While the chemists have not yet been able to isolate and identify the various vitamines they have succeeded in demonstrating many of the properties of these substances and it is the knowledge of these properties that has enabled us to produce concentrates and conduct tests. Another practical consideration involved in this matter of properties lies in the effect of cooking and commercial methods of food preparation, for not only must we learn where the vitamine resides but how to prevent injury or destruction in our utilization of the source. The properties of the vitamines may therefore be grouped under two heads: first chemical properties and second physiological properties. I. CHEMICAL PROPERTIES OF VITAMINE "A" _a_. This dietary factor's presence in butter fat and egg yolk fat indicates its solubility in the fat and it would naturally follow that the fat solvents would suffice to remove it with the fats when food sources are treated with such a reagent. Experience has shown however that while ether extraction applied to butter or egg yolk removes the vitamine with the fat this process fails when it is applied to vegetable sources such as cotton seed, corn germ, spinach, lettuce, etc. Neither does the cold or hot press method of oil extraction liberate the vitamine with the oil. Recent experiments by Osborne and Mendel, to which we have previously referred, have shown that preliminary treatment of vegetable sources with alcohol seems to loosen the bond between the source and the vitamine and that when this binding is once loosened subsequent ether extraction will take the vitamine out. That the binding is not difficult to break is shown by the fact that when vegetables are eaten as a source of vitamine the body is able to separate the complex. It is further evident that the body does separate this complex and stores it in animal fat from the experiments with cow feeds and feeding. Milk for example is rich or poor in vitamine according to the supply of the latter in the food given to the cow. The only logical conclusion to be drawn from this observation is that the cow does not synthesize this factor but splits it off from the food source and then, since it is fat soluble, is able to mobilize it in the butter fat of the milk or to a more limited extent in the body fat. This observation as to the dependence of milk content upon food has been confirmed in the case of nursing mothers and suggests the need of especial attention to the diet of the mother during the lactating period.