In patent law. Any act or substance which is known in the arts as aproper substitute for some other act or substance employed as an element in the invention,whose substitution for that other act or substance does not in any manner varythe idea of means. It possesses three characteristics: It must be capable of performingthe same office in the invention as the act or substance whose place it supplies; it mustrelate to the form or embodiment alone and not affect in any degree the idea of means;and it must have been known to the arts at the date of the patent as endowed with thiscapability. Duff Mfg. Co. v. Forgie, 59 Fed. 772, 8 C. C. A. 261; Norton v. Jensen, 49Fed. 868, 1 C. C. A. 452; Imliaeuser v. Buerk, 101 U. S. 055, 25 L. Ed. 945; CarterMaeh. Co. v. Hanes (C. C.) 70 Fed. 859; Schillinger v. Cranford, 4 Mackey (D. C.) 4G&