66 
POl^ULAR SCIENCE EEYIEW. 
of silica, and there is even still more in that of some species of 
equisetum ; while in some grasses, as the bamboo, it even occa- 
sionally occurs in a separate crystalline form. 
Chlorine is probably assimilated by plants as sodic chloride 
or common salt: a substance which is most ■widely distributed 
over the surface of the earth, although the proportion in which 
it occurs in some soils is but small. 
Fluorine, we have before said, generally accompanies phos- 
phates, and it is most likely that calcic fluoride is the combina- 
tion in which this element is supplied to plants. 
Looking generally at the whole group of non-metallic ele- 
ments found in plants, we see that with the exception of chlorine 
and fluorine, they are supplied as oxygen compounds. Taking 
the chief of these compounds, we find that they are the very 
forms into which animals are finally resolved after death ; thus 
illustrating the truth of our position wherein we stated that in 
the circulation of matter the mineral is wrought into the 
vegetable, the vegetable into the animal, and this finally re- 
lapses once more into the original simple forms of inorga,nic 
nature. Here is a list of the names and formidse of these final 
products of animal decay, which are at the same time the very 
materials of vegetable nutritive increase ; 'we wu'ite these com- 
pounds in their anhydrous condition : — 
Of the six metals invariably occurring in plants, potassium is 
usually present in larger proportion than any other. Calculat- 
ing the potassium found in plants into the form of potassic oxide, 
K,0, it rarely forms less than 20 and often more than 50 per 
cent, of their ashes. In the ash of roots and tubers the latter 
proportion is often found, while in seeds and grasses it is also 
abundant. The straw and chaff of cereals and the leaves of 
most plants contain it generally in smaller proportion, althougli 
there are exceptional cases ; poppy seed, for example, yielding an 
ash containing only 12 per cent, of potassic oxide, while the ash 
of poppy leaves has more than 37 per cent. As to the form in 
which potassium is assimilated by the plant, it is generally be- 
lieved that all neutral salts of this metal are indifferently taken 
up. Potassic chloride, nitrate, and sulphate have all been 
found in certain samples of fertile soils; and in some ex- 
periments, at all events, when these salts have been used as 
manures, an increase of potassium has been found in the crops. 
Still, although soils generally contain what one would fancy to 
be little more than the necessary minimum of available potassic 
Carbonic acid or anhydride . 
Sulphuric „ „ 
Phosphoric „ „ 
Water 
