Under certain conditions, however, a difficultly soluble substance may be produced in a solution, in a concentration far beyond its solubility, ‹without the separation of a precipitate› (evolution of a gas or formation of a separate liquid) ‹and also without the formation of a supersaturated solution›. Thus, when hydrogen sulphide is passed into an aqueous solution of arsenious oxide, the liquid acquires the yellow to orange color of ‹arsenious sulphide› and becomes opalescent. But no ‹precipitate› is seen (‹exp.›), in spite of the fact that the sulphide is extremely insoluble and is formed practically quantitatively according to the equation As_{2}O_{3} + 3 H_{2}S ⇄ As_{2}S_{3} + 3 H_{2}O. The orange liquid passes through a [p126] filter unchanged (‹exp.›). But if some hydrochloric acid or a salt (‹e.g.› sodium chloride) solution is added to a portion of it, a heavy precipitate of arsenious sulphide is immediately produced (‹exp.›); its quantity is a good indication of the great amount of sulphide that is ‹not› precipitated before the addition of the acid or salt. If some pure arsenic sulphide (solid) is added to another portion of the orange liquid, in order to overcome any possible condition of supersaturation, the liquid is found to remain clear (but opalescent), excepting for the few particles of added sulphide (‹exp.›); even when it is vigorously shaken (‹exp.›), or allowed to stand for days, no precipitate is formed. We are therefore not dealing with a case of supersaturation. A liquid, in which a very insoluble substance appears thus to be in solution far beyond its usual degree of solubility, and yet does not show at all the behavior of a supersaturated solution, is said to contain the substance in the «colloidal condition». After a few more instances of the colloidal condition have been presented, the significance of the condition and the meaning of the term used to designate it will be explained.