Parr I. Sect. i. § 1.] VOLCANIC PRODUCTS. 201 
to corroborate the deductions of C. Sainte-Claire Deville that the 
~ nature of the vapours evolved depends on the temperature or degree of 
activity of the volcanic orifice, chlorine (and fluorine) emanations 
indicating the most energetic phase of eruptivity, sulphurous gases 
a diminishing condition, and carbonic acid (with hydrocarbons) 
_ the dying out of the activity. A “solfatara,” or vent emitting only 
gaseous discharges, is believed to pass through these successive 
stages. Wolf observed that on Cotopaxi while hydrochloric acid, 
and even free chlorine escaped from the summit of the cone, sul- 
phuretted hydrogen and sulphurous acid issued from the middle 
and lower slopes. Fouqué’s studies at Santorin have shown also 
that from submarine vents a similar order of appearance obtains 
among the volcanic vapours, hydrochloric and sulphurous acids 
being only found at points of emission having a temperature above 
100° C., while carbon dioxide, sulphuretted hydrogen and nitrogen 
occur at all the fumaroles, even where the temperature is not higher 
than that of the atmosphere.” 
The following are the chief gases evolved at volcanic fumaroles. 
Hydrochloric acid is abundant at Vesuvius, and probably at 
many other vents whence it has not been recorded. It is recognis- 
able by its pungent, suffocating fumes, which make approach difficult 
to the clefts trom which it issues. Sulphuretted hydrogen 
and sulphurous acid are distinguishable by their odours. ‘The 
liability of the former gas to decomposition leads to the deposition 
of a yellow crust of sulphur, and perhaps also to the production of 
the sulphuric acid observed at active vents. Allusion has already 
been made (p. 53) to the emission of free hydrogen or of com- 
bustible compounds of this gas by Vesuvius. At the eruption 
of Santorin in 1866 these gases were also distinctly recognised by 
Fouqué, who for the first time established the existence of true volcanic 
flames. These were again studied spectroscopically in the following 
year by Janssen, who found them to arise essentially from the com- 
bustion of free hydrogen, but with traces of chlorine, soda, and 
copper. Fouqué determined by analysis that immediately over the 
focus of eruption free hydrogen formed thirty per cent. of the gases 
emitted, but that the proportion of this gas rapidly diminishes with 
distance from the active vents and hotter lavas, while at the same © 
time the proportion of marsh gas and carbon dioxide rapidly increases. 
The gaseous emanations collected by him were found to contain 
_ abundant free oxygen as well as hydrogen. One analysis gave the 
following results: carbon dioxide 0°22, oxygen 21°11, nitrogen 21:90, 
hydrogen 56°70, marsh gas 0:07, = 10000. This gaseous mixture, 
on coming in contact with a burning body, at once burns with a 
sharp explosion. Jouqué infers that the water-vapour of volcanic 
vents may exist in a state of dissociation within the molten magma 
whence lavas rise.° Carbon dioxide rises chiefly (a) after an 
1 Neues Jahrb. 1878, p. 164. 2 “Santorin et ses éruptions,” Paris, 1879. 
> Fouqué, op. cét. p. 225, 
