Scientific American Supplement, No. 344, August 5, 1882

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Experiments of a very definite kind have not yet been made as to the
nature of the arc produced by induced currents developed in alternating
current machines; but, from the experiments made with electric candles,
we are forced to admit that the current reacts as if it were alternately
reversed through the arc, since the carbons are used up to an equal
degree; and, moreover, Mr. Pilleux's experiments show that effects
analogous to those of induction coils are produced by the reaction of
magnets upon the arc. There is, then, here a doubtful point that it
would be interesting to clear up; and we believe that it is consequently
proper to introduce in this place Mr. Pilleux's note:

"Having at my disposal," says he, "a powerful vertical voltaic arc of 12
centimeters in length, kept up by alternately reversed currents, and one
of the most powerful permanent magnets that Mr. De Meritens employs for
magneto-electric machines, I have been enabled to make the following
experiments:

"1. When I caused one of the poles of my magnet to slowly approach the
voltaic arc, I ascertained that, at a distance of 10 centimeters, the
arc became flattened so as to assume the appearance of those gas jets
called 'butterfly.' The plane of the 'butterfly' was parallel with the
pole that I presented, or, in other words, with the section of the
magnet. At the same time, the arc began to emit a strident noise, which
became deafening when the pole of the magnet was brought to within a
distance of about 2 millimeters. At this moment, the butterfly form
produced by the arc was _greatly spread out, and reduced to the
thickness of a sheet of paper_; and then it burst with violence, and
projected to a distance a great number of particles of incandescent
carbon.

"2. The magnet employed being a horseshoe one, when I directed it
laterally so as to present successively, now the north and then the
south pole to the arc, the 'butterfly' pivoted upon itself so as not to
present the same surface to each pole of the magnet."

By referring to the accompanying figure, which we extract from our note
on the Ruhmkorff apparatus, it will be seen that the aureola which
developed as a circular film from right to left at D, on the north pole
of the magnet, N.S. (Fig. 1), projected itself in an opposite direction
at C, upon the south pole, S, of the same magnet; but, between the two
poles, these two contrary actions being obliged to unite, they gave rise
in doing so to a very characteristic helicoid spiral whose direction
depended upon that of the current of discharge through the aureola,
or upon the polarity of the magnetic poles. On the contrary, when the
discharge took place in the direction of the equatorial line, as in Fig.
2, the circular film developed itself in the plane of the neutral line
above or below the line of discharge, according to the direction of the
current and the magnetic polarity of the magnet.

There is, then, between Mr. Pilleux's experiments and my own so great an
analogy that we might draw the deduction therefrom that induced currents
in alternating machines have, like those of the Ruhmkorff coil, a
definite direction, which would be that of currents having the greatest
tension, that is to say, that of direct currents. This hypothesis seems
to us the more plausible in that Mr. J. Van Malderem has demonstrated
that the attraction of solenoids with the currents, not straight,
of magneto-electric machines is almost as great as that of the same
solenoids with straight currents; and it is very likely that the
difference which may then exist should be so much the less in proportion
as the induced currents have more tension. We might, then, perhaps
explain the different effects of the wear of the carbons serving as
rheophores, according as the currents are continuous or alternating, by
the different calorific effects produced on these carbons, and by the
effects of electric conveyance which are a consequence of the passage of
the current through the arc.

We know that with continuous currents the positive carbon possesses a
much higher temperature than the negative, and that its wear is about
twice greater than that of the latter. But such greater wear of the
positive carbon is especially due to the fact that combustion is greater
on it than on the negative, and also to the fact that the carbonaceous
particles carried along by the current to the positive pole are
deposited in part upon the other pole. Supposing that these polarities
of the carbons were being constantly alternately reversed, the effects
might be symmetrical from all quarters, although the only current
traversing the break were of the same direction; for, admitting that the
reverse currents could not traverse the break, they would exist none the
less for all that, and they might give rise (as has been demonstrated
by Mr. Gaugain with regard to the discharges of the induction spark
intercepted by the insulating plate of a condenser) to return discharges
through the generator, which would then have, in the metallic part of
the circuit, the same direction as the direct currents succeeding,
although they had momentarily brought about opposite polarities in the
electrodes. What might make us suppose such an interpretation of the
phenomenon to have its _raison d'etre_, is that with the induced
currents of the Ruhmkorff coil, it is not the positive pole that is
the hottest, but rather the negative; from whence we might draw the
deduction that it is not so much the direction of the current that
determines the calorific effect in the electrodes, as the conditions of
such current with respect to the generator. I should not be
surprised, then, if, in the arc formed by the alternating currents of
magneto-electric machines, there should pass only one current of the
same direction, and which would be the one formed by the superposition
of direct currents, and if the reverse currents should cause return
discharges in the midst of the generating bobbins at the moment the
direct currents were generated.--_Th. Du Moncel_.

