ON THE LIMITS OF VAPORISATION,

BY M. FARADAY, F.R.S.,

Director of the Laboratory of the Royal Institution, &c. &c.

I WAS induced some time since to put together a few remarks and experiments on the existence of a limit to vaporisation, which were favoured with a place in the Philosophical Transactions for the year 1826. When the experiments there mentioned were published, I arranged some others bearing upon the same subject, but which required great length of time for the developement of their result. Four years have since elapsed, during which, the effects, if there had been any, have been accumulating, and it is the object of this brief paper to give an account of them.

The point under consideration originally was, whether there existed any definite limit to the force of vaporisation. Water at 220° sends off vapour so powerfully, and in such abundance as to impel the steam-engine; at 120' it sends off much less; at 40°, though cold, still vapour rises; below 32°, when the water becomes ice, yet the ice evaporates; and there is no cold, either natural or artificial, so intense as entirely to stop the evaporation of water, or in the open air prevent a wet thing from becoming dry.

The opinion of many, among whom were the eminent names of Sir H. Davy and Mr. Dalton, was, that though the power of evaporating became continually less with diminution of temperature, it never entirely ceased, and that therefore every solid or fluid substance had an atmosphere of its own nature about it and diffused in its neighbourhood; but which being less powerful as the body was more fixed, and the existing temperature lower, was, with innumerable substances, as the earths, metals, &c., so feeble as to be quite insensible to ordinary or even extraordinary examination, though in certain cases they might affect the transmission of electricity; or, rising into the atmosphere, produce there peculiar and strange results, The object of my former paper was to shew that a real and distinct limit to the power of vaporisation existed, and that, at common temperature, we possess a great number of substances

which are perfectly fixed. The arguments adduced, were drawn first from the power of gravity, as applied by Dr. Wollaston, to shew that the atmosphere around our globe had an external limit, and then from the power of cohesion; either of these seemed to me alone sufficient to put a limit to vaporisation, and experiments upon the sufficiency of the latter force were detailed in the paper.

The conclusion was, that although such substances as ether, alcohol, water, iodine, &c., could not as such be entirely deprived of their vaporising force, by any means we could apply to them, but still, if in free space or in air, would send off a little vapour, yet there were other bodies, as iron, silver, copper, &c., most of the metals, and also the earths, which were absolutely fixed under common circumstances, the limit of their vaporisation being passed; and further, that there were a few bodies, the limits of whose vaporisation occurred at such temperatures as to be within our command, and therefore passable in either direction. Thus mercury is volatile at temperatures above 30°, but fixed at temperatures below 20°, and concentrated sulphuric acid, which boils at temperatures about 600°, is fixed at the ordinary temperature of the atmosphere.

It is well known in the practical laboratory that vaporisation may be very importantly assisted so as to make certain processes of distillation effectual, which otherwise would fail. Thus with the essential oils, many of them which would require a high temperature for their distillation if alone, and be seriously injured in consequence, will, when distilled with water, pass over in vapour with the vapour of the water at a much lower temperature, and, being condensed, may be obtained in their unaltered state.

It has been supposed that the vapour of the water, either by affinity for the vapour of the essential oil or in some other way, has increased the vaporising force of the latter at the temperature applied, and so enabled it to distil over; but there is no doubt that if air or any other similar elastic medium were made to come in contact with the mass of essential oil at 212° in equal quantity, and in a manner to represent the vapour of water, it would, according to well known laws, carry up the vapour of the essential oil perhaps to an equal extent,

and pass it forward; only the facility with which the carrying agent is condensed when it consists of steam, allows of the condensation of every particle of the essential oil vapour, whereas the permanency of the elastic state of the air would cause it to retain a large proportion of the vapour of the oil when cold, and consequently a diminished result would be obtained.

There are, nevertheless, some appearances which seem to favour the idea that occasionally water favours vaporisation beyond what air, equal to the bulk of the vapour of the water, would do in the manner referred to above; and it was to ascertain whether substances which, from a consideration of the general reasoning already referred to, and the high temperature at which they sensibly volatilized, might be considered as fired at common temperatures, could have any sensible degree of volatility, in conjunction with water or its vapour, conferred upon them at ordinary temperature. It is well known that a theory of meteoric stones has been founded on the supposition that the earthy and metallic matter found in them had been raised in vapour from similar matter upon the earth's surface; which vapours, though extremely attenuated and dilute at first, gradually accumulated, and by some natural operation in the upper regions of the atmosphere became condensed, forming those extraordinary masses of matter which occasionally fall to us from above. The theory has in its favour the remarkable circumstance, that, notwithstanding many substances occur in meteoric stones and iron, yet there is none but what also occur on this our earth *; and it also has a right to the favouring action of water, if there be such an action; because vaporisation is one of the most important, continual, and extensive operations that goes on between the surface of the globe and the atmosphere around it.

