Plants absorb sulphates from the soil and use the sulphur in the synthesis of proteins. Minute quantities of inorganic sulphates may be taken by man in food and drink, but by far the greater part of the sulphur concerned in metabolism enters the body in organic combinational and so far as known , chiefly as protein. The metabolism of sulphur is therefore a part of the protein metabolism, and in many respects the metabolism of sulphur tends to run parallel with that of nitrogen. In a series of ten experiments (each of 3 to 5 days' duration) upon man,* in which the food consisted of bread and milk in varying amounts and proportions, the percentage absorption from the digestive tract was nearly the same for the sulphur as for the nitrogen of the food, and the excretion of the end products ran so closely parallel that in every case in which the body stored nitrogen it also stored sulphur, and vice versa.†

* Benedict, Publication No. 203, Carnegie Institution of Washington.

It is well known that individual proteins show relatively much greater differences in sulphur than in nitrogen content, so the ratio of nitrogen to sulphur varies widely, as is shown by the following examples selected from the data for pure proteins compiled by Osborne:

Thus, while many proteins approximate the usually assumed average of 16 per cent nitrogen and 1 per cent sulphur, there are considerable deviations from this ratio in both directions.

Under ordinary conditions, however, no protein is eaten in a pure state, but only as the material containing it is used as an article of food. It is therefore the proportion of sulphur to the total protein of the food which determines the ratio of sulphur to nitrogen available for nutrition.

* Bulletin 121, Office of Experiment Stations, U. S. Department of Agriculture, † Exceptions to such parallelism of nitrogen and sulphur balances have, however, been reported in certain pathological conditions.

The proportion of sulphur to total protein has been deter mined in most staple foods, of which the following are representative : *

Taking these figures as typical, it would appear that in those staple foods which contribute the greater part of the protein of the diet, the ratio of protein to sulphur does not differ greatly, and that in most cases of ordinary mixed diet there would be. consumed not far from 1 gram of sulphur in each 100 grams of protein. We may therefore expect that in health and on an ordinary diet the sulphur requirement will usually be covered when the protein supply is adequate.

When proteins (or their cleavage products) are oxidized in the body, the sulphur becomes converted for the most part into sulphuric acid, which, of course, must be neutralized as rapidly as it is formed. The greater part of the sulphuric acid formed in metabolism appears in tie urine as inorganic sulphates; a smaller part is found combined with organic radicles in the form commonly known as "ethereal" or "conjugated" sulphates. The amount of ethereal sulphate or the ratio of ethereal to inorganic sulphate is quite variable, depending mainly upon the amount and character of the intestinal putrefaction, which in turn is apt to be considerably influenced by the food. On ordi-nary mixed diet about one tenth or one twelfth of the sulphate sulphur in the urine ordinarily appears as ethereal sulphates; but when the meat in the diet is replaced by milk, the putre-faction is usually lessened and the proportion of ethereal sulphates lowered. In one case of a healthy man who had been on a bread and milk diet for a week, only one thirtieth of the sulphate sulphur was in the form of ethereal sulphates.

* In the data here given, nitrogen and sulphur were determined in the same specimens. Average percentages of protein and sulphur in nearly all important food materials may be found in Tables I and II, respectively, of the Appendix.

Not all of the metabolized sulphur is eliminated as mineral or "ethereal" sulphate; a part is given off in less completely oxidized forms. This "unoxidized" or "neutral" sulphur usually constitutes in healthy persons on full diet from 5 to15 per cent of the total sulphur eliminated. In Folin's experiment upon very low protein diet, although the total sulphur metabolism was markedly decreased, the quantity of neutral sulphur excreted remained about constant, so that the relative proportion of sulphur appearing in this form was increased.