SUPPLY OP MINERAL INGREDIENTS. 137 



means of the blowpipe (potash being also present in the tissues), 

 into a bead of nearly the same appearance as glass ; and the fol- 

 lowing curious instance shows the same effect upon a large scale. 

 A melted mass of glassy substance was found on a meadow 

 between Mannheim and Heidelberg in Germany, after a thun- 

 der-storm. It was at first supposed to be a meteor ; but, when 

 chemically examined, it proved to consist of silex combined with 

 potash, in the form in which it exists in grasses ; and, upon 

 further inquiry it was ascertained, that a stack of hay had stood 

 upon the spot, of which nothing remained but the ashes, the 

 whole having been ignited by the lightning. 



202. Now the various substances which are thus required 

 by plants for their healthy growth, are generally contained in 

 the soil in sufficient amount, to supply the majority of Plants 

 with the necessary material ; and some, when exhausted from 

 the soil, may be supplied again from the water of the district, in 

 which they are dissolved. It is partly by thus renewing what 

 has been withdrawn, that the irrigation or flooding of meadows 

 with water, is very serviceable. Even where this water is of 

 ordinary purity, containing scarcely any organic matter, and but 

 very little of mineral ingredients, the irrigation of meadows is 

 very serviceable in improving their productiveness. From three 

 to five perfect crops of grass have thus been obtained every year, 

 by covering the fields with river-water, which is conducted over 

 it in spring by numerous small canals ; so that the quantity pro- 

 duced in all was more than four times that, which would have 

 been obtained from one not so watered. In the neighbourhood 

 of Edinburgh, the stream which conveys the fluids, collected 

 from the sewers of the town into the sea, has been diverted, so 

 as to cover much of the low meadow land that surrounds the 

 town on three sides ; and the large quantity of organic as well 

 as mineral matter, winch the fluid contains, is so beneficial to 

 the growth of grass, that the previous produce has been eight or 

 ten times multiplied. This proceeding is very injurious, how- 

 ever, to the health of the town ; since the offensiveness of the 

 putrefying matter is very much increased, by being diffused 

 through a large quantity of water. 



