Rodolfo Amedeo Lanciani.

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This secona portion, or that coming from tiie "spot," will have
commonly a greater or lesser d^ree of intensity than that re-
flected from the adjacent regions of the plate, so that we shall
nave either a briffht spot on a dark ground, or a dark spot on a
light ground, and an attempt to equalize the two illuminations,
so as to effect the disappearance of the " spot," will be success-
ful only under the following conditions :

First All portions of the "spot" must be equally illumin-
ated, and it must have no texture, that is, must not perceptibly
coninst of minute grains mingled with others having a less or
greater degree of brilliancy.

^ ♦ Fogg. Axmalen, Ba&d cxIt, p. 145. f Vol xxxri, Julj, 1863.



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0. K Bood on Photameinc Mcperiments. 147

Second, All those portioDs of the ground sarroundinK and
in contact with the "spot," must be equally illuminated, and
the texture of the ground must likewise be uniform.

Third, The transition fix)m the ground to the " spot " must
be perfectly sharp and sudden, so that not the fiiintest border
can be seen surrounding it

Fourih, K it be required to render the invisibility of the
" spot " more than momentary, it is equally essential that the
ratio subsisting between the two sources of illumination shoxild
be laiily constant

If either of the first three precautions be neglected, the dis-
appearance of the spot' becomes entirely impossible, while it is
only by a peculiar arrangement of apparatus that the fourth can
actually be realized. (See second part of this article.)

Screen. — ^A plate of colorless ^lass of ffood quality is coated
with photographic collodion and immersed for a few minutes in
a solution of nitrate of silver, (" bath,") as though it was the in-
tention to take a picture, (" negative ") ; it is then exposed for a
minute or less to ordinary dayli^t, and a solution of photo-sul-

Shate of iron poured over it This produces a dense opaxjue
eposit of silver in the substance of the film, when the plate is to
be washed well in plain water and dried. Its surface will be of
a grey tint, and will vary somewhat in its power of diffuse re-
flection according to the sample of collodion used. A small
amount of light is also regularly reflected by the upper surface
of the collodion film, and in using the plate it is always so placed
that this latter portion shall produce no effect on the result The
collodion film is now removed neatiy, by the aid of a needle,
fixnn a portion of the plate, so that a square with sides ^^ of an
inch is laid bare, care being taken to leave the edees clean and
well defined, in which there is no particular difficmty, provided
tiie collodion was originally of the proper quality to cause it to
adhere welL K the "spot" is made much smaller than the
above mentioned dimensions it becomes an annoying object for
observation, while if it be larger, it is difficult to illummate it

1.

A- » II ft j-



B c



N '



uniformly. This plate is seen at P, in figure 1, tiie collodion
side being turned toward the eye of the observer, and the other
side, except iust opposite the spot, covered by a coating of lamp-
black mixea with weak shellac varnish, so as to leave a black,
non-reflecting sur&ce. At the distance of an inch firom the
collodion plate, there is fia^stened^ parallel to it, a plate of colorless



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148 0. K Rood an Photometric Experimettk.

glass, G, finely ground on each side ; this is destined to receive
the light from L. If only one sur£eu^ of the glass be ground, the
texture becomes plainly visible, particularly when, as in the ex-
periments detailed in the second part of this paper, the spot is
magnified Only so much of the ground ^lass plate is allowed
to remain bare as is necessary, the remaining portions on both
sides are covered by black non-reflecting paper. Plates P and
G are then as it were roofed over, and enclosed on all sides by
a small well fitting blackened case, destined to prevent the light
fix>m penetrating oetween them, and finally, to guard against
possible errors by reflection fix)m the walls of the room, plate
G is provided with a projecting blackened tube which cuts off
stray reflected light P and G, constituting thus the screen, are
fSastened at the end of a pair of long parallel iron bars, B, B,
made like the ways of a lathe, and provided with a scale gradu-
ated in millimeters. The length of this iron frame is six feet
Source of illumination. — At E is the eye of the observer, the
fiwe being protected from light by the blackened screen, S. E,
the center of the " spot," and the center of the mirror experi-
mented on, all lie in the same line, which is of course at the
same time the axis of the instrument At H are two small gas
flames issuing firom circular apertures, and destined to illuminate
the collodion plate on the side next to the observer; both are
fed fix)m the same source. The gas-burner at H consists of two
thin brass tubes, half an inch m diameter and one inch long,
connected together bv a glass tube ; the circular apertures for
the flames are placed at equal distances on either side of the
spot, and as fsir from it as is found most advantageous in any
particular set of experiments ; their distance fix)m each other is
seven inches. The direct light fix>m the two flames is pre-
vented fix)m reaching the eye through the observing aperture,
by small blackened screens, the same means being employed to
arrest it in its course toward L. K instead of two, only a single
gas flame is employed at H, the ground around the spot will be
unequally illuminated, and exact observations become impossi-
ble. Of course the direct light fix)m these two burners which
penetrates through the "spot," must not be allowed to reach
that portion of the ground class opposite it; the distance of
the flames apart must be so cnosen that this becomes impossible.

