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in the same way Daguerre's sensitive preparation absorbs all
the rays which have any chemical action on it, and reflects
the yellow only, which does not afiect it. In this particu*
lar lies the secret of its vast sensitiveness, compaitd with
the comqion preparations of the chloride and bromide of

(42.) 2nd. That as a body warmed Ijf the rmft qfthe sm^ &c.
After a beam of light has made its impression on the
iodide, if the plate be laid aside in the dark before mercu*
rialistng, that impression decays away with more or less
rapidity; first the faint lights disappear, then those that are

Having brought three plates to the same condition of iodi*
zation, and received the image of a Ms-flame in the camera
on each for three minutes, I mercurialised one, A, forthwith ;
theseccmd, B, I kept an hour, the third, C forty«>eight hours;
the relative appearance of these three images is represented
in fig. 2.

(43.) Those who are in the habit of taking DaguerreOi>
types, know how much they suffer when the process of mer*
curialixation is deferred. To show this effect in the extreme,
I took four plates, and having prepared all alike, I exposed
half of the surface of each to a bright sky for eight seconds.

Np. 1. inercurialized immediately, c^mc out« black solarised*

2« .«. ••• ... in fi?e hours, .,. ... white.

3 twenty-two hours, ... same el^t.

4 one hundred and forty-four, no efiecl«

(44.) This last plate, on being submitted twice more to the
vapour of mercury, gave an indistinct mark. On exposing
a corner of it to the sun it blackened instantly, these results
showing that the peculiar condiiion brought on by the action


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S04 Dr. Draper on some Analogies between

of the light gradually disappears, the compound all the
time retaining its sensitiveness.

(45.) Similar results are mentioned by Daguerre in the
case of the changes produced on surfaces of resinous bodies,
and I have noticed them in a variety of other cases. Now to
whatever cause these phsenomena are due, whether to any
thing analogous to radiation, conduction, &c., it is most active
during the first moment after the light has exerted its agency,
but it must also take effect even at the very time of exposure;
and it is for these reasons that it comes to pass, that when
light of a double intensity is thrown upon a metallic plate
the time required to produce a given effect is less than one

(46.) I could conceive the intensity of a ray so adjusted,
that in [falling upon a given sensitive preparation, the loss
from this cause, tliis casting off of the active agent, should ex-
actly balance the primitive effect, and hence no observable
change result Hereafter we shall find, that one cause of the
non-sensitiveness of a number of bodies is to be traced di-
rectly to the circumstance, that they yield up these rays as
fast as they receive them.

(47.) It needs no other observation than a critical exami-
nation of the sharp lines of a Daguerreotype proof with a
magnifying glass, to show that the influence of the chemical
rays is not propagated laterally on the yellow iodide of silver.
Of the manifestations which these rays may exhibit, after they
have lost their radiant form and become absorbed, we know
but little. If they conform to the analogous laws for heat,
and if the absorbing action of bodies for this agent is in-
versely as their conducting power, we perceive at once wky a
photographic effect, produced on yellow iodide of silver, re-
tains the utmost sharpness without any lateral spreading; the
absorbing power is almost perfect, the conducting should
therefore be zero.

(48.) 3rd. TTiatj as when rays of heat fall on a mass ofccid
ice^ &c. Sec.

Although in the sun the iodide of silver blackens at once,
this is only the result of a series of preliminary operations.

When we look at a Daguerreotype, we are struck with the
remarkable gradation of tint, and we natflrally infer that
the amount of whitening induced by mercurialization, is in
direct proportion to the amount of incident light ; otherwise
it would hardly seem that the gradation of tones could be so

(49.) But in truth it is not so. When the rays begin to act
on it, the iodide commences changing, and is capable of be-


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the Chemicdl Bays and those of Radiant Heat. 200

ing; whitened by mercury. Step by step this process goes on^
an increased wliiteness resulting from the prolonged action
or increased brilliancy of the light, until a certain point is
gained, and now the iodide of silver apparently undergoes
no further visible change; but another point being gained, it
begins to assume, when mercurialized, a pale blue tint, be-
coming deeper and deeper, until it at last assumes the bril-
liant blue of a watch-spring. This incipient blueness goes
under the technical name of solarization.

(50.) The successful practice of the art of Daguerreo-
typing, therefore, depends on limiting the action of the sun-
ray to the first moments of change in the iodide ; for if the
exposure be continued too long, the high lights become sta-
tionary, whilst the shadows increase unduly in whiteness, and
all this happens long before solarization sets in.

