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q. I have seen a device called a storm glass which consists of a glass tube filled with a liquid containing crystals. The shape of the crystals is supposed to change according to the pressure. How does this device work? Apparently, Darwin used one on board the Beagle.



a. The storm glass is often found as part of an Admiral Fitzroy's barometer, examples of which are fairly common in antique shops and can be bought for a few hundred pounds. Robert Fitzroy was captain of the Beagle during its most famous voyage, so it is likely that Darwin would have used the storm glass.

A letter published in School Science Review (June 1987) suggested that the storm glass was invented by an alchemist for Italian sailors. The author, P. J. Towse, also suggested schools could make a storm glass by mixing 10 grams of camphor, 40 cubic centimetres of ethanol, 25 grams of potassium nitrate, 25 grams of ammonium chloride and 33 cubic centimetres of distilled water, then placing the mixture in a corked test tube.

Changes in the liquid are supposed to show how the weather will change. Clear liquid means bright weather; dim liquid means rain. If it is dim with small stars, expect thunderstorms. Large flakes mean it will be overcast or, in winter, snowy. Crystals at the bottom mean frost and threads in the upper part mean it will be windy. If the liquid contains small dots, expect humid or foggy conditions, and if it contains small stars on sunny winter days, expect snow in a few days.

Presumably some of the observations can be accounted for by changes in solubility--as the ambient temperature and pressure change--and by the varying speed at which this happens. But some of it, I suspect, is pure alchemy.


a. About forty years ago I found a recipe for the liquid in a storm glass. I prepared a sample, sealed it in a glass tube and left it on the roof of a factory in south London for six months. I examined the tube every day and tried to correlate atmospheric conditions with changes in the crystals in the tube. I could find no correlation, but as the tube was sealed, its contents could only react to temperature changes and not pressure.

South Africa

a.  I send this letter to you with some urgency. It concerns the answer to the question about the storm glass. There is the possibility that somebody may be seriously hurt trying to make this device without having first thought about what they are going to do.

Camphor is a pungent organic chemical, which is combustible. It has a history of use as a plasticiser in explosives and is readily oxidisable; ammonium chloride decomposes when heated, releasing toxic fumes and potassium nitrate is an oxidising agent. Mixing these three chemicals together in a dry state could very probably result in the potassium nitrate oxidising the camphor, causing it to combust and the ammonium chloride to decompose. The consequences may be fatal. If this device really must be made, probably the safest way to do this is to dissolve the potassium nitrate and ammonium chloride in the water, then add the ethanol and finally the camphor.



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(taken from "The Weather Book" circa 1850)



Having often noticed peculiar effects on certain instruments, used as weather glasses, that did not seem to be caused by pressure, or solely by temperature, by dryness, or by moisture - having found that these alterations happened with electric changes in the atmosphere that were not always preceded or accompanied by movement of mercury in a barometer, and that, among other peculiarities, increase or diminution of winds, in the very 'heart' of the trades, caused effects on them, while the mercurial column remained unaltered, or showed only the slight inter-tropical diurnal change (as regular there as a clock), we have long felt sure that another agent might be traced.

Considerably more than a century ago what were called 'storm glasses' were made in this country. Who was the inventor, is now very uncertain; but they were sold on old London Bridge, at the sign of the "Looking Glass".

Since 1825 we have generally had some of the vials, as curiosities rather than otherwise, for nothing certain could be made of their variations until lately, when it was fairly demonstrated that if fixed, undisturbed, in free air, not exposed to radiation, fire or sun, but in the ordinary light of a well-ventilated room, or, preferably, in the outer air, the chemical mixture in a so-called storm glass varies in character with the direction of the wind - not its force, specially, though it may so vary (in appearance only) from another cause, electrical tension.

As the atmospheric current veers towards, comes from, or is only approaching from the polar direction, this chemical mixture - if closely, even microscopically watched, - is found to grow like fir, yew, or fern leaves - or like hoar frost - or even large but delicate crystallisations.

As the wind, or great body of air, tends more from the opposite quarter, the lines or spikes - all the regular, hard, or crisp features, gradually soften and diminish till they vanish.

Before and in a continued southerly wind the mixture sinks slowly downwrd in the vial, till it becomes shapeless, like melting white sugar.

Before or during the continuance of a northerly wind (polar current), the crystallisations are beautiful (if the mixture is correct, the glass a fixture, and duly placed); but the least motion of the liquid disturbs them.

When the main air-currents meet, and turn towards the west, making easterly winds, stars are more or less numerous, and the liquid dull, or less clear. When, and while they combine by the west, making westerly wind, the liquid is clear, and the crystallisation well defined, without loose stars.

While any hard or crisp features are visible below, above, or at the top of the liquid (where they form for polar wind) there is plus electricity in the air; a mixture of polar current co-exisiting in that locality with the opposite, or southerly.

When nothing but soft, melting, sugary substance is seen, the atmospheric current (feeble or strong as it may be) is southerly with minus electricity, unmixed with and uninfluenced by the contrary wind.

Repeated trials with a delicate galvanometer, applied to measure electric tension in the air, have proved these facts, which are now found useful for aiding, with the barometer and thermometers, in forecasting weather.

Temperature affects the mixture much, but not solely; as many comparisons of winter with summer changes of temperature have fully demonstrated.

A confused appearance of the mixture, with flaky spots, or stars, in motion, and less clearness of the liquid, indicates south-easterly wind, probably strong - to a gale.


Clearness of the liquid, with more or less perfect crystallisations, accompanies a combination, or a contest, of the main currents, by the west, and very remarkable these differences are - the results of these air currents acting on each other from eastward, or entirely from an opposite direction, the west.

The glass should be wiped clean, now and then, - and two or three times in a year the mixture should be disturbed, by inverting and gently shaking the glass vial.

The composition is camphor - nitrate of potassium and sal-ammoniac - partly dissolved by alcohol, with water, and some air, in hermetically sealed glass.

There are many imitations, more or less incorrectly made.

Those camphor glasses used by the writer lately were prepared by Messrs. Negretti and Zambra. There are numerous others, some of which are inexact in chemical composition; and are not nearly so sensitive.