Supersulphate of alumina and potash. Sulphate of zinc.

In the progress of these experiments I could not avoid observing, that the principal characteristic difference between these two sets of salts was, that the first contain water of crystallization, whereas the second are anhydrous. So generally true is this, that it was with some surprise that I found bichromate of potash, an anhydrous salt, to form an exception. Of all the rest of the salts in the first table, the crystals are hydrates. In the second table there are two exceptions, sulphate of ammonia, and sulphate of magnesia and potash, both of which are crystalline hydrates. These three exceptions (and many others may yet be found) annul the distinction between hydrated and anhydrous salts, and invalidate any general rule which might be founded on it.

Whilst operating on sulphate of soda, I frequently observed a deposition of a portion of the salt, in the form of brilliant transparent crystals, which became white and opaque when crystallization was induced in the remaining liquid. This phenomenon is described by Mr Faraday in the Journal of Science, vol. xix.; he finds the crystals to contain eight only, instead of ten, atoms of water of crystallization. Mr Faraday refers to their production in closed vessels only. I have generally experimented with open vessels, and all the instances I have seen have

been in such. In one case the vessel was an open dish four inches wide.

But there is another phenomenon in the habitudes of sulphate of soda with water, which, so far as I know, has not been noticed by any writer. Under certain circumstances, a cold supersaturated solution has the power to dissolve an additional quantity of the crystallized salt. Not only does it dissolve it, but the solution is greatly facilitated by agitation, unless, by some capricious incident, the agitation should excite it to crystallize. To illustrate this point, four ounces of sulphate of soda may be dissolved in four ounces and a half of hot distilled water in a glass-flask. The superfluous salt must be allowed to crystallize, and the vessel containing both salt and mother-water is to be placed in a dish containing sand, and exposed to a temperature of 120° or 130° F. in a common kitchen-oven. When all the salt, with the exception of about a drachm, is dissolved, the flask is to be removed and carefully cooled. If this is successfully done, it is not accompanied by any deposition of crystals. In this stage, the flask contains a cold supersaturated solution, along with the portion of salt which remained undissolved by the heat of the sand-bath *. It is now to be gently inclined to one side, so as to elevate the undissolved crystals into the superior part of the liquid. After standing an hour or two in this position, the most elevated part of the salt will have been dissolved; and, the vessel being inclined in another direction, another part of the salt is in its turn raised to the superior part of the liquid, and there dissolved.

In repeating the experiment, I have generally shaken the vessel briskly, and found the crystals to dissolve with much greater rapidity in consequence; though this very agitation has sometimes induced crystallization before the solution had been completed.

Thus, a solution of sulphate of soda more than saturated, and which has stood two, three, or four days in a cool room, actually continues to exert a solvent power on salt of its own kind.

It was suggested to me, that the undissolved salt might not be sulphate of soda, but some accidental impurity, soluble in Furnishing another proof that the presence of crystals in a supersaturated solution does not necessarily determine crystallization.

virtue of the known power possessed by saturated solutions to dissolve a little of another different salt. I had no reason to doubt the purity of the salt which I employed, and I tried many different specimens, and always obtained the same result. To put it still further to the test, I prepared the solution as above described, and when cool, poured the supersaturated liquid into another vessel, and crystallized it. I then returned the mother water into the flask containing the undissolved salt; but it had no solvent effect on it. So that the very same salt which would have dissolved in a cold supersaturated solution, was insoluble in a solution simply saturated, which is the condition of mother water after the deposition of crystals.

Doubtless this power of solution possessed by the supersaturated liquid, has its limits; as the first stage, or simple saturation, is the limit of the solvent power of cold water; but what is the limit of supersaturation, yet remains to be ascertained. And the inquiry is beset with some difficulties; for, independently of the great liability of strong solutions entirely to crystallize, the process is sometimes interrupted by the deposition of the brilliant quadrangular crystals, containing eight proportionals of water of crystallization. Sulphate of soda may indeed be considered capable of three stages of saturation; the first is the limit of the solvent power of cold water; the second is the li quid which has deposited quadrangular prisms; and the third contains a still greater quantity of salt. The next experiment illustrates these three stages.

13. A supersaturated solution of sulphate of soda, with a portion of undissolved salt remaining at the bottom of the vessel, was allowed to stand at rest for four days. It was then briskly agitated, and most of the salt was dissolved. All this time it had existed in the third stage of saturation, although not at the limits of that stage. The next day there had been a considerable deposition of brilliant and transparent crystals. The remaining liquid was now in the second stage of saturation. The whole was again repeatedly shaken in the course of three hours, without effecting any change. Two hours afterwards, the liquid suddenly became nearly solid, without any apparent cause'; the small quantity of fluid now remaining was in the first stage of

saturation.

I never could calculate with any certainty on the phenomena which these experiments would present. They were often total failures, and it was only by frequent repetition of them that their peculiarities could be observed. The following is an instance of the irregularities I met with.

14. A pound of crystallized acetate of soda was fused in its water of crystallization, and poured into a clean glass retort. Six hours afterwards, it was cold and perfectly fluid, with the exception of a mass of crystals about the size of a hazel nut, which lay at the bottom of the retort, and a few smaller masses which were floating in the liquid. It was several times shaken without its shewing any tendency to crystallize. A small crystal of the same salt being then dropped in, the entire mass became solid in a few seconds. The evolution of sensible heat which attends the transition from the liquid to the solid state, was in this case very considerable.

15. Sulphuric acid sufficient to decompose the acetate of soda was then added, and the acetic acid was drawn off by distillation. Water was poured on the residuary sulphate of soda when cold. The next day about two-thirds of the salt had dissolved; but on attempting to pour it out of the retort, it suddenly crystallized, and became a semifluid mass.

SUNDERLAND.

On the Magnitude of the Ultimate Particles of Bodies; Infusory Animals not formed immediately from Dead Matter; Extraordinary Minuteness of the Infusoria; Improved Arrangement of the Infusoria; Marvellous Multiplication of the Infusoria; Estimate of the relative value of the Microscopes of Chevalier, Ploessel, and Schiek. By Prof. C. G. EHRENBERG of Berlin.

Magnitude of Ultimate Particles.-Within these few years, the atomists have become pretty confident in their doctrines relating to the minute particles of bodies. They have not rested satisfied with viewing atoms as ideal unities of an infinite degree of minuteness, but have sought for approximative numerical ex

pressions of their magnitudes. Nay, many bold theoretical experimenters of the present day seem to have fallen little short, in their own imagination, of in reality seizing the ultimate elements of bodies, and of reconstructing them at pleasure.

Newton long ago taught us to believe that the elements of colour were of tolerable magnitude. His words are," Could the power of the microscope be so increased as to represent objects at a foot distance, magnified 500 or 600 times above what they appear to the naked eye, I imagine that we might discover some of the coarser elements which enter into the formation of colours; and that, with a microscope which magnified 3000 or 4000 times, we might recognise them all, even those which form the black colour." Supposing that Newton has correctly estimated the natural power of human vision, his elementary particles, as appears from the following observations, would, for the red colour, not exceed a minuteness of 3" in diameter, between which and 1000", all coloured particles, not excepting the black, would be included*. But it is probable that Newton's estimate of the acuteness of vision is below the truth, when these elementary particles would become considerably larger. But, as has been already observed by Herschel, in his Optics, it must not be forgotten that Newton made a marked distinction between the elements of colours and atoms, as well as later philosophers, although he does not expressly say so. In the above passage, Newton does not speak of atoms, but of colouring particles.-(Traité d'Optique, 1704; lib. ii, part iii; Ed. Franc. 1720, p. 357.)

The small magnitudes which have been required for the explication of the phenomena of light upon the theory of undulation, are exactly enough determined by calculation, but they can only be considered as hypothetical, not as really observable quantities, as the whole theory, however great its probability, still requires more complete demonstration. The smallest length of a wave of light on this theory, calculated from the most exact data, does not exceed 100000", or about "". But, as the particles of ether must be considerably smaller than their undu

1

8000

• The general reader may be informed, that 0" is the mark for inch, 0"" for line or 12th part of an inch.