Encyclopaedia Britannica > Volume 8, DIA-England

ELECTRICITY.
636
Phenome- In the course of these interesting experiments Mr Pear-
na and sall observed the curious fact, that the specimens of fluor
Laws. Spar> though colourless in their natural state, received a
bluish tint when electrified, and the acquired phosphores¬
cence was proportional to the depth of the tint. When a
number of fragments were used, the larger fragments were
of a blue colour, and emitted a blue light when heated,
whereas the smaller fragments emitted only a pale yellow
light. Mr Pearsall thinks it probable that the phosphores¬
cent property is communicated by electricity only to the
surface, which he considers as explaining the fact that
fragments of different dimensions emit differently coloured
lights.
In resuming the investigation of this subject, Mr Pearsall
found, that bodies not naturally phosphorescent, such as
statuary marble in its natural or calcined state, ivory
when its carbonaceous part was removed, calcined mother
of pearl, calcined oyster shells, calcined petuncles, egg
shells, and lime, were not only rendered phosphorescent
by heat after being strongly electrified, but acquired this
property with a beauty, a variety, and an intensity of colour
superior to those which occur in specimens that possess a
natural phosphorescence. We regret that our limits will
not permit us to give in detail a second series of experi¬
ments which Mr Pearsall performed with twelve different
varieties of fluor spar, all of which gave distinct phospho¬
rescence previous to their being electrified ; but the gene¬
ral result of them may be thus expressed. When the na¬
tural spars emit by heat a light of different colours, the
electric action produces only one of them; but when
the mineral yields only one natural colour by heat, this is
replaced, when electricity is applied, by a phosphorescence
of various colours, among which the primitive tint does not
appear. As the colours change with the number of elec¬
trical discharges, Mr Pearsall found the following to be the
order of progression. The specimen was a green fluor.
1 discharge, pale purple light when heated.
2 pale green, changing into purple.
3 the same colours, more intense and durable.
4 purple, with increased intensity.
6 green, brighter and deeper.
10 green bright ;fineand more durable purple.
20 deep and more durable colours.
40 very rich colours, the purple at last inclin¬
ing to red.
100 green colour, highly brilliant, and becom¬
ing yellowish. The purple had now a
superb tint.
160 an intense light nearly white, followed with
a brilliant green light, then with a dura¬
ble purple, and then with a yellow ac¬
companied with violet tints.
This specimen had been successively heated and elec¬
trified nearly fifteen times, and had suffered no deteriora¬
tion in its phosphorescent property.
Mr Pearsall next shows that the property communicated
by electricity was preserved even for three months, when
the specimen was kept in the dark. Out of twelve frag¬
ments, two had completely lost their acquired phosphores¬
cence by exposure to the sun for twenty-one days, five
had nearly lost it, and six had experienced a modification
in their colours by this exposure.
Mr Pearsall now examined the influence of electricity
on the natural phosphorescence of bodies, and he found
that an augmentation of intensity was produced, of which
it is difficult to give an idea. Specimens of fluor whose
pyro-phosphorescence was feeble or uncertain, were raised
to the rank of highly phosphorescent bodies, and some of
them even rivalled the Siberian fluor. At the end of fifty
days some of these specimens still preserved the excess
of phosphorescence which had been communicated to Phenonu
them, while others continued to exhibit the same order of na and
colours. Laws.
Mr Pearsall has brought forward several experiments to
prove that the phosphorescence of bodies, and the modi¬
fications it experiences, depend on their structure and
mechanical condition. Phosphate of lime, for example,
which in the form of apatite has an intense natural phos¬
phorescence, has none when aggregated from a precipita¬
tion of it in a solution of muriatic acid, nor when obtained
from powdered or calcined apatite. A calculus of phos¬
phate of lime, however, gave green, yellow, and orange
light when heated after having been calcined and expos¬
ed to twenty electrical discharges. Mr Pearsall also se¬
veral times observed that the power of phosphorescence
returned after it had disappeared.
With the view of showing that the phosphorescence was
not owing to any radiating matter which was carried along
with the sparks, Mr Pearsall inclosed coloured chlorophane
in glass tubes hermetically sealed, and found them phos¬
phorescent after 225 discharges. He found Voltaic elec¬
tricity capable of producing phosphorescence in some
cases and not in others; so that it differs greatly from
common electricity in this property.
In explaining the preceding phenomena, Mr Pearsall
considers the intense electricity of the Leyden jar as alter¬
ing the structure upon which phosphorescence depends,
by the vibratory motion which it communicates, and which
allows the particles to take a new arrangement. When the
body has had a new structure communicated to it by the
vibrations or shocks of each electrical discharge, the action
of heat is supposed by our author to permit the body to
return to its primitive structure; and he conceives that the
vibrations of the atoms during these changes of structure
may produce light.
Sect. IV.— On the Changes produced hy Electricity on
Odoriferous Bodies.
It has been recently discovered by M. Libri of Florence Influenc
that electricity exercises a curious influence over odorife-on odorj
rous bodies. Having caused a continued current of elec-“i™us
tricity to traverse a piece of camphor, the odour of this
substancebecame more and more feeble, and at last entirely
disappeared. When the camphor has suffered this change,
and is withdrawn from all electrical influence, and put in
communication with the ground, it will remain without
odour for some time, but it will afterwards resume its for¬
mer properties slowly and gradually. M. Libri seems to
have obtained a similar result with other odoriferous bodies;
but he has not, so far as we know, given any more parti¬
cular account of his researches.
Sect. V.— On the Magnetic Effects of Electricity.
During almost every period of the history of electricity, Magnet
philosophers have pointed out strong resemblances be-effec^
tween the phenomena which it exhibits and those of mag-e cc L
netism. Some of the most striking points of resemblance
were, that each consisted, as it were, of two powers or di¬
rections of powers, of an opposite nature, and subjected to
similar laws of attraction and repulsion ; that the action ot
magnetism has a great analogy with that of electricity;
that the distribution of the forces in an electrified body
differs very little from that of the forces in a magnet; and
that the pyro-electrical tourmaline has the strongest re¬
semblance to an artificial magnet.
These views were powerfully confirmed by the fact, often