508 
DTiS. T. AXDEESOX AXD .T. S. FLETT OX THE ERUPTIOXS OF THE 
some part liquid, each surrounded on all sides by films of expanding gases, and thus 
the mixture of the ingredients from the first was perfect. Around each gram of dust 
there was a film of gases ready to expand enormously when the mass reached the 
upper air. 
The amount of expansion of which these gases are capable is so great as to he 
almost incredible. The small black l)all of cloud, which we saw emerge on July 9th, 
in a feAv minutes was a great black mass, wiiich covered more than a square mile, and 
the white steam, Avhich shoots upwards in the air when the cloud is dead, is still 
actively expanding at a great rate. This leads us to wonder whether, when the cloud 
emerges, it may not be partly at least composed of molten droplets, which, when they 
cool and pass into the solid condition as the cloud rolls on, give out gases which, till 
then, had been physically occluded or alrsorbed in the liquid. The mere pressure 
within the crater would almost seem insufficient to compress so great a volume of gas 
into so small a sjoace, especially when we rememljer the very high temperature of the 
mass. 
As this turbulent mixture of expanding gases and fine dust pours down the .surface 
of the mountain, the small, solid grains are unable, at first, to rest on the ground, 
even when they may have sunk to the base of the cloud, and they are swept up again, 
and borne along till they reach .some sheltered hollow, or the violence of the expansive 
forces lessens and the turmoil diminishes. 
As noted by Professor Lacroix and his colleagues,"^ the cloud cools more rapidly 
when it passes over the sea than over the land, as witness the more complete 
destruction of the south end of St. Pierre than of the “ Roraima ” and other ships, 
which were lying off the shore. The explanatioji is simple ; when the hot dust falls 
on the sea it can rise no more. The sea also, owing to its higher specific heat, rises 
more .slowly in temperature than the land surface, and, though it may be raised to 
lioiling point, it cannot exceed this. There is no such limit to the possible temperature 
of dry, bare earth. By the trapping of the dust and the cooling of the gases in 
contact with the water the cloud soon loses its heat, and with that its power of 
buoying up the solid matter it contains. 
The temperature of the magma, when it rises within the crater, can be fixed within 
certain limits, though these are not very jirecise. We know that it was bright red 
hot, for all who have seen it at night have described it as incandescent. When the 
cloud struck the north end of St. Pierre the dust was hot enoucii to set fire to 
combustible articles, but did not fuse tlie copper wires of the telephone apparatus, or 
melt objects of brass or tin. And in the liquid within the crater were floating- 
crystals of hypersthene and plagioclase felspar in great abundance, and in such 
perfection of crystalline form as to indicate that they had formed by crystallization 
out of a cooling molten mass. Professor JoiA" gives the melting ])oint of labradorite 
* “ Snr I’Eruption de la IMartiiiique,” ‘ Comptes Eendus,’ vol. cxxxv., p. 424. 
