12 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Experiments illustrating the Corrosive Action of Deposits. 



In the autumn of 1891 I made a few experiments in order to 

 elucidate the action of these deposits. In the first instance I 

 took some of the Chelsea deposit,* and after shaking it up with 

 a little distilled water, painted the right-hand halves of the leaves 

 of a number of stove plants therewith. These included Cucurbita 

 Pepo, Bouvardia, Dalechamjna, Heterocentron roseum, Dendro- 

 bium nobile, and Hydrangea hortensis. 



From day to day the leaves were moistened with water. 

 Within a week the Bouvardia and Heterocentron showed slight 

 corrosions, and in ten days the Cucurbita and Dalecliampia. The 

 Dendrobium and Hydrangea (both possessing highly cuticularised 

 leaf -surfaces) resisted the action. I also determined that even 

 the soluble constituents of the deposit in several instances caused 

 a corrosion. 



From the substances contained in the deposits I then 

 selected sulphuric acid, powdered metallic iron, red oxide and 

 magnetic oxide of iron as possible causes of this corrosion. 

 With these I experimented separately. 



To the powdered metallic iron and to the oxides a little dis- 

 tilled water was added, and a thin layer of the paste so formed 

 spread upon parts of the upper surfaces of leaves of the follow- 

 ing plants : Cucurbita, Bicinus, Dalechampia, Bouvardia, Bose, 

 Clerodendron, Tecoma, Begonia sanguinea and B. Haageana, 

 Dendrobium nobile, Bhododendron, and Pteris sp. 



Neither of the oxides had any apparent effect on the subjacent 

 parts of the leaves with the sole exception of the magnetic oxide 

 upon a leaf of Dalechamjria. But the action here, after fifteen 

 days, was a very slight one indeed. Observations were continued 

 without result for five weeks. With metallic iron the result was 

 totally different. In four days the Bouvardia and Begonia san- 

 guinea showed a slight discoloration of their epidermal cells. In 

 seven days Cucurbita and Begonia Haageana exhibited the same 

 symptoms. Within twenty-four days the iron had destroyed the 

 subjacent epidermis in all cases ; but I did not find that the 

 action, was continued appreciably deeper into the substance of 

 the leaf, as the iron had become entirely oxidised. 



With sulphuric acid I experimented upon the same species, 



* Cf. Analysis, p. 4. 