The light fix)m the movable burner on the other side of the
screen at L will be used direct and reflected The smdl singk
movable burner which supplies it, is similar to those already
described, and to it is attached a flexible india-rubber tube,
which is supplied firom the same source that feeds H ; it is con-
nect^ by a wooden column, (for the sake of insulating the heati)
to the arm A, fig. 2.



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0. N. Bood on Phoiametrie Eeperimenis. 149

I pass now to the joint support of the arm, A, and of the
mirror to be experimented on. it consists of a block of brass,

1



B, four inches square (see figure 2, which is one-third of the
reed size^ ; its under surface is cut in such a way as to fit the
two parallel iron bars and to slide on them easily but steadily,
and iina foundation block is farther provided with a vernier, V ,
to read off the distances on the milhmeter scale. C is a gradu-
ated circle six inches in diameter, and is provided with a clamp
at cL The hollow massive cylinder, e, supports the arm A, and
also carries the axis of the support of the mirror. The screws,
1, 2, 3, serve to bring the mirror into its correct position ; it is
pressed against them^ a band of india-rubber attached to the
edges of the mirror. It will be seen that owing to this arrange-
ment, all the difierent parts have motions quite independent,
and yet by the clamping screws can at any moment be connected.
Finally, attached to this stand is a long light rod of wood, R,
reaching to the observer, and enabling him, by varying the dis-
tance between this movable piece and the fixed screen, to effect
compensation.

Fleocible Oas tubes. — It occurred to me that by splitting a
stream of gas and sending one portion to L, fig. 1, the other to
H, it would be possible to secure a uniform ratio between the
two illuminations, as it would seem that any cause which in-
creased the pressure in one of the branches of the tube ouj^ht
to be equally operative in the other, so that after a compensation
had been effected it should be permanent In practice this was
not found to be the case; the compensation point gradually
shifted its position in the course of the evening away from H,
sometimes to the extent of 50 or 60 millemeters. This difficulty
was remedied by a suggestion from Mr. Lewis Rutherfurd, who
suspected that the trouble came fix)m the unequal length of the



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150 0. N. Rood on Photometric ExperimerUs.

two branches of the supply tubing, in point of fact, on giving
them equal lengths ana diameters, and making the compensa-
tions slowly, so that the branch attached to L was not much
shaken or set in oscillation, this source of error became so
greatly reduced as in no way to interfere with the observations.

Mode of measuring the am/mnt of light reflected from a mir-
ror. — ^To adjust the apparatus, it is nrst necessary that that di*
ameter of the circle joming the 0*^ and 180*^ points, should be
made parallel to the axis of the instrument, which is eflfected
by the use of the vernier at X, fig. 2, the circle is then clamped
Next, the vernier attached to the mirror is placed at 90°, and
the mirror itself is made to assume a position at right angles
to the axis of the instrument, by the aid of its three screws,
and a small gas flame placed on a support which rests on the
parallel bars at a distance of two or three feet from the mirror,
it being so contrived that the center of this small flame shall
just lie in the axis of the instrument The mirror is adjusted
till it reflects the light of this small flame back to the eye
through the flame itself, securing thus the coUimation with the
desired accuracy. The arm A is then set at any desired angle
with the mirror, and the two clamped together, when it is easy
to arrange matters so that the reflected light shall be sent from
the mirror along the axis of the instrument to the fixed screen
at G. The small shade at S, fig. 2, prevents the direct light
from reaching the same destination. After a compensation has
been eflfected with reflected light, the arm A is made parallel
with the axis of the photometer by the aid of its vernier, and
another compensation is made with the direct light, the small
shade being now placed behind the flame so as to be out of the
way and to prevent light fix)m reaching the mirror.

