308 CARNEGIE INSTITUTION OF WASHINGTON. 



in many ways surprised us, for it has been found that animals are 

 able to retain some of the inorganic elements so tenaciously that the 

 effect of excluding these from the diet can be learned only by using 

 ingredients of the food, all of which are free from even traces of the 

 element under observation. Although we prepared the materials for 

 our diets with great care, examination of the ash of large quantities 

 showed the presence of minute amounts of all of the inorganic elements 

 which we were studying. Some of these were unavoidably introduced 

 by the vitamine-containing product which it was necessary to add to 

 the food and which of course could not be treated chemically. The 

 method has consisted in observing the comparative growth of animals 

 on rations in which differences in the quantities of the inorganic 

 elements represented practically the sole variations between the 

 different food mixtures; the effect of removing from the diet all but 

 very small quantities of a given element was thus measured. A 

 lowering of the intake of calcium and phosphoric acid promptly led to 

 an interruption of growth and to nutritive decline if the supply of 

 these elements was too greatly curtailed. We may emphasize anew 

 that any shortage of these elements can be supplied by additions of 

 inorganic salts quite as well as by natural foods containing them; hence 

 calcimn, phosphorus, and iron, for example, in natural foods have 

 not the unique value that is commonly ascribed to them. For feeding 

 farm animals, where the lack of calcium in their grain rations is always 

 encountered, complete nutrition can be attained upon diets supplying 

 this inorganic element in the form of its commercial salts. 



While relatively large quantities of calciiun and phosphoric acid 

 were found to be required for normal nutrition, surprisingly small 

 quantities of chlorine, sodium, potassium, and magnesium, as con- 

 trasted with the customary intake of them, were adequate for growth. 

 Rats gained well even when their diet contained less than 0.04 per 

 cent of either sodium or chlorine, only 0.03 per cent of potassium, or 

 only slightly more than 0.01 per cent of magnesium. When the 

 quantities of sodium, chlorine, and magnesium in the food were 

 simultaneously decreased to these low levels, rats still grew at a normal 

 rate. When, however, both sodium and potassium were limited to 

 these small amounts growth ceased. Calcium, phosphorus, and 

 sodium or potassium, all of which are supposed to play an important 

 part in the maintenance of neutrality, are needed daily in considerable 

 quantities, the requirem.ent for the other inorganic elements being very- 

 small. Perhaps this explains why either sodium or potassium must 

 be present in fairly large proportion, while either alone can be reduced 

 to insignificant quantities without seriously impairing growth. 



To learn to what extent the instinct and appetite of rats are capable 

 of guiding them to a proper choice between foods of unUke nutritive 

 value, a number of animals were allowed to choose freely between two 



