Already to the versatile image of amino acids does also belong the fact that the solubility of neutral amino acids [i.e. amino acids containing as many COOH groups as NH2 groups] "is on one occasion increased and on another decreased as a result of neutral salts. They behave individually and are, as to their overall behavior, rather close to proteins". The protein-neutral-salt complexes are viewed by Pfeifer generally as to be chemical compounds. Also the bone substance is held to be a Ca-salt-protein compound (not a mixture), and, with its "continuous" transition, very well fits the Unimol picture.