(51.) Let us examine this important phsenomenon more
minutely. Having carefully cleaned and iodized a silver plate
three inches by four in size, it is to be kept in the dark an
hour or two.

By a suitable set of tin-foil screens, rectangular portions of
its surface, half an inch by one-eighth, are to be exposed at a
constant distance to the rays of an Argand gas-burner (the
one I have used is a common twelve-holed burner), the first
portion being exposed fifteen seconds, the second thirty se-
conds, the third forty-five seconds, the fourth sixty seconds,
&c. &C.

We have thus a series of discs or spaces upon the plate.

a, i, c, dj fig. S, each of which has been affected by known
quantities of Tight; b being affected twice as much as a^ having
received a double quantity of light; c thrice as much as a.

having received a triple quantity, &c. &c.

The plate now is exposed to the vapour of mercury at
170^ Fahr. for ten minutes ; the spaces or discs all come out
in their proper order, and nothing remains but to remove the

(52.) An examination of one of these plates thus prepared,
shows us^ that, commencing with the first space a, we dis-
cover a gradual increase of whitening effect until we reach
the seventh ; that a perfect whiteness is there attained ; that,
passing on to the sixteenth, no increase of whitening is to be
perceived, although the quantities of light that have been in-

* It is impossible to represent these changes in a drawing, which is
simply black and white ; it will be understood that the characteristic di-
stinction of the spaces from the sixteenth to the twentieth, for example,
depends on their aasuming a blue tint, which continually deepens in inten-


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906 Dr. Draper on iome Analogies beimem

cidentand absorbed, have been continoallv increasing ; but
as soon as tlie light thus latent has reached a certain qnan-
tity, risible decomposition sets in^ indicated by a blueness, and
the sensitive surface once more renders evident the increments
of incident light

(53.) Or, by presenting a plate covered with a screen to a sky
that is clear or uniformly obscured, and with a regular motion,
withdrawing the screen deliberately firom one end to the otlier,
and then suddenly screening the whole ; it is plain that those
parts first uncovered will have received the greatest quantity
of light, and the others less and less. On mercurialixing, it
will be seen that a stain will be evolved on the plate, as is
i^epresented in fig. 5 ; from a to 6 the changes have been suc-
cessive ; from 6 to c no variation in the amount of whitening
is perceptible ; at d solarisation is commencing, which be-
comes deeper and deeper to the end, «, of the stain»

(64*. ) The plate from which the drawing of fig* 5 is taken,
gives from 6 to d ten parts, from bxoc seventeen parts, from d
to e twelve parts ; we perceive therefore how lar^ an amount
of light is absorbed, and its effects rendered latent, lietween
the maximum of whiteness being gained, and sdariffation
setting in,

(B5.) 4th, That it depends on the chi&mical nutute of the
ponderable material u>hat rays shall be absorbed.

I had prepared a number of observations in proof of ibis,
very much of the same kind as those which have some time
ago been published in the Phil. Trans, by Sir J. Herschei.
These refer chiefiy to the variable lengths of the stains^ im-
pressed by the prismatic solar spectrum on different chemical
bodies, and the points of maximum action notioed in them.
For the present I content myself with referring to that ex*
eel lent memoir for proofs substantiating this proposition.

(56.) 5th. That whilst the specific rays thus absortied de-
pend upon the chemical nature of the body, die absolute
amount is regulated by its optical qualities, such as depend
on the condition of its surfaces, and interior arrangement.

I took a polished silver plate, and having exposed it to the
vapour of iodine, found that it passed through the following
changes of colour :~lst, lemon yellow t 2nd, golden yellow :
Srd, reddish yellow : 4th, blue: 5th, lavender: 6th, metallic :
7th, yellow: 8th, reddish: 9th, green, 8cc. &c. the differ-
ences of colour being produced by the differences of thick-
ness in the film of iodide, and not by any difference of che-
mical quality*

(570 ^^ ^ ^ common remark, originally made by M* Da^
guerre, that of these different tints that marked S is the


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the Chemical Bays and those tf Radiant Heat. 807

most sensitive^ and photogenic draughtsmen generally suppose
that the others are less efficient from the circumstance of the
film of iodide being too thick. Some suppose^ indeed^ that
the first yellow alone is sensitive to light We shall see in a
few moments that this is very far from being the case.

