that had imbibed it; whereas, where nitrate of strontian was employed, the functions of life appeared to go on for a considerable time without material obstruction.

Upon the whole, then, I see nothing, so far as experiments have yet gone, to invalidate the conclusion, to which the preceding facts appear to lead, that the roots of plants do, to a certain extent at least, possess a power of selection, and that the earthy constituents which form the basis of their solid parts are determined as to quality by some primary law of nature, although their amount may depend upon the more or less abundant supply of the principles presented to them from without.

Note on Taking the Dew-Point directly, by the Method of Leroi and Saussure.

THE following observations aim at nothing new in point of principle, and are merely intended to facilitate the long known and simple mode of finding the dew-point, by the cooling action of saline solutions on polished surfaces. The arrangement is by no means suggested as entering into competition with more costly and ingenious hygrometric instruments; but will probably be found useful in the absence of such aids.

The objects proposed are, first, to reduce within very narrow limits the quantity of materials, and size of the vessel used, and thereby extend the circumstances under which this method may be adopted; and, secondly, to avoid the trouble of any transference of the materials from one vessel to another, or of any successive additions of the substances in the course of the same experiment, by producing the reduction of temperature simply by a gentle and carefully regulated agitation of the vessel.

wide, and is farther The bottle, when filled to an ounce. The outside is

A little hollow ball, or bottle of thin brass, like that in the opposite page, is employed, having a diameter of 14 inch. The neck is 1% to 1% of an inch long, and about expanded into a funnel shape at top. the top of the neck, contains about preserved bright and polished; and it will save trouble in cleaning, although not answer the purpose better, to have the outside gilt. A small thermometer is used, having a very fine

wide; a

capillary bore, and an oblong bulb 1%
to of an inch long, by
very light and narrow scale, four inches

long, and ranging from 100° Fahr. to
near zero, is attached from 2 to 3
inches from the extremity of the bulb:
and a cork capable of fitting into the
neck of the bottle slides on the bare
part of the stem. The more similar the
circumstances, the more constant the re-
sults are likely to be.

As the cooling material, equal parts of nitre and sal ammoniac, are employed, the salts being rather finely pounded, well dried, and thoroughly mixed together; and a stock of this mixture is kept

ready prepared in a close bottle. A little glass tube is provided, capable of holding, when filled to a mark, 40 grains of the mixed salts. This quantity is called a measure of the salts. Another tube is in like manner provided, holding, when filled to a mark, oz. measure. This is called a measure of water.

Suppose, then, that we introduce into the brass bottle one measure of the salts. If we next, taking care that the neck is not obstructed by salt, pour into the bottle very gently one measure of water, at the temperature of the surrounding air, and immediately, but cautiously, introduce the thermometer into the bottle, so that the bulb shall reach the bottom, and the cork fit into the neck, the mercury will fall a few degrees, rarely exceeding five or six; and the farther reduction of temperature we have entirely at our command, By an agitation of the bottle, at first very gentle and cautious, and afterwards slowly and gradually increased, holding it by the upper part of the neck, the farther fall of the thermometer will be so regular and gradual, that we can easily mark the temperature at which the deposition of moisture occurs on the polished external surface of the brass ball.

With the proportion of salts and water just indicated, we can usually obtain a reduction of temperature of from 15° to 20° below that of the water used. This will include by far the VOL. XIX. NO. XXXVII —JULY 1835.

M

greater number of cases of the dew-point which will fall under our observation. By employing 14 measure of the salts with the same proportion of water as before, a few degrees greater fall will be commanded; and with two measures of salt and one of water, a still greater cold is procured if wanted..

The point that chiefly requires attention is a moist state of the atmosphere, of which the operator can easily judge by the actual or recent fall of rain, and by the general appearances of the heavens. In such cases, it is necessary to use water of a temperature from 5° to 10° above that of the air, which is readily accomplished, simply by grasping the measure containing the water in the hand for a minute or two before pouring it into the brass bottle. In this way, the initial fall of the temperature is so much retarded, that we can easily, during the subsequent gentle agitation, mark the dew-point, even when differing little from the temperature of the air; and whenever we find that a too rapid fall takes place at first, we have always this method in our power as a corrective.

On the other hand, when very fine weather, a high barometer, and other general appearances, indicate a very dry state of the atmosphere, it may be proper to employ 1 measure of salts and one measure of water, to ensure a greater reduction of temperature.

