Слово "soluble". Англо-русский словарь Мюллера

Some are concerned simply with planning the logistics of future wars; others devise larger and larger rocket bombs, more and more powerful explosives, and more and more impenetrable armour-plating; others search for new and deadlier gases, or for soluble poisons capable of being produced in such quantities as to destroy the vegetation of whole continents, or for breeds of disease germs immunized against all possible antibodies; others strive to produce a vehicle that shall bore its way under the soil like a submarine under the water, or an aeroplane as independent of its base as a sailing-ship; others explore even remoter possibilities such as focusing the sun's rays through lenses suspended thousands of kilometres away in space, or producing artificial earthquakes and tidal waves by tapping the heat at the earth's centre.

“From their lightness and transparency, I should imagine that they are soluble in water,” I remarked.

On one of them, carve: "Problems are soluble."

I have said enough to convince you that ciphers of this nature are readily soluble, and to give you some insight into the rationale of their development.