68 
POPULAR SCIENCE REYIEW. 
in weighable quantities. The proportion of manganese in plants 
is probably seriously influenced by that in the soil ; it is a vari- 
able quantity. But little is known of the forms of combina- 
tions in which iron and manganese exist in plants, but they are 
usually found in the soil, if it be a really productive one, in the 
form of ferric oxide, FegOg, and manganic binoxide, MnOg. It 
is well known that many unoxidised or partially oxidised com- 
pounds of iron, when present to a large extent, are injurious to 
plants, even to the extent of causing absolute sterility. Such 
combinations are iron pyrites, FeSg, magnetic oxide of iron 
Fe 304 , and ferrous oxide and sulphate, FeO and FeS 04 . 
Of the rarer metals, csesium, rubidium, copper, &c., which have 
been recognised in the ashes of certain plants, we have already 
spoken. It is probable that in the course of careful inquiry in 
this direction we may discover other metals, the presence of 
which is as yet wholly unsuspected. 