* * * * *

VOLCKMAR'S SECONDARY BATTERIES.

The inventive genius of the country is now directed to these important
accessories of electric enterprise, and no wonder, for as far as can at
present be seen, the secret of electric motion lies in these secondary
batteries. Among other contributions of this kind is the following, by
Ernest Volckmar, electrician, Paris:

The object of this invention is to render unnecessary the use in
secondary batteries of a porous pot which creates useless resistance
to the electric current, and to store in an apparatus of comparatively
small weight and bulk considerable electric force. To this end two
reticulated or perforated plates of lead of similar proportions are
prepared, and their interstices are filled with granules or filaments of
lead, by preference chemically pure. These plates are then submitted to
pressure, and placed together, with strips of nonconducting material
interposed between them, in a suitable vessel containing a bath of
acidulated water. The plates being connected with wires from an electric
generator are brought for a while under the action of the current, to
peroxidize and reduce the whole of the finely divided lead exposed to
the acidulated water. The secondary battery is then complete. It will be
understood that any number of these pairs of plates may be combined to
form a secondary battery, their number being determined by the amount
of storage required. The perforated plates of lead may be prepared by
drilling, casting, or in other convenient manner, but the apertures, of
whatever form, should be placed as closely together as possible, and
the finely divided lead to be peroxidized is pressed into the cells or
cavities so as to fill their interiors only.

* * * * *

THE MINERALOGICAL LOCALITIES IN AND AROUND NEW YORK CITY, AND THE
MINERALS OCCURRING THEREIN.

By NELSON H. DARTON.

There will be many persons in the city of New York and its suburbs who
will not have the time or facilities for leaving town during the summer,
to spend a part of their time enjoying the country, but would have
sufficient time to take occasional recreation for short periods. I have
sought by this paper to show a pleasurable, and at the same time very
instructive use for the time of this latter class, and that is in
mineralogy. In the surrounding parts of New York are many mineralogical
localities, known to no others than a few professional mineralogists,
etc., and from which an excellent assortment of minerals may be
obtained, which would well grace a cabinet and afford considerable
instruction and entertainment to their owner and friends, besides acting
as an incentive to a further study of this and the other sciences. These
localities which I will discuss are all within an hour's ride from New
York, and the expenses inside of a half dollar, and generally very much
less. I could detail many other places further off, but will reserve
that for another paper.