Mode of registering t^te observations, — ^These were always ro-
istered with a sharp point on a slender fillet of paper, attach^
to the long wooden rod R T, fig. 9, used for moving the mirror.
This point was one end of a small lever placed at N, fig. L
In conseauence of this, at the end of a set of experiments two
groups 01 dots were found on the paper, admitting of the most
exact measurement on the following day. Before removing
the paper from the rod two dots were always made on it in the
neighborhood of these groups, the corresponding positions of
the vernier V, fig. 2, being at the same time noted A glass
slide, ruled with millimeter lines, was then to be placed over the
detached fillet of paper, and the observations recorded in the
note-book. In the determinations given below, the observa-
tions on the direct and reflected light were made alternate, so
as to avoid errors due to the shifting of the compensation point



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0, N. Rood en Photamefyie Experiments.



151



Observations on the amount of light reflected by a glass plate
silvered by Liebig's process, the suver side being iifl<Bd. The
light was reflected at an angle of 45^



nitUmee when
mliTor wMOMd.

946

949

950

951

952

955

956

957

958

961

961

962

965

966

967

967



DtoUnee Willi
free flame.

1297
1301
1802
1802
1804
1305
1806
1808
1309
1309
1310
1812
1312
1315
1318



16)15828

957-68
Correction 191*



1148-68



15)19610

1307-8
105-

1202-8



Result 91-26 per cent reflected.



DliUnee when
mirror wee need.

936

940

941

942

943

944

946

946

947

948

949

950

952

958

954

955

956

958

959

960

961

962

965



23)21867



Correction



960-7
191-



1141-7



DIeUBcewith
freefleme.

1286

1287

1288

1288

1290

1292

1292

1293

1295

1296

1297

1298

1298

13ol

1303

1303

1304

1305

1307

1307

1307

1308

1312

1812

1312

1815



26)33796



1299-8
Ccrrectloo 105-



1194-8

Result 91*8 per cent reflected.

These flgures are taken of course directly from the note-
book, and in making an examination of them it is to be re-
membered that the corrections, 191 and 105, (for false positions
of the vernier), are to be applied to each of the distances in the
respective columns, before a correct judgment of the closeness
of mdividual observations can be made.



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162



0. Ni Hood on Pfiotameirie Mcperiments.



When the same mirror was used at 5^, i a, the light being
reflected &om it nearly perpendioularly, the results given below
were obtained.



2.



Dlttonoe
■Irroriri

} 955

958

958

958

960

962

962

963

963

965

965

969

970

971

972

973

977

977

980

983

983

21)20324

967-8
191

1158-8



DIttMioewlth
free flame.

1297
1304
1304
1305
1305
1307
1308
1310
1310
1313
1313
1313
1314
1314
1315
1318
1318
1328



18)23596

1310-8
105

1205-8



Dtotanoe when
mirror wee wed.

954

955

956

957

957

958

958

961

961

962

962

963

963

964

965

967

968

969

972



19)18272

961-6
191-



1298
1301
1302
1303
1303
1304
1305
1306
1307
1308
1308
1309
1310
1312
1312
1313
1315
1316

18)23532

1307-4
105



1152-6



1202*4



Result, 91 '88 per cent reflected.

Result, 92*35 per cent reflected.

In the second part of this paper I shall detail the results of
several sets of experiments made with a truly constant illumin-
ation, in other respects the mode of experimenting being quite
similar to that used in obtaining the above results, when it will
be seen that about two-thirds of the scattering in the figures was
due to the shifting of the compensation point during the experi-
ments, and not to any defect m the screen.

In the same way experiments were made on another mirror
sUvered by Liebig's process, the glass side being used, with the
result that out of a hundred rays 78*01 were reflected, the angle
being 45^; while an amalgam mirror tested at the same angle
reflected only 44 68 per cent



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T. & Hunt on the Chemiairy of Copper. 168



Art. XVHL — Contributions to ^ Chemistry of Copper; by
T. Stebry Hunt, LLD., F.RS. Part 1.

[Read before the Ameiieao Aaeodatioii for the AdvanoemeDt of Science at Suleni,

August 26, 1869]

§ 1. The reaemblances between silver and copper in its ca-
prous form have already attracted the attention of chemists.
The ordinary chlorid of silver (argentic chlorid) and the dichlo-
rid of copper (cuprous chlorid) have many properties in com-
mon. Both of these chlorids are white, readily fusible, and
blackened by exposure to light ; both of them are insoluble in
water but mssolve in ammonia and in aqueous solutions of
other chlorids, in which however the cuprous is far more solu-
ble than the argentic chlorid. A saturated solution of chlorid
of sodium holds at 90^ Centigrade, IB^O per cent of cuprous
chlorid, at 40*" C, 11 7, and at 11"", 8*9 per cent A solution
containing fifteen per cent of chlorid of sodium retains at 90^
C, 10*3 per cent of cuprous chlorid, at 40°, 6*0 per cent, and
at 14**, 8 '6 per cent; wnile a solution with only five per cent
of chlorid of sodium holds of the cuprous chlorid at 90®, 2*6,
and at 40** only 11 per cent These determinations are from
single observations and therefore require verification. From
the sparing solubility of the cuprous chlorid in dilute solutions
of cmorid of sodium it follows that the denser saturated solu-
tions are copiously precipitated by dilution with water, which
causes the separation of white cuprous chlorid in a crystalline
condition.

