From this summary of the speculative views which have been entertained upon the subject of the physical condition of the earth's interior, it will be clear that at present we have not sufficient evidence for arriving at anything like a definite solution of the problem. The conditions of temperature and pressure which exist in the interior of a globe of such vast dimensions as our earth, are so far removed from those which we can imitate in our experimental enquiries, and it is so unsafe to push the application of laws arrived at by the latter to the extreme limits required by the former, that we shall do well to pause before attempting to dogmatise on such a difficult question. In the next chapter we shall endeavour to grapple with a somewhat more hopeful task, to point out how far observation and experiment have enabled us to offer a reasonable explanation of the wonderful series of phenomena which are displayed during outbursts of volcanic activity. CHAPTER XII. THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN THE CAUSES OF VOLCANIC ACTION. Every completed scientific investigation must consist of four series of operations. In the first of these an attempt is made to collect the whole of the facts bearing on the question, by means of observation and experiment; the latter being only observation under conditions determined by ourselves. In the second stage of the enquiry, the attention is directed to classifying and grouping the isolated facts, so as to determine their bearings upon one another, and the general conclusions to which they appear to point. In the third stage, it is sought to frame an hypothesis which shall embrace all the observed facts, and shall be in harmony with the general conclusions derived from them. In the fourth stage, this hypothesis is put to the most rigid test; comparing the results which must follow, if it be true, with the phenomena actually observed, and rejecting or amending our hypothesis accordingly. Every great scientific theory has thus been established by these four processes--observation, generalisation, hypothesis, and verification.