70 CHAP. IX. EFFECT OF TRACES OF CHEMICAL SUBSTANCES 



The noticeable fact in the action of thyroid extract is 

 that no reversal or diminution of activity below normal took 

 place for a considerable range in the dilution (fig. 17). 



Fig. 17. Curves exhibiting the effect of infinitesimal traces of 

 HNO3, of Thyroid Gland, and of Formaldehyde, on Photo- 

 synthetic Activity of Hydrilla 



The dilutions are n parts in a billion. 



Effect of Iodine 



The effect of traces of iodine is somewhat similar to 

 the foregoing. One part of iodine in a billion produced the 

 maximum enhancement of about 50 per cent. ; but dilution 

 of one part in a million depressed the rate below the normal. 



Effect of Formaldehyde 



The effect of traces of the very poisonous agent formalde- 

 hyde is of much theoretical interest. A solution of one part 



