In conclusion, these considerations will also indicate the precautions to be observed in the precipitation of difficultly soluble substances. If this is not properly carried out, endless trouble in [p125] the analytical laboratory results. Except when heating is, for some special reason, undesirable—as inducing a chemical change (like hydrolysis) that is not wanted—solutions are brought to the boiling-point, and ‹the precipitant added drop by drop›, in order not to supersaturate the solution too strongly. The solution is thus allowed time to deposit its excess as far as possible on the ‹first crystals formed›, which it will do rather than to form new, minute ones (see supersaturation, p. 121). ‹Constant stirring› is prescribed in order to bring older crystals, as far as possible, into contact with all parts of the slightly supersaturated solution. After the precipitation is complete, it is usually desirable to allow the mixture to "digest" for some time, for the reasons given above—stirring and a high temperature during the process being desirable. (See further Chap. VIII, p. 147, ‹in regard to the use of an excess of precipitant›.) THE COLLOIDAL CONDITION When a difficultly soluble substance is formed in a solution beyond the point of saturation of the solution, the substance in question ‹separates› from the solution ‹in a new phase›, according to the principles just laid down. Ordinarily, if the substance is a solid, a ‹precipitate› is formed; if a gas, a gas escapes; if a liquid, a liquid separates out, which is immiscible with the solution in which it is formed. Occasionally, the condition of supersaturation which precedes the separation is somewhat persistent, but this resistance to the separation of the phase may be overcome by vigorous agitation of the solution or, as in the case of supersaturation with a crystallizable salt, by inoculation of the solution with a particle of the new phase (p. 123).