144 
THE GEOLOGIST. 
earth, was considered. This sihcon has only of late been carefully studied. 
It is certainly one of the most remarkable and important elements which 
we possess. It occurs, as will be remembered, in three distinct forms ; 
the pulverulent state, or amorphous condition cf silicon, which is ex- 
tremely divided, and perfectly non-crystalline. It is in the form of a 
chocolate-brown powder, and it is indeed only recently that we have seen 
silicon in the crystallized state. We have it next in the graphitoidal 
state, or state resembling graphite. It occurs in the production of alu- 
minium, or, at all events, it was first discovered in the making of aluminium 
by a particular process. It appears in the form of six-sided prisms, having 
a more or less metallic lustre and a dark bluish-black colour. Then we 
have what is termed the octahedral form of silicon. In many characteristics 
it is very similar to the graphitoidal. In colour, for example, one can hardly 
distinguish between the two. It crystallizes in the regular cubical system 
to v\ hich the regular octahedral belongs. JSow, it is a curious thing that 
there is a remarkable analogy between silicon and carbon. Ever)^ chemist 
knows that there are very small chemical relations existing between the 
two ; but with regard to the particular states in which the elements occur, 
we find an analogy obtaining. Thus, we have common charcoal, which is 
the amorphous or non-crystalline form of carbon ; we have the amorphous 
or non-crystalline form of silicon ; we have the graphitic form of carbon, 
exactly corresponding to the so-called graphitoidal form of sihcon ; lastly, 
we have the diamond, which is the crystalline form of carbon, and corre- 
sponds exactly to the octahedral form of silicon. In fact, silicon in this 
state has been called the silicon diamond. 
How this silicon can be obtained belongs rather to purely chemical in- 
vestigation. There is a compound, well known to chemists, called silico- 
fluoride of potassium or sodium. The transparent, colourless gas, consisting 
of fluorine and silicon, combines with potassium or sodium, forming a defi- 
nite white salt — silico-fluoride of potassium or sodium. If we bring in 
contact with that salt some sodium or potassium, and also add a little 
common zinc, and heat the mixture, silicon is separated— displaced from 
its combination with fluorine by the sodium, and is immediately caught by 
the metallic zinc. The temperature should be such as to keep the zinc in 
a molten state. The silicon so separated dissolves in this molten zinc, 
and, on solidification, it separates more or less completely from the metallic 
mass in a definite, beautiful, distinct crystalline form, the silicon being 
dissolved through the mass of zinc. Common hydrochloric acid dissolves 
the zinc and leaves the silicon unacted upon. In this way these crystals 
can be obtained. Notwithstanding the powerful afiinity of silicon for 
oxygen, and the great ex-ercise of force w hich it requires to separate the 
oxygen from the silicon in the state in which they are combined in silica, 
yet, when the silicon is separated, it is astonishing how remarkably stable 
it is. It may be exposed to the air for an indefinite time without under- 
going the least change. You may heat it, indeed, to a very high tempe- 
rature, even w ith access of air, yet it shall not be oxidized. You may 
expose it to the action of various strong chemical reagents, and yet it 
shall undergo no change. It is extremely remarkable that a body which 
possesses such a strong affinity for oxygen, ar.d requires the exercise of so 
much force to separate it from oxygen, should be so stable w hen separated 
as w e find it to be. Exposed to a high temperature it fuses. A specimen 
of fused silicon was exhibited, furnished by Mr. Matthey, of Hatton 
Garden. This silicon is now playmg a very important part in certain 
metallurgical operations. For instance, in the smelting of iron we find 
