Haycraft considers2 that 'quality' in taste depends upon the nature of the atoms found in the sapid molecule. A study of the periodic law demonstrates that similar tastes are produced by combinations which contain elements such as lithium, sodium, potassium, which show a periodic recurrence of ordinary physical properties. Among the carbon compounds, those which produce similar tastes are found to contain a common ' group' of elements. Thus organic acids contain the group CO.OH, the sweet sub stances CH2.OH. There is no relation between quality of sensation and gross molecular weight, except that substances of either very small or very great molecular weight are not tasted at all.