Intelligence and Miscellaneous Articles. 513 
in the same volcano at different times, and between the eruptions of 
different volcanoes, is a subject deserving the most attentive con- 
sideration. The violence of a volcanic outburst does not bear any 
relation to the quantity of material ejected. The union of water 
with lavas may be compared with the solution of a gas in water; but 
there is reason to believe that in their deep-seated sources lavas con- 
tain little or no water. If igneous matter be extruded through dry 
strata the eruption might take place without explosive manifesta- 
tions. But if igneous matter be extruded through water-bearing 
beds, a kind of dialysis would take place between the igneous and 
aqueous masses. In this way the tension of the steam in the fluid 
rock may at last become so great that a fissure will be formed at the 
surface and volcanic action will follow. 
In this way the violence of a volcanic eruption will be determined 
by the quantity of water contained in the strata through which the 
lava passes in its passage to the surface, and by the temperature at 
which it reaches the surface. 
This theory explains the acknowledged sequence of volcanic out- 
bursts of different degrees of violence, and the intervals which occur 
between them. It also explains the differences between the central 
and lateral eruptions of a great volcano, and the phenomena attend- 
ing its extinction. 
The structures of the igneous rocks, whether of basic or acid 
composition, are greatly modified by the presence in them of volatile 
ingredients. 
The succession of events indicated by the structure of Monte 
Somma and Vesuvius, Roccamonfina, Monte Vulture and Monte 
Nuovo, show that after a long cessation of volcanic activity we have 
an extensive production of fragmentary and scoriaceous material, 
and that this is gradually succeeded by the eruption of lava-streams. 
The water and other volatile substances, such as sulphates and 
chlorides, which are given off abundantly in volcanic eruptions, 
may act as solvents for the various minerals which constitute lavas. 
LX. Intelligence and Miscellaneous Articles. 
APPLICATION OF PHOTOGRAPHY TO ELECTRICAL MEASUREMENTS. 
BY JOHN TROWBRIDGE AND HAMMOND VINTON HAYES, 
i the study of electromotive force and of voltaic cells it is 
often desirable to have long-continued observations. The com- 
plete history of the action, for instance, of the Daniell cell with 
different strengths of solution extending over hours or days, if it. 
could be presented to the eye as a curve, would be valuable to those 
who desire to know the behaviour of such a cell while it is doing 
work under definite conditions. Such curve could be obtained by 
patient observation, but it would be unprofitable labour for one to 
spend his time in watching the excursions of a galvanometer- 
needle, if the needle can be made to record its movements by any 
device. 
