LITMUS, or LACMUS, in the arts, is a blue pigment, formed from archil. It is brought from Holland at a cheap rate; but may be prepared by adding quicklime and putrid urine, or spirit of urine distilled from lime, to the archil previously bruised by grinding. The mixture having cooled, and the fluid suffered to evaporate, becomes a mass of the consistency of a paste, which is laid on boards to dry in square lumps. It is only used in miniature paintings, and cannot be well depended on, because the least approach of acid changes it instantly from blue to red. The best litmus is very apt to change and fly.