"If, as we yet can scarcely doubt, there are catalytic influences which are the cause of the miasmic spread of disease, it is possible that an substance, such as , even in vanishingly small quantities, may indeed not be without definite influence upon such processes in the air. From daily and long continued spectrum observation it would be easy to learn whether the variation in the intensity of the spectral line Naα, produced by the sodium combination in the air, is related in any degree to the appearance and the spread of endemic diseases."