In many cases the best guarantee of the purity of a chemical substance is the method by which it has been prepared. If the processes are well devised and carefully conducted, they constitute in themselves a qualitative analysis on a large scale, the exactitude and reliability of which go far beyond what could be obtained by ordinary testing. This, however, does not apply to many organic preparations, and is eminently inapplicable in the case of compound ethers. If in the preparation of such an ether, we start with a pure alcohol and a pure acid, and carefully distil, wash, and redistil the ether produced, the product ultimately obtained may contain considerable quantities of alcohol, free acid, and water. The purity of a compound ether can be established only by direct Experiment. Free acid is easily tested for, and even quantitatively determined by the well-known acidimetric processes; the presence or absence of water may be proved by an elementary analysis, but an admixture of alcohol is not easily established.