55 



beyond the mere unnatural enlargement. In the 

 course of a few years, less in number in proportion 

 to the advanced age of the tree, and the unfavourable 

 circumstances under which it is vegetating, the swell- 

 ins; is greatly increased in size, and the alburnum has 

 become extensively dead ; the superincumbent bark 

 cracks, rises in discoloured scales, and decays even 

 more rapidly than the wood beneath. If the caries 

 is upon a moderately-sized branch, the decay soon 

 completely encircles it, extending through the whole 

 alburnum and bark. The circulation of the sap being 

 thus entirely prevented, all the parts above the disease 

 of necessity perish. In the apple and the pear the 

 disease is accompanied by scarcely any discharge ; but 

 in the elm this is very abundant. The only chemists 

 who have examined these morbid products are Sir H. 

 Davy and Vauquelin ; the former's observations being 

 confined to the fact, that he often found carbonate of 

 lime on the edges of the canker in apple-trees.* 



Vauquelin has examined the sanies discharged from 

 the canker of an elm with much more precision. He 

 found this liquor nearly as transparent as water, some- 

 times slightly coloured, at other times a blackish 

 brown, but always tasting acrid and saline. From 

 this liquor a soft matter, insoluble in water, is de- 

 posited upon the sides of the ulcer. The bark over 



* Elements of Agric. Chemistry, 2nd ed. p. 246. 



