MODERN THEORIES IN CHEMISTRY. 
171 
the difference of potential necessary for the discharge of a 
thunderstorm can be accounted for by estimating the number of 
particles of water vapour in a single inch of saturated steam at 
about three hundred millions of millions of millions, and that 
these form but one average rain drop. Eate of lightning, 288,000 
miles per second, compared with that of light. -Zigzag lightning, 
sheet lightning, and ball lightning. Periods and localities of 
thunder-storms. Induction and the return stroke. The pealing 
and rolling of the thunder explained : (1) by the difference 
between the rates of sound and light; (2) by the reflection of 
sound from clouds ; (3) by the refraction of sound when travelling 
through layers of atmosphere of different densities; (4) by the 
combination of some or all these, with the introduction of forked 
lightning. The distance at which thunder is heard is proved to 
be small. Lightning prints illustrated by Mr. Pidgeon's account 
of a thunder-storm at Torquay, 25th February, 1875. Fulgurites. 
St. Elmo's fire. Lightning-conductors. • Their utility illustrated 
by the effects produced after their general introduction to Pieter- 
maritzburg, in Natal. The conditions to be observed to secure 
a good conductor. Laws laid down by the " Lightning-rod 
Conference." 
MODERN THEORIES IN CHEMISTRY. 
SYLLABUS OF LECTURE BY LOUIS SEXTON. 
(Read 15th November, 1888.) 
Modern chemistry the outcome of alchemy. Sketch of the 
history of alchemy. Its foundations in the various cosmogonies. 
At first a "sacred art," afterwards an independent study. Its 
two tendencies leading to the endeavour to transmute metals and 
to theories of the constitution of matter : the first, the origin of 
practical chemistry ; the second, that of theoretic chemistry. The 
introduction of the use of the balance in experiments, leading 
to the discovery of oxygen and to Dalton's law of multiple pro- 
portions. Dalton's application of the atomic theory to explain it. 
Growth of modern chemistry from this time. Re-opening of the 
old question — What are the elements'? Growth of the idea of 
evolution, as evinced by Prout's hypothesis, Mendeljeef 's law, &c. 
Recent opinion, as shown by Mr. Crooke's address to the British 
Association, in 1886. 
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