The usual mode of operating will, however, be with one measure of the salts and one measure of water, either of the temperature of the air, or some degrees above it, as appearances may suggest. After a little experience has given the requisite dexterity in the manipulation, and a moderate degree of foresight respecting the general state of the atmosphere, we shall not often find it necessary to repeat the operation. In a series of observations at the distance of some hours from one another, the preceding observation will of course be some guide to the subsequent *.

With the ball, thermometer, and measures, and a small bottle of

• In a very few instances, when the dew-point is high, the layer of salt causes a deposition on the bottom of the bottle, before it extends to the sides. This appearance, which occurs only very rarely, is to be overlooked, because the thermometer is scarce affected by the cold which occasions it; and we must wait until, during the gentle agitation, we observe the deposition on the middle part of the sides of the bottle, as we are then sure that the cold has been equally diffused, and the thermometer duly affected.

the mixed salts, we are ready for finding the dew-point in all situations, and a very few minutes will suffice for the purpose. Within-doors, we have only to call for a jug of water, and a small cup to receive the liquid used in rincing out the brass ball once or twice after the operation. In the open air, a spring or brook will supply all our wants. In situations where water may not be found, as on the summit of mountains, a phial of the liquid carried with us will remedy the deficiency; and if its temperature has risen a little too high, we have only to employ 14 or 2 measures of the salts, as may be necessary. The writer of this note has, for a year or two past, used this little apparatus in a great variety of situations, and, amongst others, during the barometric measurement of heights; and having thus had sufficient experience of its usefulness, he has thought that it might be worth while to give this short notice respecting it.

During frost, water at 33°, and 1, 14, or 2 measures of salts, as circumstances may point out, will give the requisite cold. For more severe climates, the vessel might be of polished platinum, and the materials an acid and ice, or the proper salts. AB. CONNELL.

SCIENTIFIC INTELLIGENCE.

NATURAL PHILOSOPHY.

1. Velocity of Electricity.-The following notice of Professor Wheatstone's very interesting experiments on the velocity of Electricity, will enable the reader to form a general conception of them. Two very great difficulties present themselves in this inquiry, the one arising from the circumstance that the sensation of light on the retina continues longer than the impression which produces it, as is illustrated by the well known experiment of a luminous circle being produced by a point of light in rapid rotation; this prolongation, very manifestly, is a great obstacle to the exact measurement of the duration of the impression the second difficulty is the prodigious extent of conductor that could be used for the purpose of exhibiting any appreciable difference in the transit of a spark from one point to another. The immense velocity of electricity makes it impossible to calculate it by direct observation; it would require to be

any

many thousands of leagues long, before the result could be expressed in the fractions of a second. It being thus impossible to arrive at any result by direct experiment, the Professor had to task his ingenuity that he might attain his object in some other way. After many fruitless attempts, he availed himself of the following expedient, which has already yielded many interesting results, and which he hopes still farther to improve, and render practically useful. He placed a double metallio mirror at the extremity of an axis of rotation, to which, by means of a large spinning-wheel, he could give an exceedingly rapid motion. The musical tone produced by the vibration of a card attached to the axis, supplied the number of revolutions, A luminous point, seen by the reflection of the two faces of the moving mirror, produced, in every semirevolution, two luminous circles, proceeding in the opposite direction to the mirror. All inequalities of the flame were made manifest by the circles appearing more or less distinct. By means of this arrangement, he discovered a series of luminous condensations and dilatations in the flame of hydrogen, when it produced sound in passing through a glass tube. He also observed that the electrical aura, as manifested in erecting feathers, down, &c. was not a continuous stream, but a rapid succession of very minute sparks. He then took a wire half a mile long, which he cut in the middle, and arranged each half in such a way, that both extremities and the central ends were in a line parallel to the mirror, so that the three sparks appeared on one straight line. As this sensation on the retina continued longer than the impression, instead of seeing points, which the three sparks should have produced, he observed three arcs of a circle, the origins of which should commence on the same right line, if the transit at the three sections were instantaneous. He imparted a velocity of 800 revolutions in a second to the mirror. Even at this degree of velocity, the origin of the first and third arcs continued on the same right line, but the middle one was somewhat in advance or behind the other two, according to the direction which was given to the rotation. Here the Professor remarks, that this experiment does not agree with the view that electricity is a single fluid, entering at the one end and issuing at the other, as Franklin's theory requires. As the sparks at the two extremities are always consentaneous, whilst that in the middle appears somewhat