Special interest has attached to the possibility of tin poisoning on account of the widespread use of canned foods.[38] It is established chemically that tin is attacked, not only by acid fruits and berries, but by some vegetables having only a slightly acid reaction. More tin is found in the drained solids than in the liquor, and the metal is largely in an insoluble form.[39] It has been the general opinion based on experiments by Lehmann[40] and others that the amounts of tin ordinarily present in canned foods "are undeserving of serious notice," and this view has found expression in the leading textbooks on hygiene.[41] Certainly there has not been any noticeable amount of tin poisoning observed coincident with the enormous increase in the use of canned foods. An instance of poisoning by canned asparagus observed by Friedmann,[42] however, is attributed by him to the tin content, and this view is rendered probable by the negative result of his bacteriological and serological examinations. Canned asparagus apparently contains an unusually large amount of soluble tin compounds.[43] There seems some ground for the assumption that certain individuals are especially susceptible to small quantities of tin and that the relative infrequency of such cases as that cited by Friedmann can be best explained in this way. Lacquered or "enamel-lined" cans are being used to an increasing extent for fruits and vegetables that are especially likely to attack tin.[44] Intentional addition of tin salts to food substances does not appear to be common, although protochloride of tin is said sometimes to be added to molasses for the purpose of reducing the color. The chlorides are regarded as more definitely poisonous than other compounds of tin, and for this and other reasons the practice is undesirable. Sanitarians insist that chemical substances likely to be irritating to the human tissues in assimilation or elimination should not be employed in food. Each new irritant, even in small quantity, may add to the burden of organs already weakened by age or previous harsh treatment. COPPER Danger is popularly supposed to attend the cooking and especially the long standing of certain foods in copper vessels on account of the verdigris or copper acetate that is sometimes formed, but Professor Long, of the Referee Board of Consulting Scientific Experts,[45] points out that this substance is far less toxic than it was once imagined to be, and he considers it likely that the cases of illness attributed to "verdigris poisoning" reported in the older literature should have been explained in some other way.