10 Prof. J. W. Judd—The Lavas of Krakatoa. 
points of various salts is a question which was very carefully investi- 
gated by Guthrie. He showed that a certain mixture of the nitrates 
of lead and potash had its fusion-point reduced from 8° to 4° C. by 
the introduction into it of a quantity of water equal to ;45th part of 
its weight. Still more striking are the results obtained by the study 
of the fusion-point of nitre as influenced by the presence of water. 
The following table abridged from that given by Dr. Guthrie will 
serve to explain this subject :— 
Fusion-points oF Mixtursrs or Nirre anD WATER. 
Nitre. Water, Fusion-points, 
100 te) 600 320° C. 
98°86 wee 1:14 am 300%mae 
95-11 Ba 4:89 a NO 
89°94 Be 10-06 He HDI 26, 
84°69 ue 15°31 uy NGI. 
70°14 i 20°86 an TRO 
75-02 £5, 24:08 Be INGO 
70°03 hae 29°07 es 97°°6 ,, 
There is no question here of the formation of definite compounds 
of nitre and water, but in the words of Dr. Guthrie ‘‘ fused nitre 
and fused ice are miscible in all proportions,’ the result of the 
increase of water being a proportional lowering of the fusion-points. 
“The phenomenon of fusion” is shown by Guthrie to be “nothing 
more than an extreme case of liquefaction by solution.” Indeed, it 
is impossible to define where solution ends and fusion begins ; when, 
for example, one part of water is present in 555 parts of a salt, it 
would be hard to consider the latter as dissolved in the former. 
Now it is impossible to doubt for one moment that the presence 
of water has the same effect on the fusion-points of mixed silicates 
that it has on other salts, though owing to the high temperatures and 
pressures required, experiments on these are more difficult than in 
the case of salts of more easy fusibility. The Zeolites consist of 
mixtures of silicates of alumina and of the alkalies or lime, in the 
same proportions as in the Felspars; but while the former contain 
water, the latter are anhydrous; as is well known, the Zeolites 
fuse at much lower temperatures than the Felspars. Nor is this true 
only of definite compounds as is shown by studying the fusibility of 
such indefinite mixtures of silicates and water as the tachylytes, 
hydrotachylytes, palagonites, ete. 
The consideration of these points leads to a clearer understanding 
of the method of action of water in volcanic vents. That there are 
serious objections to the notion of water giving rise to volcanic 
eruptions by finding its way through open fissures to masses of 
incandescent lava, has been pointed out by Scrope and other authors; 
and Daubrée and others have advocated the gradual percolation of 
water through the interstices of solid rock-masses as being an 
agency more in accordance with the phenomena observed. But it 
has always been considered necessary to assume a considerable rise 
in the temperature of the subterranean mass as one of the exciting 
causes of a volcanic outburst. 
A careful consideration of the results arrived at by Dr. Guthrie’s 
