144 



THE GEOLOGIST. 



earth, was considered. This silicon 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 which the regular octahedral belongs. JNow, it is a curious thing that 

 there is a remarkable analogy between silicon and carbon. Every 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 silicon ; 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- 

 fiuoride 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 atfinity of silicon for 

 oxygen, and the great ex-ercise of force which 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 with access of air, yet it shall not be oxidized. You may 

 expose it to the action of various strong chemical reagents, and } r et it 

 shall undergo no change. It is extremely remarkable that a body which 

 possesses such a strong affinity for oxygen, and requires the exercise of so 

 much force to separate it from oxygen, should be so stable when separated 

 as we 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 playing a very important part in certain 

 metallurgical operations. For instance, in the smelting of iron we find 



