but sufficient to burn off any air, if air existed, still acts as a nucleus.

In my paper in the 'Proceedings' the following experiment is given :

"It is said that rough bodies are most favourable to the liberation of vapour. The hot carbonic disulphide was touched with a rat's-tail file, and it produced furious boiling. The file was then held in the flame of a spirit-lamp, and while hot placed in the upper part of the tube, so that it might cool down to about the temperature of the liquid and yet be sheltered from the air. On touching the surface of the disulphide with the end of the file, there was no liberation of vapour; and the file was slowly passed to the bottom of the liquid, but still there was no action. The file was now taken out and waved in the air; on reinserting it into the liquid, there was a burst of vapour, arising from some mote or speck of dust caught by the file from the air. The file was quickly cleaned by the liquid, and it became inactive as before. It was again taken out and waved in the air; and on once more putting it into the liquid, boiling set in again."

But, according to M. Gernez, the file instead of catching nuclear particles caught air, which it reintroduced into the hot liquid. He does not attempt to explain the following experiment, although he refers to the paper containing it*.

Wood-spirit, boiling at 140° F., contained in a test-tube, was plunged into a flask of hot water. A clean glass rod that had been exposed during an hour to the air of my garden was inactive. It was drawn through the hand that had been made slightly greasy with lard, and when reinserted it produced such a burst of vapour as to turn out half the contents of the tube.

A similar result was produced with ether and bisulphide of carbon.

Surely I am not exceeding the bounds of philosophical discretion in insisting that a theory of boiling that does not include within its generalizing influence such results as the above must be defective.

:

I have endeavoured to show + that solids, in their action on gaseous or vaporous solutions, admit of being arranged into four classes (1) Vitreous bodies and certain metals which, in a catharized or chemically clean state, have no nuclear action, because there is perfect adhesion between them and the solution. (2) Bodies that are not wetted by the water of the solution, but to which the gas or vapour can adhere. Such are fatty or oleaginous bodies and also resinous bodies; these act as nuclei.

"Reply to the Rev. Father Solaro in Les Mondes, Dec. 21, 1871," Phil. Mag. March 1872.

* Phil. Mag. April 1873.

(3) Porous bodies, such as charcoal, coke, the various woods, meerschaum, chalk, &c.; these act as permanent nuclei. (4) Soluble substances, such as sugar, gamboge, &c., which act as nuclei by diminishing adhesion between the gas or vapour and the water. A body in Class 1 is said to be chemically unclean when it is contaminated by a body in Class 2.

M. Gernez regards these bodies merely as vehicles for conveying air into the body of the liquid; he maintains that such bodies have no nuclear action per se, that porous bodies, such as charcoal, and resinous and fatty bodies, such as shellac, stearine, &c., have no specific action, because when deprived of air by repeated boiling they become inactive. I have not been able to deprive fragments of cocoa-nut-shell charcoal of their nuclear character, although employed in distillations for days and weeks together. My first paper contains some numerical results showing how singularly efficacious cocoa-nut-shell charcoal is in separating vapour from boiling liquids and increasing the amount of the distillate; and on this account the use of porous nuclei in such operations is strongly recommended. M. Gernez also, in his practical applications at the end of his memoir, gives similar advice.

I felt the point in dispute to be of such importance that I repeated the directions of M. Gernez, namely to boil what I call a permanent nucleus a number of times at short intervals. For this purpose I selected a fragment of cocoa-nut-shell charcoal. Mr. Hunter has taught us how powerfully and rapidly this substance absorbs vapours at various high temperatures-such as 255-4 times its volume of the vapour of a mixture of 10 cubic centims. of absolute alcohol and 30 cubic centims. of water, 150 times its volume of methylic alcohol, 261-8 of a mixture of 10 cubic centims. of methylic alcohol and 20 cubic centims. of water, 117 of bisulphide of carbon, 87 of ether, 116 of acetic ether. These measurements were all taken at 100° C. The mean of several observations for vapour of water gave 43.9 volumes absorbed at 127° C. Accordingly water was boiled over the flame of a spirit-lamp in a clean tube in the presence of a triangular fragment of cocoa-nut-shell charcoal. It was boiled eleven times, at intervals of rather more than five minutes, as M. Gernez directs. While the tube was still warm it was put under the receiver of an air-pump; and the exhaustion had not proceeded far when a violent burst of vapour from the charcoal turned out about one third of the water from the tube. The tube was again boiled, plugged with cotton-wool, and left for about three hours. It was again placed under the receiver, and * Journal of the Chemical Society, vol. iii. 1865, p. 215. See also vols. v, & vi.

the exhaustion was made as perfect as possible. Not a single bubble was visible either by daylight or by candlelight. Had there been a bubble in any part of the tube it must have been detected, since the bubbles have the lustre of mercury. On again holding the tube over the flame, the charcoal gave a few crackling sounds and then emitted vapour in abundance, and the boiling was as easy as at first. Next morning abundance of airbubbles were given off during the boiling, showing that air had been absorbed during the night.

I do not see any à priori reason why the charcoal, as a nucleus, should be exhausted. Its absorptive powers remain, whether for vapour or gas, whatever number of times the same specimen be used. In Mr. Hunter's experiments the air was expelled by raising the charcoal to a red heat and quenching it under mercury before introducing it into the vapour. When used as a nucleus in boiling, the air is gradually displaced by the vapour; it is continually absorbing vapour and emitting it under the continued action of the heat. Its action resembles those catalytic processes in which a body remaining apparently unchanged brings about continuous changes in another bodyas, for example, where the presence of a small quantity of black oxide of manganese enables chlorate of potash to liberate its oxygen at a moderate heat. There is probably a continuous formation of permanganic acid at the expense of the chlorate, and a continuous decomposition under the continued action of the heat, the function of the oxide being to transfer oxygen from the state of combination to the free state. So also the charcoal, by its strong absorptive power, transfers vapour from the liquid, but cannot retain it at the high temperature, so that there is a continual absorption and a continual emission. When after re-. peated boilings the liquid is left to cool, a moderate heat is sufficient to start the charcoal as a nucleus; and if left during twelve or fourteen hours, air is absorbed, and the action is then the same as with a fresh piece of charcoal.

