Both tin and lead poisoning may arise from the prolonged use of preserved meats, vegetables, or fruits. In tin cans the lead is derived from the action of various organic acids upon the solder (which in this country is usually 50 per cent lead), and the tin used for coating the inside of the can eventually is dissolved in a similar manner. (See also Canning, p. 282).

Of late years attention has been directed to this subject by chemists, and a number of cases are reported from time to time of serious metallic poisoning. As a general rule, if the canning is properly conducted and the meats or vegetables have not decomposed, there is no danger in their use provided they are thoroughly cooked, and provided also that the cans have not been kept too long. The amount of tin dissolved is directly proportionate to the age of the contents of the cans. The food in cans that have been unopened for two or three years is very apt to produce poisonous effects. Professor Bettink declares that malic acid is the solvent of the tin when fruits and vegetables are canned.

Hehner examined many canned foods, and found tin present, in most of them as a stannous hydrate, which existed in sufficient quantity to be fatal when given to guinea pigs.

Ungar and Bodländer have reported a number of cases of poison from canned asparagus. Tin present in such food in an insoluble form may be dissolved and absorbed during digestion.

Professor Beckurts reported that a sulphate of tin is formed by the action of vegetable albuminoids or of meat albumin on the can. This is very liable to take place with canned asparagus, and two cases of severe poisoning of this nature have come under my personal observation. Violent gastro-intestinal symptoms occurred, such as severe vomiting and diarrhoea, accompanied by faintness and vertigo. The symptoms came on two or three hours after eating the asparagus from cans which had been kept for a long time.

Johnson (in The Medico-Legal Journal, vol. iii, p. 53) reports cases of dangerous poisoning from eating tinned tomatoes which were accompanied by symptoms of violent gastritis, stupor, and coma.

Winckel reported the cases of 270 soldiers who had been made ill from eating lettuce and meat preserved in tins. In their cases it was estimated that the quantity of tin in solution was from 19 to 72 milligrammes per kilogramme. Roos advises the coating of such cans on the inside with insoluble varnish, which prevents the albuminoids or vegetable acids from coming in contact with the tinned surface and dissolving the metal. He found upon opening a can of asparagus preserved for thirty-one years that the inside tin coating was wholly dissolved in the liquid. A can of beef preserved for eight years, weighing 976 grammes, contained 77 milligrammes of oxide of tin, and a can of asparagus preserved only four months contained 11 milligrammes of oxide of tin and 6 of copper. Preserved apples, corn, apricots, meats, and soups have also all been found to possess the same solvent action upon tin cans. This is almost entirely prevented in those instances in which a coating of varnish has been put over the tinned surface.

Pears cooked in a tin stewpan have been known to cause severe poisoning.

One hundred and fifty cases of severe metallic poisoning occurred among the soldiers of a Tyrolese regiment who ate food from a tin-lined copper kettle.

In general, food cooked or allowed to stand for any length of time in either brass or copper vessels not kept absolutely clean is dangerous to life.