the temperature rises. At 2590° the brilliancy is more than thirty-six times as great as it is at 1900°.

Thus, therefore, the theoretical anticipation which we founded on the analogy of light and heat is completely verified; and we discover that as the temperature of a self-luminous solid rises, it emits light in a greater proportion than would correspond to the mere difference of temperature. To place that analogy in a still more striking point of view, I will here introduce some experiments I have made in relation to radiant heat. No chemist, so far as I am aware, has hitherto published results for high temperatures, or endeavored to establish, through an extensive scale, the principle of Delaroche, that "the quantity of heat which a hot body gives off in a given time by way of radiation to a cold body, situated at a distance, increases, other things being equal, in a progression more rapid than the excess of the temperature of the first above that of the second."

As my object on the present occasion is chiefly to illustrate the remarkable analogy between light and heat, the experiments now to be related were arranged so as to resemble the foregoing; that is to say, as in determining the intensities of light emitted by a shining body at different temperatures, I had received the rays upon a screen placed at an invariable distance, and then determined their value by photometric methods; so, in this case, I received the rays of heat upon a screen placed at an invariable distance, and determined their intensity by thermometric methods. In this instance the screen employed was in fact the blackened surface of the thermo-electric pile. It was placed at a distance of about one inch from the slip of incandescent platinum, a distance sufficient to keep it from any disturbance from the stream of hot air arising from the metal; care also was taken that the multiplier itself was placed so far from the rest of the apparatus, that its astatic needles could not be affected by the voltaic current igniting the platinum, or the electro-magnetic action of the wires used to modify the degrees of heat.

The experiments were conducted as follows:-The needles of the thermo-multiplier standing at the zero of their scale, the voltaic current was passed through the platinum, which immediately rose to the corresponding temperature, and radiated its heat to the face of the pile. The instant the current passed, the needles of the multiplier moved, and kept steadily advancing upon the scale. At the close of one minute, the deviation of the needle and the temperature of the platinum were simultaneously noted, and then the voltaic current was stopped.

Sufficient time was now given for the needle of the multiplier to come back to zero. This time varied in the different cases, according to the intensity of the heat to which the pile had been exposed in no instance, however, did it exceed six minutes, and

in most cases was much less. A little consideration will show that the usual artifice employed to drive the needles back to zero by warming the opposite face of the pile, was not admissible in these experiments.

The needles having regained their zero, the platinum was brought again to a given temperature, and the experiment conducted as before. The following table exhibits a series of these results.

Table of the Intensity of Radiant Heat emitted by Platinum at differ

ent Temperatures.

In this table the first column gives the temperatures of the platinum in Fahrenheit degrees; the second and third two sets of experiments, expressing the arc passed over by the needle at the close of a radiation lasting for one minute, each number being the mean of several successive trials; and the fourth the mean of the two. It therefore gives the radiant effect of the incandescent platinum upon the thermo-multiplier for the different temperatures.

Of course it is understood that I here take the angular deviations of the needle as expressing the force of the thermo-electric current, or in other words, as being proportional to the temperaThis hypothesis, it is known, is admissible.

tures.

It therefore appears that the quantity of heat radiated by incandescent platinum at 980° being taken as unity, it will have increased at 1440° to 2·5; at 1900° to 7·8; and at 2360° to 17.8, nearly the rate of increase is therefore very rapid. Further, it may be remarked, as illustrative of the same fact, that the increased quantity of heat radiated by a mass of platinum in passing from 1000° to 1300°, is nearly equal to the amount it gives out in passing from common temperatures up to 1000°.

I cannot here express myself with too much emphasis on the remarkable analogy between light and heat which these experiments reveal. The march of the phenomena in all their leading points is the same in both cases. The rapid increase of effect as the temperature rises is common to both.

It is not to be forgotten, however, that in the case of light we necessarily measure its effects by an apparatus which possesses special peculiarities. The eye is insensible to rays which are not comprehended within certain limits of refrangibility. In these experiments, it is requisite to raise the temperature of the platinum almost to 1000° before we can discover the first traces of light. Measures obtained under such circumstances are dependent on the physiological action of the visual organ itself, and hence their analogy with those obtained by the thermometer becomes more striking, because we should scarcely have anticipated that it could be so complete.

Description of the apparatus employed in the foregoing experi

ments.

The source of light is in all instances a slip of platinum foil 1.35 inch long, andth of an inch broad, ignited by the passage of a voltaic current, and placed in such a position that its dilatation could be measured by the movements of an index over a graduated scale.

