ob

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metres long could constantly be brought to a white heat. Doubtless these are very vague indications for estimating the temperature and the intensity of the current; but they suffice to give an idea of the conditions of the experiment.

The thermoheliometer being placed on a level with the carbons, and the blackened thermometer at a distance of 395 millims., there was found, at the end of half an hour, a difference of 3°.63 between the temperature of the other thermometers and that of the blackbulb; this difference remained constant for an hour, the fluctuations being insignificant.

The temperature produced by solar radiation was determined about noon on several days in July, and with the same instrument. For the difference there was found 17°-16, a quantity which agrees very well with that which was obtained several years since. To this value, however, must be applied the correction due to the absorption produced by our atmosphere; taking into account the height of the sun at the time of the observation, we are led to the value 17°.37.

Substituting these values in the preceding formula and calculating the diameters a and d from the dimensions and distances of the radiant areas, we get

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so that the solar radiation would be thirty-six and a half times that of the carbons. Nevertheless this valuation is below the truth; for we know that the correction made for the absorption by the atmosphere is too small. M. Soret found directly, on Mont Blanc, 21°-13; probably the value at the upper limit of our atmosphere would be about 27°. These two values would give respectively

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These ratios differ considerably from those which have been given by other observers. In the fear that there might be some cause of enormous error in my electric light, I compared it with the light of a stearine taper; I found that it was equal to 1450 tapers of commerce; it possessed therefore quite the intensity ordinarily exhibited by a good pile. In another series of experiments, made after the pile had been worked for some time, I found I, I, × 47·5, a result not very far from that at which I arrived above with the temperature 21°13 obtained directly by M. Soret.

Adhering, then, to this temperature of 21°13, which is incontestable and certainly less than the reality, and supposing that the temperature of the radiating surface of the carbons is 3000° (a number which is not exaggerated, since the whole extent of the platinum submitted to the experiment was fused), and supposing the radiation proportional to the temperature, we obtain for the potential temperature of the sun 133780°. This value may even

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