When the writer of this article had the pleasure of seeing Monsieur Arago at Paris in the course of last summer, he mentioned to him a series of experiments made on the refractive power of water at different tem peratures, in order to determine if its maximum density was above 32°. He filled a prism with water at the tem perature of 32°, and observed the angle of deviation pro duced by refraction, while its temperature rose front 32° to 212°. The angle of deviation was greater at 32°, and it gradually diminished to 212°, exhibiting no marks whatever of a variation of refractive power at 40°, or at any point between 32° and 212°. Hence Monsieur Arago concluded, that since the refractive power always increases with the density, the density of water must be al a maximum at 32°. This conclusion might have been admitted to have considerable weight, before Dr Hope had established the opposite conclusion by direct expe riment; but, independent of this circumstance, we have no hesitation in saying, that M. Arago's conclusion is not legitimately deducible from his experiments. If is assumed in his reasoning, that the refractive power of bodies increases with their density, a doctrine which re quires to he established by direct experiment, before it can be admitted as a valid argument in favour of any other position. Nay, it has actually been proved by Al bert Euler from numerous experiments, that the refrac tive power of glass is increased by heat. An augmen tation of temperature of 60° of Reanmur diminished the focal length part, and an augmentation of 33° pro duced a diminution of Al. Euler concludes, without sufficient evidence, that the refractive power of all fluids is increased with heat ; but though this is obviously er roneous, yet the experiments which he has made com pletely overturn the assumption of M. Arago, that the density at different temperatures may be inferred from the refractive power, and leave the subject of the maxi mum density of water in the same state as it had been left. by Dr Hope.
No experiments, so far as we know, have been made, with the view of explaining this singular property of water, nor does it readily appear how such experiments could be made. We have often thought that an inci pient crystallization takes place at the point of maximum density, and gradually increases till the congelation is completed. But in what manner are we to render this incipient crystallization apparent ? It is not visible to the eye ; nor can it be detected by the application of any known instrument. It has often occurred to us, that it may be rendered visible, if it does exist, by the action of the water in depolarising a ray of polarised light, a property which is possessed by the water as soon as it is converted into ice. We have shewn from numerous experiments, that a piece of glass which has no depolarising structure, receives it by an augmenta tion of temperature, and again loses it when the tem perature diminishes, and this communication and de struction of the depolarising virtue takes place when the glass is perfectly solid. Hence it is reasonable to infer, that water, though still in a fluid state in passing from 40° to 32°, may be gradually receiving that peculiar arrangement which is necessary to depolarise light; and which belongs to crystallized bodies. Several months ago, we tried this experiment in a very rude manner, by melting a quantity of snow between two plates of glass, and, after the snow was completely melted, a con siderable quantity of light was depolarised. As this effect, however, might have been produced by some in visible portions of unmelted snow, we do not consider the experiment as worthy of any confidence. We have, however, begun a series of experiments, for the purpose of determining this interesting point.
We shall now conclude this branch of the subject, with a tabular view of the different experiments that have been made on the expansion of mercury, water, the acids, and alcohol, which have been collected and arranged in a tabular form by Dr Thomas Young.
The formula used in the preceding Table is 113456— (if-F the temperature being p, or reckoning the degrees from 60,1 —.00058f—.000000625f From a diligent comparison of the observations of De Luc, and other natural philosophers, M. Biot of the In stitute of France has succeeded in discovering the law of the expansion of fluids ; and by means of a very simple formula, he has been enabled to calculate the true dilata tion corresponding to any given temperature. These interesting investigations have not yet been published ; but M. Biot has had the kindness to communicate to the Editor a general account of his labours.
In all fluids whose expansions have hitherto been ex amined, the true dilatation J'T reckoned from is capa ble of being expressed by the formula T indicating the temperature, and a, b, c, three coeffi cients constant for each liquid, but varying in different liquids. M. Biot has determined the value of these co efficients for eight or ten fluids, which are most Ire qucntly employed. The preceding formula is applica ble with equal accuracy at all temperatures, the fluid exists in some state of fluidity, and continues to be heated. It may, therefore, be extended even beyond ebullition, if the fluid is contained in a tube where it can not boil. The preceding formula likewise agrees with a set of fine experiments made by M. Charles, with an instrument of his own invention, for rendering sensible, and measuring the maximum expansion of water. A full account of M. Riot's investigations, will be found in the third volume of the Memoires d'Arcueil, which is now in the press ; and also in a complete treatise on natural philosophy, with which M. Biot is at present occupied, and to the appearance of which we look forward with the highest expectations.
On the Expansion of Gases.
The subject of the expansion of gases, has been inves tigated by Priestley, Roy, Saussure, Monge, Guyton, Duvernas, and Prony ; but the most accurate are those which have recently been made by Gay Lussac, and our countryman Mr Dalton. The results obtained by some of these eminent chemists, are given in the following Ta ble drawn up by Dr Thomas Young.
It follows from these experiments, that all the gases undergo the same expansion from the same increase of temperature, the expansion for each degree of Fahren heit being according to Dalton, and according to Gay Lussac. An account of the expansion of the va pour of water, will be found in our article STEAM.
For farther information on this subject, see Ellicott, Phil. Trans. vol. xlvii. p. 185. Ferguson's Lectures, vol. 1: p. 20. Smeaton, Phil. Trans. vol. xlviii. pt. 2. p. 612. Roy, Phil. Trans. vol. lxxv. p. 473. Troughton, Nichol son's Journal, vol. ix. p. 225. Wollaston, Phil. Trans. 1805. Croune, Birch's Hist. of Royal Soc. vol. iv. p. 263. Dc Luc Recherches sur les Modifications de l'At mosphere, torn. i. p. 225. Biot Menzoires d'Arcueil, tom. 3. (not yet published). Lefevre Gineau Mem. de l'In stitut. Dalton, Manchester Memoirs, vol. v. p. 59. Dalton, Xicholson's Journal, vol. x. p. 95 ; xiii. 377; and xiv. p. 128. Hope, Edinburgh Transactions, vol. v. p. 379. Gough, Vicholson's Journal, vol. xiii. p. 189. Mushet, Phil. Magazine, vol. xviii. p. 1. Gay Lussac, .1nnales de C'hinzie, torn. xliv. p. 87, or Nicholson's Jour nal, Oct. vol. iv. p. 207. Brewster, Phil. Trans. 1814, part 2, and 1815, page 1. Young's Natural Philoso phy, tom. ii. page 390. See also CHEMISTRY, vol. V. part ii. (3)