Socin demonstrated the superiority of the iron of egg yolk over iron chloride by dividing a number of mice into groups, some of which were fed on a mixture of iron-free food and iron chloride, while others received the same iron-free food with the addition of egg yolk. None of the mice fed without organic iron lived for more than thirty-two days, while some of those receiving egg yolk lived as long as the experiments were continued (sixty to ninety-nine days), and gained in weight.

Gottlieb, recognizing the fact that iron might be absorbed and used by the body, yet finally excreted with the feces, determined the intestinal elimination of iron in dogs before and after subcutaneous and intravenous injections of known amounts of iron salts. From the results obtained it was estimated that practically all of the injected iron was eliminated by the intestines.

Voit studied the metabolism of iron in dogs by direct observations of absorption and elimination in isolated sections of the small intestine. Opening the peritoneal cavity, he separated the desired section, removed the contents, closed the ends, and left the sac thus formed in its normal position after having reunited the remainder of the intestine. Under these conditions the isolated section of intestine, while not coming in direct contact with anything taken by the mouth, would still receive its proportional share of anything eliminated from the body through the intestinal wall. By killing and examining animals which had been kept for some time after such an operation, Voit was able to compare the amount of iron eliminated through the intestinal wall with the amounts contained in food and feces, and thus to infer the extent to which the iron taken by the mouth was absorbed and returned to the intestine for elimination. In fasting, the daily elimination found for each square meter of intestinal surface was 6 milligrams in the feces and the same amount (per square meter of surface) in the isolated loop of intestine. On food poor in iron the feces contained in each of two cases 10 milligrams, the isolated loops 6 and 9 milligrams, of iron per square meter of intestinal surface; while on food rich in iron the corresponding figures for two experiments were 43 and 78 milligrams in the feces, and 8 and 6 milligrams in the isolated portion of the intestine. Hence it appears that the iron eliminated in the feces during fasting or on food poor in iron came from the body through the intestinal wall, while most of the extra iron given with the food in the last two experiments passed through the alimentary canal without being absorbed and metabolized.

Stockman, in a paper upon the metabolism of iron, published in 1893, while discussing mainly the therapeutics of chlorosis (a type of anemia occurring in girls and young women) undertook to solve the question of the absorption of inorganic iron. He reasoned as follows:

If inorganic iron preparations given hypodermically will cure chlorosis, there can in such cases be no possibility of the iron exerting its effect by the stimulation of the alimentary canal or by combining with hydrogen sulphide in the intestine.

If iron sulphide given by the mouth cures chlorosis, it must be through absorption of the iron, since ferrous sulphide has no stimulating effect and cannot take up more sulphur.

If bismuth, manganese, etc., take up hydrogen sulphide as readily as iron, but are inert in chlorosis, a further indirect evidence of absorption of iron is obtained.

Stockman made experiments and observations upon hospital patients (of which he cites nine cases) which appeared to substantiate each of the three propositions, and thus to establish the fact that inorganic iron preparations cure chlorosis through being absorbed and utilized in the formation of hemoglobin.

During the years 1894-1897 several investigators studied the absorption of different forms of iron by microchemical methods. Suitable stains having been found for the identification of iron in the microscopic sections of tissue, it was possible by examination of the intestinal wall and the various organs and tissues of the body to follow the absorption, storage, and elimination of the iron given medicinally or occurring in the food. Macallum investigated in this manner the behavior of inorganic salts of iron, iron albuminates, and the iron compound of the egg yolk, and found that iron taken in any of these forms may be absorbed from the small intestine.

Woltering compared microchemically and by quantitative determination the amounts of iron in the livers of mice, rabbits, and dogs, fed with and without sulphate of iron, and reported an increase in the iron content of the liver and in the hemoglobin and red corpuscles of the blood as the result of feeding the iron salt.

Gaule, using principally microchemical methods, found no reaction for iron in the chyle under normal conditions; but a distinct reaction appeared in the lymph nodes, and extended to the spleen soon after the feeding of iron salt to rabbits. This absorption of inorganic iron was followed by an increase in the number of red corpuscles and percentage of hemoglobin in the blood.

In the meantime, Kunkel and Egers studied especially the influence of iron salts upon the regeneration of blood after hemorrhage. Kunkel kept two dogs on a limited milk diet, but gave one of them, in addition to the milk, iron in the form of albuminate. Each of the animals was bled every seven days, about one third of the total blood being taken each time. The iron in the drawn blood was determined and ascertained to be greater than the amount supplied by the milk, but less than the total iron received by the dog which was fed with albuminate. The experiment was continued seven weeks, at the end of which time the blood and organs of the dog which had been kept on milk alone were poorer in iron than those of the dog which had received the iron albuminate. Only one animal was fed in each way, and no determinations of hemoglobin are recorded. According to Egers, the regeneration of blood after severe losses (one third of the estimated total) is very slow on food poor in iron, unless medicinal iron is also given, when the rate of regeneration becomes better, but not so good as on a diet supplying an abundance of food iron alone. Even when the diet was rich in food iron, however, Egers found that medicinal iron appeared to aid the regeneration of blood after hemorrhage.