Analogy is of great use to the mind in many cases,especially in natural philosophy,and that part of it chiefly which consists in happy and successful experiments.But here we must take care that we keep ourselves within that wherein the analogy consists.

For example,the acid oil of vitriol is found to be good in such a case,therefore the spirit of nitre or vinegar may be used in the like case.If the good effect of it be owing wholly to the acidity of it,the trial may be justified;but if there be something else besides the acidity in the oil of vitriol,which produces the good we desire in the case,we mistake that for analogy which is not and suffer our understanding to be misguided by a wrong supposition of analogy where there is none.