§ 2. The aqueous solutions of the chlorids of calcium, mag-
nesium, zinc, manganese, cobalt, ferrosum and cupricum, also
freely dissolve cuprous chlorid, and it is probable that this
property is shared by other soluble chlorida The strong af-
finity of cuprosum for chlorine enables cuprous oxyd to decom-
pose all the chlorids just named, with the exception of those of
sodium and calcium, with separation of the corresponding oxyds
and formation of cuprous chlorid. In the case of zmc and
manganese, insoluble oxychlorids of these metals are formed
at the same time. These reactions require farther study, and
the same may be said of the cupric and cobaltic chlorids with
cuprous oxyd. I have, however, partially investigated the be-
havior of cuprous (Jxyd with magnesic and ferrous chlorids,
and obtained the results about to be described.

§ 3. The cuprous oxyd for these experiments was prepared
by gently heating a solution of sulphate of copper mixed with
cane sugar and an excess of caustic soda, until the whole of the
copper was thrown down as a bright dense cinnabar-red powder



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154 T. S, Sunt on the Chemistry of Copper.

which was carefully washed and dried. A concentrated solu-
tion of chlorid of magnesium dissolves this oxyd in the cold,
and more readily when heated, with separation of hydrated
oxyd of magnesium and cuprous chlorid, which latter is held
in solution by the excess of magnesic chlorid. By filtering
the liquid while hot, and washing with a strong solution of
chlorid of sodium, the hydrate of magnesia may be separated,
and the dissolved copper subsequently precipitated by metallic
iron from the colorless filtrate, ferrous chlorid being formed.
Experiment shows that the reaction between the red oxyd of
copper and chlorid of magnesium may be represented as follows ;
Cu,0+MgCl=Cu2CH-MgO.

§ 4. A solution of magnesian chlorid nearly saturated when
hot with cuprous oxyd, and allowed to cool in contact with the
precipitated magnesian hydrate, deposits a j)ortion of orange
colored oxyd, or perhaps an oxychiorid, which disappears as
often as the solution is heated. The solid cuprous chlorid is
moreover decomposed when digested with water and magnesia,
hydrated cuprous oxyd and magnesic chlorid being formed.
Trtie double chlorid of cuprosum and magnesium is however
stable, even in the cold, in presence of magnesian hydrate, pro-
vided a considerable excess of magnesian chlorid be present
From a filtered solution of cuprous oxyd in chlorid of magne-
sium water precipitates a large portion of the cuprous chlorid,
in this case colored orange-yellow from adhering oxyd, due to
the reaction of a little magnesia, which remains dissolved or
suspended in the concentrated solution even after filtration. A
solution of magnesian chlorid of specific gravity 1*23, retains
in solution at 12® Centigrade, about 710 per cent of cuprous
chlorid. A solution of magnesian sulphate with chlorid of
sodium may be employed to dissolve cuprous oxyd. This, like
all similar solutions of cuprous chlorid, rapidly absorbs oxygen
from the air and deposits a pale green cupric oxychiorid.

§ 5. With ferrous chlorid and cuprous oxyd it might be ex-
pected, from analogy with the magnesian salt, that we should
obtain cuprous chlorid and ferrous oxyd, but the reaction is
complicated by the tendency of the latter to pass to the state
of ferric oxya When ferrous chlorid in solution with chlorid
of sodium is heated with a sufficient quantity of cuprous oxyd,
the whole of the iron is precipitated as ferric oxyd, mingled
with metallic copper, while cuprous chlorid remains in solution.
Experiments made with an excess of ferrous chlorid show that
one third of the copper is reduced, while two thirds are dis-
solved as dichlorid. This reduction may be effected directly by
ferrous oxyd ; if to a solution of cuprous chlorid in chlorid of
sodium, we add hydrated ferrous oxyd recently precipitated by
an alkaline base and still suspended in the liquid, it is at once



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T. S. Sunt on the Chemistry of Copper. 155

converted into ferric oxyd, with precipitation of metallic cop-
per. The first stage in the action of ferrous chlorid on cuprous
oxyd may be represented as similar to that of magnesic chlorid :
Cu,0 + FeCl= Cu,Cl + FeO. In the second stage CU2CI+
8FeO=Cu, -f FeCH-Fe,0 3. It follows from this that one-third
of the cuprous chlorid formed in the first stage is reduced to
the metallic state, and the final result may be represented as
follows: 3Cu,0+2FeCl=2Cu,CH-2Cu+Fe,0,.