The course which I will pursue in my explanations I have purposely made
very simple, avoiding--or when using, explaining--all technical terms.
The apparatus and tests noticed are of the most rudimentary style
consistent with that which is necessary to attain the simple purpose of
distinguishment, and altogether I have prepared this paper for those
having at the present time little or no knowledge or practice in
mineralogy, while those having it can be led perhaps by the details of
the localities noticed. Another reason why I have written so in detail
of this last subject is, because the experiences of most amateur
mineralogists are generally so very discouraging in their endeavors to
find the minerals, and there is everything in giving a good start
to properly fix the interest on the subject. The reason of these
discouragements is simple, and generally because they do not know the
portion of the locality, say, for instance, a certain township, in which
the minerals occur. And if they do succeed in finding this, it is seldom
that the portion in which the mineral occurs, which is generally some
small inconspicuous vein or fissure, is found; and even in this it
is generally difficult to recognize and isolate the mineral from the
extraneous matter holding it. As an instance of this I might cite thus:
Dana, in his text book on mineralogy, will mention the locality for
a certain species, as Bergen Hill--say for this instance, dogtooth
calespar. When we consider that Bergen Hill, in the limited sense of the
expression, is ten miles long and fully one mile wide, and as the rock
outcrops nearly all over it, and it is also covered with quarries,
cuttings, etc., it may be seen that this direction is rather indefinite.
To the professional mineralogist it is but an index, however, and he
may consult the authority it is quoted from--the _American Journal of
Science_, etc.--and thus find the part referred to, or by consulting
other mineralogists who happen to know. Again, the person having found
by inquiry that the part referred to is the Pennsylvania Railroad, and
as this is fully a mile long and interspersed with various prominent
looking, but veins of a mineral of little value, at any rate not the one
in question, they are few who could suppose that it occurred in that.
Apparently a vein of it would not be noticed at all from the surrounding
rock of gravelly earth, but there it is, and in a vein of chlorite. This
is so throughout the long and more or less complete stated lists of
mineralogical localities. Thus I will, in describing the mineral, after
explaining the conditions under which it occurs, give almost the
exact spot where I have found the same mineral myself, and have left
sufficiently fine specimens to carry away, and thus no time will be lost
in going over fruitless ground, and further, this paper is written up to
the date given at its end, insuring a necessary presence of them.

In order that one not familiar with mineral specimens should not carry
off from the various localities a variety of worthless stones, etc.,
which are frequently more or less attractive to an inexperienced eye,
the following hints may be salutary.

There are the varieties of three minerals, which are very commonly met
with in greater or less abundance in mineralogical trips: they are of
calcite, steatite, and quartz. They occur in so many modifications of
form, color, and condition that one might speedily form a cabinet of
these, if they were taken when met with, and imagine it to be of great
value. The first of these is calcite. It occurs as marble, limestone;
calcspar, dogtooth spar, nail head spar, stalactites, and a number of
other forms, which are only valuable when occurring in perfect crystals
or uniquely set upon the rock holding it. The calcspar is extremely
abundant at Bergen Hill, where it might be mistaken for many of the
other minerals which I describe as occurring there, and even in
preference to them, to one's great chagrin upon arriving home and
testing it, to find that it is nothing but calcite. In order to avoid
this and distinguish this mineral on the field, it should be tested with
a single drop of acid, which on coming in contact with it bubbles up or
effervesces like soda water, seidlitz powder, etc., while it does not do
so with any of the minerals occurring in the same locality. This acid
is prepared for use as follows: about twenty drops of muriatic acid are
procured from a druggist in a half-ounce bottle, which is then filled up
with water and kept tightly corked. It is applied by taking a drop out
on a wisp of broom or a small minim dropper, which may be obtained at
the druggist's also. I do not say that in every case this mineral should
be rejected, because it is frequently very beautiful and worthy of place
in a cabinet, but should be kept only under the conditions mentioned
further on in this paper, under the head of "Calcite in Weehawken
Tunnel."

The next mineral abundant in so many forms is quartz, and is not so
readily distinguished as calcite. It is found of every color, shape,
etc., possible, and that which is found in any of the localities I am
about to describe, with the exception of fine crystals on Staten Island,
are of no value and may be rejected, unless answering in detail to the
description given under Staten Island. The method of distinguishing the
quartz is by its hardness, which is generally so great that it cannot be
scratched by the point of a knife, or at least with great difficulty,
and a fragment of it will scratch glass readily; thus it is
distinguished from the other minerals occurring in the localities
discussed in this paper.

The other minerals so common are the varieties of steatite. This is
especially so at Bergen Hill and Staten Island. They occur in amorphous
masses generally, and may be distinguished by being so soft as to be
readily cut by the finger nail. I will detail further upon the soapstone
forms in discussing the localities on Staten Island, and the chloritic
form under the head of "Weehawken Tunnel." The surest method of avoiding
these and recognizing the others by their appearance, which is generally
the only guide used by a professional mineralogist, is to copy off the
lists of the various minerals I describe, and, by visiting the American
Museum of Natural History on any week day except Mondays and Tuesdays,
one may see and become familiar with the minerals they are going
in quest of, besides others in the cases. This method is much more
satisfactory than printed descriptions, and saves the labor of many of
the distinguishing manipulations I am about to describe, besides saving
the trouble of bringing inferior specimens of the minerals home.