I give one or two more examples of permanent nuclei. A bit of gutta percha was boiled with water in a clean tube; it softened under the heat; and a glass rod was introduced so as to press it against the side of the tube near the bottom and leave a smear, while the greater portion was removed adhering to the rod. This smear was singularly active in liberating bubbles of vapour; and after the lamp was removed, bubbles were dragged upwards from it so as to become pear-shaped. The tube was boiled eight times, at intervals of from 5 to 20 minutes; but the smear continued to maintain its activity. Next morning the tube was boiled again, and the smear was active as before.

The inside of a tube was extensively smeared with lead soap.

The boiling was singularly vigorous, every part of the smear originating rapidly ascending bubbles of vapour, which boiled

over.

Such experiments as these may be multiplied to any extent. Their results seem to me to be eloquent in favour of the action of nuclei in liberating vapour. Such action may be explained by M. Verdet's theory, or by that of M. Gernez, or by my own. I say they are active because the vapour adheres to them and the water does not; but I think it can hardly be maintained that their action is due to air, seeing that in the course of these repeated boilings air is expelled while the nuclei continue as active as ever, and it is only after twelve hours or so of repose that air is reabsorbed in any appreciable quantity.

The following cases of nuclear action are taken from a paper read by me before the Society of Arts on the 7th of April, 1869, entitled "On the Theory of Boiling in connexion with some Processes in the Useful Arts."

1. In some of the old colliery engines of Scotland, when the steam becomes slack and there is no time for cleaning out the boiler, the engine-men are accustomed to throw into it about a bushel of the radicles of barley, produced during the malting and separated in cleaning the malt. The effect of these radicles. (or comings as the men call them) becomes immediately apparent on again raising the steam; for not only is there an abundant supply for producing the full working-speed of the engine, but an excess going to waste at the safety-valve. This singular effect will continue during several days*.

2. In the distillation of ardent spirits on a large scale it is customary, when converting the wash into what are called "low wines," to throw a lump of soap into the still every time it is charged. This has the effect not only of causing the steam to rise more quietly, but the vapour is more free from the residual matter of the process.

3. The sugar-boilers are accustomed to throw a lump of butter into the vacuum-pan for the purpose of preventing that irregular boiling which displays itself in furious bursts, separated by apparently almost passive intervals. Solid paraffin is now preferred to butter in producing regular boiling.

4. Professor Oersted + noticed that if a metallic wire be suspended in a boiling liquid, it instantly becomes covered with bubbles of vapour. To show the application of this fact, 10 lbs. of brass wire, of a line in diameter, were put into a still containing ten pints of brandy; seven pints came over at a heat which, in the absence of the wire, would have sent over only four. * Edinburgh Philosophical Journal, O. S. vol. ii. p. 340. + Gehlen's Journal, vol. i. p. 277.

5. It is well known in the laboratory that in distilling ether, alcohol, and similar liquids in glass vessels, the vapour is given off with difficulty. At one moment the liquid does not boil at all, at another it bursts into a mass of vapour and liquid which fills the vessel and occasions such a bumping of the retort as to endanger its safety, and, indeed, sometimes to break it. The vapour forms, in fact, with a sort of dull explosion which is very marked in distilling sulphuric acid. These bumpings are called soubresauts by the French. They are mitigated if not prevented by introducing into the retort some solid matter not acted upon by the liquid. It is recommended to use sharp, or angular, or rough pieces of metal, glass &c., the points being, it is supposed, favourable to the generation of steam. Silver, platinum, or copper (in the form of foil or wire or filings), or bits of cork or cartridge paper are recommended by Dr. Faraday as "promoters of vaporization"*.

It seems to me in such cases as the above that the various substances act by the adhesion to them of the vapour and its consequent separation under the continued action of the heat, and that they cease to act as soon as they become chemically clean, in which case the vaporous solution adheres to them as a wholet.

Throughout his memoir M. Gernez avoids the use of the term chemically clean, and attributes the varied behaviour of solid nuclei simply and solely to the adhering air. Air and air only is the means by which bubbles of vapour can escape from a liquid at or above the boiling-point; and for this purpose the minutest speck is all-sufficient. Get rid of this speck of air, and boiling becomes impossible; the liquid becomes more and more superheated, and then suddenly goes off with an explosion. But to get rid of this speck of air is the difficulty. It persists in remaining; and so efficacious is it that, although it may not be more than a millimetre in diameter, it will continue its action during twentyfour hours and liberate upwards of half a million of bubbles of vapour, each bubble about five millimetres in diameter. However marvellous this result may appear, the proposition at the head of the subdivision 3° on the next page (p. 380) is still more so, namely that "each bubble of vapour is formed at the *Chemical Manipulation, 1830, p. 199.

† A friend, writing from the laboratory of a manufacture (May 6, 1875), says: "We have had some difficulty in determining our boiling-points. We were very careful at first in cleaning our tubes and thermometers; and the result was that we did not get water to boil under 216° or 217° F. Now, after cleaning all the parts thoroughly, we rub the thermometer-stem and bulb with a duster in use in the laboratory, and get the boiling-points all right." I recommended him to put a bit of cocoa-nut-shell charcoal into the liquid.