Catalytic oxidation in living substances rests upon change of valency in an iron compound which is the respiratory oxygen-transferring ferment.

The concentration of the ferment iron in living substance is very small, being in the region of 1 g to 10 million g of cellular substance.

Green, red, and mixed shades of haemins are known. If magnesium is replaced by iron in chlorophyll, green haemins are obtained. Their colour is due to a strong band in the red which is already recognized in chlorophyll.

The toxic effect of carbon monoxide on man has nothing to do with inhibition of cellular respiration by carbon monoxide but is based on the reaction of carbon monoxide with blood iron.

As soon as chemists have a definite conception of the internal structure of the molecule of an organic compound, they are able to tackle the task of producing these substances by artificial methods, i.e. by synthesis, as we call it.

Most of the oils which are valued as scents are mixtures of substances; only the combined effect of these leads to the known result.