In going forth on a trip one should be provided with a mineralogical
hammer, or one answering its purpose, and a cold chisel with which to
detach or trim the minerals from adhering rocks, the bottle of acid
before referred to, and a three cornered file for testing hardness,
as explained further on. As I noticed before, the better plan of
distinguishing a mineral is by being familiar with its appearance, but
as this is generally impracticable, I will detail the modes used in
lieu of this to be applied on bringing the minerals home. These
distinguishments depend on difference in specific gravity, hardness,
solubility in hot acids, and the action of high heat. I will explain the
application of each one separately, commencing with--

_The Specific Gravity_.--In ascertaining the specific gravity the
following apparatus is necessary: a small pair of hand scales with a set
of weights, from one grain to one ounce. These can be procured from the
apparatus maker, the scales for about fifty cents, and the weights for
not much over the same amount. The scales are prepared for this work by
cutting two small holes in one of the scale pans, near together, with
a pointed piece of metal, and tying a piece of silk thread about eight
inches long into these. In a loop at the end of this thread the mineral
to be examined is suspended. It should be a pure representative of the
mineral it is taken from, should weigh about from one hundred grains to
an ounce, and be quite dry and free from dirt. If the piece of mineral
obtained is very large, this sized portion may be often taken from it
without injury; but it will not do to mar the beauty of a mineral to
ascertain its specific gravity, and it is generally only applicable
when a small piece is at hand. With more weights, however, a piece of a
quarter pound weight may be taken if necessary. The mineral is tied into
the loop and weighed, the weight being set down in the note book, either
in grains or decimal parts of an ounce. Call this result A. It is then
weighed in some water held in a vessel containing about a quart, taking
care while weighing it that it is entirely immersed, but at the same
time does not touch either the sides or bottom. Both weighings should
be accurate to a grain. This result we call B. The specific gravity is
found by subtracting B from A, and dividing A by the remainder. For
instance, if the mineral weighed eight hundred grains when weighed in
the air, and in the water six hundred, giving us the equation: 800
/ (800 - 600) = sp. gr., or 4, which is the specific gravity of
the mineral. If the mineral whose specific gravity is sought is an
incrustation on a rock, or a mixture of a number of minerals, or would
break to pieces in the water, the specific gravity is by this method of
course unattainable, and other data must be used.

_The Comparative Hardness_.--The next characteristic of the mineral to
be ascertained is the comparative hardness. In mineralogy there is a
scale fixed for comparison, from 1 to 10, 10 being the hardest, the
diamond, and Number 1 the soft soapstone. These and the intermediate
minerals fixed upon the scale are generally inaccessible to those who
may use the contents of this paper, and I will give some more familiar
materials for comparison. 8, 9, and 10 are the topaz, sapphire, and
diamond respectively, and as these and minerals of similar hardness will
probably not be found in any of the localities of which I make mention,
we need not become accustomed to them for the present. 7 is of
sufficient hardness to scratch glass, and is also not to be cut with the
file before mentioned, which is used for these determinations. 6 is
of the hardness of ordinary French glass. 5 is about the hardness of
horse-shoe or similar iron; 4 of the brown stone (sandstone) of which
the fronts of many city buildings, etc., are built; 3 of marble; 2 of
alabaster; and 1 as French chalk, or so soft as to be readily cut with
the finger nail. The method of using and applying these comparisons is
by having the above matters at hand, and compare them by the relative
ease with which they can be cut by running the edge of the file over
their surface. One will soon become familiar with the scale, and it
may of course then be discarded. As it is one of the most important
characteristics of some of the minerals, it should be carefully
executed, and the result carefully considered. It is of course
inapplicable under those conditions with minerals that are in very small
crystals or in a fibrous condition.