In September, 1826, several stoppered bottles were made perfectly clean, and several wide tubes close at one extremity, so as to form smaller vessels capable of being placed within

This very striking circumstance does not prove that aërolites in any way originate from our planet; but then, if we could by other arguments deduce that they were extraneous, it would lead to the conclusion that the substances which have been used in the construction of this our globe, are the same with those which have been used extensively elsewhere in the material creation.

the bottles, were prepared. Then selected substances were put into the tubes, and solutions of other selected substances into the bottles: the tubes were placed in the bottles so that nothing could pass from the one substance to the other, except by way of vaporisation. The stoppers were introduced, the bottles tied over carefully and put away in a dark safe cupboard, where, except for an occasional examination, they have been left for nearly four years, during which time such portion of the substances as could vaporise have been free to act and produce accumulation of their specific effects.

No. 1. The bottle contained a clear solution of sulphate of soda with a drop of nitric acid,-the tube, crystals of muriate of baryta. One half or more of the water has passed by evaporation into the tube, and formed a solution of muriate of baryta above crystals, but both that and the remaining solution of sulphate of soda is perfectly clear; there is not the slightest trace of sulphate of baryta in either the one or the other, so that neither muriate of baryta nor sulphate of soda appear to have volatilised with the water.

No. 2. Bottle, solution of nitrate of silver; tube, fused chloride of sodium. All the water has passed from the nitrate of silver to the salt; but there is no trace of chloride of silver either in one or the other. No nitrate of silver has sublimed with the water, nor has any chloride of sodium passed over to the nitrate.

No. 3. Bottle, solution of muriate of lime; tube, crystals of oxalic acid. The water here remained with the muriate of lime. In the tube, the oxalic acid when put in had formed a loose aggregation, with numerous vacancies, and with a very irregular upper surface about an inch below the upper edge of the tube. No particular appearances occur in the vacancies; but at the top there has evidently been a sublimation of the oxalic acid, for upon the crystals and glass new crystals in exceedingly thin plates and reflecting colour have been formed; these rise no higher in the tube than to the level of the most projecting part of the original portion of oxalic acid; no appearance of sublimation is evident above this, and it seems as if the most elevated parts of the salt have given off vapour, which has sunk and formed crystals on the neighbouring

lower surfaces, but that no vapour has risen to the upper part of the tube. On examining the solution by a drop or two of pure ammonia, it was however found that a slight precipitate of oxalate of ammonia occurred. The experiment shews, therefore, that oxalic acid is volatile at common temperatures, and had not only formed crystals in the tube, but has passed over to the solution of lime.

No. 4. Bottle, solution half sulphuric acid, half water; tube, crystallized common salt. No water has passed to the salt. On opening the bottle, the clear diluted sulphuric acid was examined for muriatic acid, but no trace could be found. Hence chloride of sodium has not been volatilised under these circumstances.

No. 5. Bottle, solution of muriate of lime; tube, crystals of oxalate of ammonia. The oxalate of ammonia appeared quite unchanged. The solution of muriate of lime was perfectly clear; but when a little pure ammonia was added to it, a very faint precipitate of oxalate of lime was produced.

No. 6. Bottle, little solution of potash; tube, white arsenic in pieces and powder. This bottle was opened because of the appearances, in October, 1829, and had then remained three years undisturbed. The arsenious acid was to all appearance unchanged. The solution of potash was turbid and foul. On chemical examination, it proved to have acted powerfully on the glass. It had dissolved so much silica as to become a soft solid, by the action of an acid, and it had also dissolved a considerable quantity of lead; but there was no trace of arsenious acid in it; so that this substance, although abundantly volatile at 600°, had not risen in vapour when aqueous yapour and air was present at common temperatures.

No. 7. Was some of the sulphuric acid used in these experiments, preserved for comparison,

No. 8. Bottle, solution half sulphuric acid, half water; tube, pieces of muriate of ammonia. When this bottle was opened, the pieces of muriate of ammonia presented no appearance of change; there was no moisture about them, nor any appearances of dissection that I could distinguish. The diluted sulphuric acid being examined by sulphate of silver, gave no appearances of muriatic acid; so that muriate of ammonia appears fixed under these circumstances.