456 Prof. Ritchie’s Experimental and Physical 
than those commonly employed, and made of a tube of iron 
instead ofa solid mass. A continuous coil of eighty or a hun- 
dred yards, or even more according to the effect intended to be 
roduced, is rolled about one of the ends, whilst two or three 
coils of thirty, forty, or fifty yards long are rolled about the 
other end. The ends of the last are collected together and 
soldered to a thick wire which fits into the cylinder formed 
by the spiral, each end of the single coil being terminated by 
similar pieces. 
When brilliant phenomena of light are required, we fix a 
star of platina foil on the solid axis, and if we wish to double the 
effect we fix another similar star on the hollow axis, and con- 
nect the ends of the compound coil with the two axes by means 
of the spiral cylinder. If we wish to exhibit chemical or phy- 
siological effects we connect the continuous coil, or employ 
both coils as a continuous one. 
When the short coil is employed the light is exceedingly 
brilliant and the shock scarcely sensible; with the long coil 
the light is feeble, but the shock unpleasantly powerful, even 
without wetting the hands. 
The following simple addition to the revolving lifter will 
supersede the apparatus which I formerly described for de- 
tonating a mixture of oxygen and hydrogen by the magneto- 
electric spark*. DE is a thick copper or brass wire, about 
the size of a quill, and bent into the annexed form. It is 
screwed into the end of the brass axis so as to have good me- 
tallic contact. EP is a wire having a loop at E through 
which the wire passes, the other end resting on a small disc 
of copper connected with the wire W. T is a glass tube open 
at the lower end and closed at the other by a sound cork, or 
a piece of wood cemented in it. ‘The wire W dips into the 
interior compartment I of the cup for holding mercury. A 
small spiral spring is fixed on the wire a little above P in order 
to secure good contact with the disc of copper. When the lifter 
is made to revolve, the end of the wire is raised from the disc 
at every revolution, and a brilliant spark appears at the point 
P, which will detonate a mixture of oxygen and hydrogen in- 
troduced into the tube. 
Though these facts, which I have endeavoured to illustrate 
by an improved apparatus, are generally known, I am_ not 
aware that any theory has been proposed to account for the 
striking difference between the physical and the chemical or 
physiological effects. 
The undulatory theory of light is already established on so 
firm a basis, that we may employ it in the explanation of all 
* [See Lond. and Edinb. Phil. Mag., vol. iv. p. 105.—Eprv.] 
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