_Action of Hot Acids_.--This very important test is never, like the
above, applicable upon the field, but applied when home is reached.
From the body of the mineral as pure and clean as possible a portion is
chipped, about the size of a small pea; this is wrapped in a piece of
stiff wrapping paper, and after placing it in contact with a solid body,
crushed finally by a blow from the hammer. A pinch of the powder so
obtained is taken up on the point of a penknife, and transferred into
a test tube. Two or more of these should be provided, about six inches
long. They may be obtained in the apparatus shop for a trifle. Some
hydrochloric, or, as it is generally called, muriatic acid, is poured
upon it to the depth of about three quarters of an inch; the tube is
then placed in some boiling water heated over a lamp in a tinned or
other vessel, and allowed to boil for from ten to fifteen minutes;
the tube is then removed and its contents allowed to cool, and then
examined. If the powder has all disappeared, we term the mineral
"soluble;" if more or less is dissolved, "partly soluble;" if none,
"insoluble;" and if the contents of the tube are of a solid transparent
mass like jelly, "gelatinous;" while if transparent gelatinous flakes
are left, it is so termed. As this method of distinguishment is always
applicable, it is very important, and its detail and result should be
carefully noticed. Care should be taken that only a small portion of
the mineral is used, and also but little acid; the action should be
observed, and is frequently a characteristic, in the case with calcspar,
which effervesces while dissolving. The acid used is hydrochloric at
first, and then, if the mineral cannot he recognized, the same treatment
may be repeated using nitric acid. Both of these acids should be at hand
and two ounces are generally sufficient.

_Action of Heat_.--This is, perhaps, the most important characteristic,
and, when taken with the preceding data, will identify any of the
minerals found in any one locality, which I will describe, from each
other. The heat is applied to the mineral by means of a candle and
blowpipe. A thick wax candle answers well, and an ordinary japanned tin
blowpipe, costing twenty cents, will serve the purpose. The substance
to be examined is held on a loop of platinum wire about one inch to the
left and just below the top of the wick, which is bent toward it. Here
it is steadily held, as is shown in Fig. 1, and the flame of the candle
bent over upon it, and the heat intensified by blowing a steady and
strong current of air across it by means of the blowpipe held in the
mouth and supported by the right hand, whose elbow is resting upon the
table. The current of air is difficult to keep up by one unaccustomed to
the blowpipe, the skill of using which is readily obtained; it consists
in breathing through the nostrils, while the air is forced out by
pressure on the air held by the inflated cheeks, and not from the lungs.
This can be practiced while not using the blow-pipe, and may readily
be accomplished by one's keeping his cheeks distended with air and
breathing at the same time.

This heat is steadily applied until the splinter of mineral has been
kept at a high red heat for a sufficient length of time to convince one
of what it may do, as fuse or not, or on the edges. The first two
are evident, as when it fuses it runs into a globule; the last, by
inspecting it before and after the heating with a magnifying glass;
sometimes it froths up when heated, and is then said to "intumesce;" or,
if it flies to fragments, "decrepitates." Upon the first it is further
heated; but in the latter case, a new splinter of mineral must be broken
off from the mass and heated upon the wire very cautiously until quite
hot, when it may then be readily heated further without fear of loss.
For holding the splinter of mineral, which should well represent the
mass and be quite small, is a three-inch length of platinum wire of the
thickness of a cambric-needle; this may be bought for about ten cents at
the apparatus shop. The ends should be looped, as is shown in Fig. 2,
and the mineral placed in the loop.

Sometimes a mineral has to be fused with borax, as I mention further
on in my tables. This is done by heating the wire-loop to redness, and
plunging it into some borax; what adheres is fused upon it by heating.
Some more is accumulated in the same manner, until the loop is filled
with a fair-sized globule. A small quantity of the mineral, which had
been crushed as for the acid test, is caused to adhere to it while it is
molten, and then the heat of the blast directed upon it for some time
until either the small fragments of mineral dissolve, or positively
refuse to do so. After cooling, the aspect of the globule is noticed as
to color, transparency, etc. Care must be taken that too large an amount
of the mineral is not taken, a very minute amount being sufficient.

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