[ 18 J 



form, even the typical grain of sand having a vei-y perceptible size 

 and shape, whei-eas the siliceous coating of a wheaten straw was 

 deposited in sufficient quantity to turn the edge of a knife, with 

 a uniformity and a fineness of division resembling that produced 

 by the electrotype process. Whether this silica was extracted 

 from sand, as indicated by the constant presence of reeds and 

 sedges on the sea shore, or separated from the soluble silicates 

 everywhere present in arable soil, a curious chemical process must 

 have taken place, which there was no means of tracing, and the 

 question was not simplified but rather increased by the fact that 

 the deposit was not always a powder. Frequently it was crystalline 

 or needle-shaped, sometimes laminated, as at the nodes of bamboo, 

 from whence it was collected in considerable quantity under the 

 name of tabasheer. 



Then, again, the importance of the change might be estimated 

 by the scale on which deposition took place. The horsetails and 

 reeds yielded in their ashes 97 per cent, of pure silica, the grasses 

 nearly as much. The diatoms were almost made of silica, and 

 when it was remembered that the deposit remained unchanged by 

 burning or natural decomposition, it would readily be conceived 

 that in this way also plants performed a most important work in 

 nature. The silica so deposited was pure, and the pui-ification of 

 salts was one of the most remai-kable functions of plants. In the 

 laboratory, purification was generally performed by ci^stallising 

 from a solution, and it was always assumed that a crystal posses- 

 sing the proper form and colour was a pure specimen of the salt 

 under examination. 



In like manner plants produced various salts in purity, as 

 those knew who wei'C dicussing the beautiful crystals called 

 raphides, found in the cells of almost all plants, but especially in 

 the liliacese and kindred oi'ders, with the crystals on the leaves of 

 the deutzia, &c. In this pure form were found the cai-bonate, 

 oxalate, phosphate, and sulphate of calcium, and those who wei"e 

 curious in the vai'ious shapes which ci-ystals of the same compound 

 assumed, might find fresh field for investigation in the fact that all 

 the varieties of form common to the bodies mentioned were found 

 in different plants, but that the same species always presented the 

 same form and composition of crystal. Whether raphides were 



