THE SPHEROIDAL CONDITION OF LIQUIDS.
--WHY THE HAND MAY BE DIPPED IN
MOLTEN METALS.--PRINCIPLES OF
HEAT-RESISTANCE PUT TO PRACTICAL
USES: ALDINI, 1829.--IN EARLY FIRE-
FIGHTING.TEMPERATURES THE BODY
CAN ENDURE.
The spheroidal condition of liquids was discovered by Leidenfrost, but M.Boutigny was the first to give this singular subject careful investigation.From time out of mind the test of letting a drop of water fall on the face of a hot flat-iron has been employed to discover whether it may safely be used.Everybody knows that if it is not too hot the water will spread over the surface and evaporate; but if it is too hot, the water will glance off without wetting the iron, and if this drop be allowed to fall on the hand it will be found that it is still cool.The fact is that the water never touches the hot iron at all, provided the heat is sufficiently intense, but assumes a slightly elliptical shape and is supported by a cushion of vapor.If, instead of a flat-iron, we use a concave metal disk about the size and shape of a watch crystal, some very interesting results may be obtained.If the temperature of the disk is at, or slightly above, the boiling point, water dropped on it from a medicine dropper will boil; but if the disk is heated to 340 degrees F., the drop practically retains its roundness--becoming only slightly oblate--and does not boil.In fact the temperature never rises above 206 degrees F., since the vapor is so rapidly evaporated from the surface of the drop that it forms the cushion just mentioned.By a careful manipulation of the dropper, the disk may be filled with water which, notwithstanding the intense heat, never reaches the boiling point.On the other hand, if boiling water be dropped on the superheated disk its temperature will immediately be REDUCED to six degrees below the boiling point; thus the hot metal really cools the water.
By taking advantage of the fact that different liquids assume a spheroidal form at widely different temperatures, one may obtain some startling results.For example, liquid sulphurous acid is so volatile as to have a temperature of only 13 degrees F.when in that state, or 19 degrees below the freezing point of water, so that if a little water be dropped into the acid, it will immediately freeze and the pellet of ice may be dropped into the hand from the still red-hot disk.Even mercury can be frozen in this way by a combination of chemicals.
Through the action of this principle it is possible to dip the hand for a short time into melted lead, or even into melted copper, the moisture of the skin supplying a vapor which prevents direct contact with the molten metal;no more than an endurable degree of heat reaches the hand while the moisture lasts, although the temperature of the fusing copper is 1996 degrees.The natural moisture of the hand is usually sufficient for this result, but it is better to wipe the hand with a damp towel.
In David A.Wells' Things not Generally Known, New York, 1857, I find a translation of an article by M.Boutigny in The Comptes Rendus, in which he notes that ``the portion of the hands which are not immersed in the fused metal, but are exposed to the action of the heat radiated from its surface, experience a painful sensation of heat.'' He adds that when the hand was dampened with ether ``there was no sensation of heat, but, on the contrary, an agreeable feeling of coolness.''
Beckmann, in his History of Inventions, Vol.II., page 122, says:
In the month of September, 1765, when I visited the copper works at Awested, one of the workmen, for a little drink money, took some of the melted copper in his hand, and after showing it to us, threw it against the wall.He then squeezed the fingers of his horny hand close together, put it for a few minutes under his armpit, to make it sweat, as he said; and, taking it again out, drew it over a ladle filled with melted copper, some of which he skimmed off, and moved his hand backwards and forwards, very quickly, by way of ostentation.
While I was viewing this performance, I remarked a smell like that of singed horn or leather, though his hand was not burnt.
The workmen at the Swedish melting-
house showed the same thing to some travellers in the seventeenth century; for Regnard saw it in 1681, at the copper-works in Lapland.
My friend Quincy Kilby, of Brookline, Mass., saw the same stunt performed by workmen at the Meridan Brittania Company's plant.They told him that if the hand had been wet it would have been badly scalded.
Thus far our interest in heat-resistance has uncovered secrets of no very great practical value, however entertaining the uses to which we have seen them put.But not all the investigation of these principles has been dictated by considerations of curiosity and entertainment.
As long ago as 1829, for instance, an English newspaper printed the following:
Proof against Fire--On Tuesday week an experiment was made in presence of a Committee of the Academy of Sciences at Paris, by M.Aldini, for the purpose of showing that he can secure the body against the action of flames so as to enable firemen to carry on their operations with safety.His experiment is stated to have given satisfaction.The pompiers were clothed in asbestos, over which was a network of iron.Some of them, it was stated, who wore double gloves of amianthus, held a red-hot bar during four minutes.
Sir David Brewster, in his Letters on Natural Magic, page 305, gives a more detailed account of Aldini, from which the natural deduction is that the Chevalier was a showman with an intellect fully up to the demands of his art.Sir David says:
