STRUCTURE.] COMPOSITION. 107 



which it is proved that the precipitate is oxalate of silver. 

 These results, which may be performed with certainty with 

 conglomerate Raphides, plainly prove their composition to 

 be oxalate of lime. 



The acicular can be demonstrated to be phosphate of lime, 

 by proceeding thus. If heated red hot, they do not dissolve 

 in acids with effervescence, a fact which essentially distin- 

 guishes the composition of the two kinds. When dissolved 

 in dilute nitric acid, if oxalate of ammonia be added, we have 

 the characteristic precipitate of lime; if with a portion of the 

 acid solution be mixed a little nitrate of silver, a white preci- 

 pitate is not obtained, as in the last case, but one of a lemon 

 colour, which is such as denotes the combination with silver 

 of phosphoric acid, which must have been furnished by the 

 acicular Raphides. 



Though phosphoric and oxalic acids united with lime 

 are found the most frequent components of these minute 

 crystals, there can be no doubt that tartaric acid enters into 

 their formation in certain plants, as in the fruit of the Grape, 

 where the crystals are found of a different figure from those 

 in the interior of the leaves or stem ; and also that magnesia 

 can be frequently detected combined with lime, and perhaps 

 never forms crystals with acid, without lime entering also 

 into their composition. 



Silica, though it frequently forms an organised part of 

 vegetables, seldom exists as crystals in their interior. In a 

 bark from Para, which is said to be manufactured into a kind 

 of pottery, silica exists in abundance in granular fragments, 

 which, however, do not put on a crystalline form. 



Conclusion. It is not known what purposes these bodies 

 fulfil in the economy of plants, but it has been conjectured, 

 since amylaceous matters are deposited, and again appro- 

 priated for the support of the carbonaceous portion of the 

 tissue, according to the necessities of the individual, that 

 these crystals may be deposits to be applied towards the 

 mineral part or skeleton of the plant, as occasion may 

 require : but it has been found from experiments that these 

 calcareous bodies are insoluble in vegetable acids, and the 

 silica of course in every thing ; consequently they cannot be 