In fig. 7, ab represents the slip of platinum, the upper end of which is soldered to a stout and short copper pin a, firmly sunk in a block of wood c, which is immovably fastened on the basis dd of the instrument. A cavity e, half an inch in diameter, is sunk in the block c, and into this cavity the pin a projects; so that when the cavity is filled with mercury, a voltaic current may be passed through the pin and down the platinum.

The other extremity of the platinum b is fastened to a delicate lever bf, which plays on an axis at g, the axis working in brass holes supported on a block h. Immediately beneath the pla

tinum strip, and in metallic communication with it, a straight copper wire dips down into the mercury cup m; on this wire there is a metal ball n, weighing about 100 grains. The further end of the index plays over a graduated ivory scale pp, which is supported on a block 4, and can be moved a little up and down, so as to bring its zero to coincide with the index at common temperatures.

The action of the instrument is readily understood. In the mercury cup e dip one of the wires N of a Grove's battery of three or four pairs, the other wire P being dipped into the cup m. The current passes through the platinum, which immediately expands, the weight n lightly stretching it. The index f moves promptly over the scale, indicating the amount of expansion, and therefore the degree of heat. Remove the wire N out of its mercury cup e, the platinum instantly becomes cold, and pulls the lever to the zero point.

When the platinum is thin, so as to be quite flexible at the point b, where it is fastened to the index, the movements take place with such promptitude and precision as to leave nothing to be desired. When the heat has been very high and long continued, the limit of elasticity of the platinum is somewhat overpassed, and it suffers a slight permanent extension. But as the ivory scale p p can slide up and down a little, the index is readily re-adjusted to the zero point.

I

The temperature of the platinum depends entirely on the force of the current passed through it. By intervening coils of brass wire of lengths adjusted beforehand, so as to resist the current to a given extent, any desired temperature may be reached. found it convenient to intervene in the course of the current one of Prof. Wheatstone's rheostats, so as to be able to bring the index with precision to any degree, notwithstanding slight changes in the force of the voltaic battery.

The following are the dimensions and measures of the instrument I have used:-Length of the platinum strip, 1.35 inch; length of the part actually ignited, 114 inch; width of ditto,

th of an inch; length of the index from its centre of motion to the scale, 7.19 inches; distance of the centre of motion of index from the insertion of the platinum at the point b, 22 inch; multiplying effect of the index, 32.68 times; length of each division on the ivory scale, 021 inch. From this it would appear, by a simple calculation, using the coefficient of dilatation of platinum given by Dulong and Petit, that each of the divisions here used is equal to 114-5 Fahrenheit degrees. For the sake of perspicuity I have generally taken them at 115°.

The Grove's battery I have employed has platinum plates three inches long and three-quarters wide; the zinc cylinders are two inches and a half in diameter, three high, and one-third SECOND SERIES, Vol. IV, No. 12.-Nov., 1847. 51

thick. As used in these experiments, it could maintain a current nearly uniform for an hour. I commonly employ four pairs.

Among writers on optics, it has been a desideratum to obtain an artificial light of standard brilliancy. The preceding experiments furnish an easy means of supplying that want, and give us what might be termed a "unit-lamp." A surface of platinum of standard dimensions, raised to a standard temperature by a voltaic current, will always emit a constant light. A strip of that metal, one inch long and th of an inch wide, connected with a lever by which its expansion might be measured, would yield at 2000 a light suitable for most purposes. Moreover, it would be very easy to form it an available photometer, by screening portions of the shining surface. An ingenious artist would have very little difficulty, by taking advantage of the movements of the lever, in making a self-acting apparatus, in which the platinum should be maintained at a uniform temperature, notwithstanding any change taking place in the voltaic current. University, New York, Feb. 27, 1847.

ART. XXXVI.—On the Changes which the Albuminous Substances undergo in the Stomach, during the process of Digestion; by Prof. MULDER, of Utrecht. (Translated from the Dutch, by Dr. Aug. VÖLCKER.)

I LAST year demonstrated,* that the fibrin of blood undergoes no change in composition by solution in muriatic acid and precipitation by carbonate of ammonia.

The results of my analysis, employing in the present instance my last experiments on the amount of sulphur in these substances, were as follows:

The phosphorus has not been determined in the dissolved portion; but as vitellin loses phosphamid under the influence of acetic acid and ammonia,† it is probable that fibrin will have been deprived of the phosphamid under the influence of muriatic acid and carbonate of ammonia.

* Scheik. Onderz., Deel iii, p. 470.

↑ Von Baumhauer in Scheik. Onderz., Deel iii, p. 284.