A similar result is obtained if ferrous chlorid is added to an
unfiltered solution of cuprous oxyd in chlorid of magnesium.
The suspended hydrate of magjnesia in this case liberates an
equivalent of ferrous oxyd, which reduces to the metallic state
one-third of the dissolved cuprous chlorid, in accordance with
the second reaction given above.

§ 6. The reducing pjower of ferrous oxyd is also shown with
cupric chlorid, which is at once converted by it into cuprous
chlorid in accordance with the equation, 2CuCl + 8FeO =
Cu,Cl + FeCl+Fe,03. The further action of ferrous oxyd
will, as we have seen, reduce the cuprous chlorid to the metallic
state: in feet, 2CuCH-6FeO=2Cu+2FeCl+2Fe,0,. If re-
cently precipitated hydrated ferrous 0x3rd or ferrous carbonate
be added to a solution of cupric chlorid' in the proportions indi-
cated by the last equation, tne whole of the copper is separated
in the metallic state, mingled with ferric oxjd, while ferrous
chlorid is found in solution. The reaction with ferrous carbo-
nate, which requires a gentle heat, is accompanied by a violent
disengagement of carbonic acid gaa This experiment is best
made by dissolving in water ferrous sulphate and sodic carbo-
nate or sodic hydrate in the proportions required, and adding
thereto a solution holding the proper amount of cupric chlorid.
Under certain conditions the cuprous precipitate is brownish-
black in color, like that obtained by neatmg ferrous chlorid
with cuprous oxyd, but more genendly it is of a bright red
color, and often coats the glass with a mirror-like film. A
warm solution of cupric chlorid with chlorid of sodium at once
convei-ts the metallic copper of the precipitate into cuprous
chlorid, which is dissolved, leaving behind only hydrated ferric
oxyd. When a solution of ferrous chlorid with chlorid of
ammonium and excess of ammonia is added to a solution
of a copper salt the precipitated films of metallic copper some-
times possess considerable brilliancy and show a bluisn translu-
cency. It is to be remarked that although the cupreous precipi-
tate thus obtained is bright red in color, that which is produced
by boiling cuprous oxyd with ferrous chlorid is nearly black.

§ 7. It was long since shown by Levol that hydrated ferrous
oxyd will reduce cupric to cuprous oxyd, and tnis, as we have
already seen, can separate fh)m its combinations ferrous oxyd,



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156 T. & Hunt on tlis Chemistry of Copper.

whose reducing power may be still further exerted upon the
cuprous combination thus formed. These facts serve to explain
the results obtained by E. Braun {Zeitschr. Chem,^ 1867, p. 568,
cited in Jahresbericht for 1867), which were not known to me
at the time of making these experiments. He found that by
digesting cupric hydrate or cupric carbonate with ferrous sul-

S)hate in solution tnere was obtamed a reddish mixture of basic
erric sulphate with cuprous oxyd, formed apparently in accord-
ance witn the equation,

2FeO, SO,+2CuO=Cu,0+Fe,03, 2S0,.
This, when boiled with a further portion of ferrous sulphate,
became black in color, and from the small amount of oxygen

Eent was supposed to contain metallic copper. By adding a
e excess oi carbonate of ammonia to a mixture of ferrous
cupric sulphates, Braun succeeded in obtaining solutions in
which all the copper was present in a cuprous form, and even
in reducing portions of it to the metallic state, a process which
we have seen is complete when the requisite amount of ferrous
oxvd is brought in contact with the chlorids of copper.

§8. In this Journal for March, 1867, page 308,1 described
briefly the reaction between cupric oxyd and ferrous chlorid, ac-
cording to the equation, 8CuO+2FCl = FejOj+CuaCl+CuCL
I was not then aware that the same had been shown by Meyer
(Berg, und Hutt Zeit, 1862, 182, cited by Kerl).* Further
studies of this reaction have riven me interesting results. The
black oxyd of copper, even after ignition, is attacked by ferrous
chlorid in the cold, but the insolubility of the resulting cuprous



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