616 Proceedings of the Royal Irish Academy. 



The facts ascertained are, ho-^ever, amply sufficient to prove that 

 ammonium sulphocyanate acts as a powerful poison on these plants, 

 notvrithstanding the large proportion of ready-formed ammonia it 

 contains, while its metamer thiocarbamide stimulates the growth of 

 similar plants, and induces healthy development of all their parts, 

 thus acting as a distinct plant food. Indeed if it were not that 

 thiocarbamide tends to revert to the sulphocyanate after some time, 

 the former might be regarded as a good organic manure for tobacco. 



I must leave to the vegetable physiologist the task of determining 

 the precise way in which nutrition is arrested by the sulphocyanate, 

 and promoted by thiocarbamide : for the present I am content to have 

 shown that their effects on the particular plants employed are widely 

 different, though the bodies compared contain the same elements che- 

 mically united, in the same proportions within molecules of the same 

 weight. The conclusion is inevitable that their strongly contrasted 

 physiological action is due to diverse molecular structure. "We thus 

 learn — 



1st — That the particular elements of which the bodies are com- 

 posed exerted less influence on their physiological activity than the 

 intra-molecular grouping of the component atoms. 



2nd — That, in some instances at least, differences of physiological 

 activity between metameric bodies can be easily detected with the aid 

 of Dlants. 