(58*) Having brought nine different plates to the different
colours just inaicateO} I received on each the image of an
uniform gas-flame in the camera, treating all as nearly alike
as the case permitted. I readily found that in No* 1 there
was a well-marked action, No. 2 still stronger, but that the
rays had less and less influence down to No. 6, in which they
appeared to be almost without action ; but in No. 7 th^ had
recovered their original power, being as energetic as in No. 2,
and from that declining again ; this is shown m fig. 6.

(59.) Hence we see, thsLt the sensitiveness of Uie iodide of
silver is by no means constant; that it observes periodical
changes which depend on the optical qualities of the film^
and not on its chemical composition ; and that by bringing the
iodide into those circumstances that it reflects the blue rays
we greatly reduce its sensitiveness, and still more so when we
ai^ust its thickness so as to ffive it a gray metallic aspect;
But the moment we go beyond this, and restore by an in^
creased thickness its original cohrn^ we restore also its sensi"
tiveness. Here then, in this remarkable result, we again per^
oeive a corroboration of our first proposition.

(60.) I may, however, observe in passing, that although
I am describing these actions as if there was an actual ab-
sorption of the rays, and that films on metallic plates exhibit
colours, not through any mechanism like interference^ but
simply because they have the power of absorbing this or that
ray, there is no difficulty in translating these observations
into the language of that hypothesis. When the diffracted
fringes given by a hair or wire in a cone of diverging light
are received on these plates, corresponding marks are ob-
tained, a dark stripe occupying the place of a yellow fringe^
and a white that of a. blue. I found, more than four years ago^
that this held in the case of bromide of silver paper, and have
since verified in a more exact way with this French prepara-
tion. Similar phasnomena of interference may be exhioited
with the chloride of silver.

(61.) We have it therefore in our power to exalt or do*
press the sensitiveness of any compound, by changing its
optical conditions. Until now, it has been supposed that the
amount of change taking place in different oodies, by thtt
action of the rays of light, depended wholly on their chemical
oonstitutioB) and hence ccnnparisons have been instituted^ «•


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SOS Dr. Draper on sofke Analogies beiweeti

to the relative sensitiveness of the chlorides, bromides, oxides^
and iodides of silver, &c. But it seems this liability to change
depends also on other principles, which, being liable to varia-
tion, the sensitiveness of a given body varies with them. Thus
this very iodide of silver, when in a thin yellow film, is de-
composed by the feeblest rays of a taper, and even moon-
light acts with energy; yet simplv by altering the thickness
of its film it becomes sluggish, blackening even in the sun-
light tardily, and recovers its sensitiveness again on recover-
ing its yellow hue.

(62.) We have now no difficulty in understanding, how in
the preparation of ordinary sensitive paper great variations
ensue, by modifying the process slightly, and how even on
a sheet which is apparently washra uniformly over, lai^
blotches appear which are either inordinately sensitive, or not
sensitive at all. If, without altering the chemical composition
of a film on metallic silver, or even its mode of aj^^egation,
such striking changes result by difference of tkicJhiess, how
much more may we expect tliat the great changes in molecu-
lar condition, which apparently trivial causes must bring
about on sensitive paper, should elevate or depress its capa-
bility of being acted on by light 1 If I mistake not, it is upon
these principles that an explanation is to be given of the
successful modes of preparation which Mr. Talbot and Mr.
Hunt have described, and the action of the mordants of Sir
John Herschel.

(63.) I therefore infer,

6th. Thai the sensitiveness of any given preparationdepends
on its chemical nature^ and its optical qualities conjointly; and
that it is possible to exalt or diminish the sensitixieness of a
given compound^ by changing its optical relations.

(64.) 7th. TAn/, as when radiant heat falls on the surface of
an opake body, the number of rays reflected is the complement
of those that are absorbed^ so in the case of a sensitive prepara-
tion^ the number of* chemical rays reflected from the surface is
the complement of those that are absorbed*

This important proposition I prove in the following way : —
I take a plate, A 6, fig. 4, three inches by four, and by
partially screening its surface whilst in the act of iodizing,
with a proper piece of flat glass, I produce upon it five trans-
verse bands, 6, c, ^, e^f; the fifth, f which has been longest
exposed, is of a pale lavender colour; the fourth a bright
blue ; the third a red ; the second a golden yellow ; and the
first uniodized metal ; the object of this arrangement being
to expose at the same time and on the same plate, a series of
£ilms of different colours and of different thicknessi and to


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the Chemical Bays and those of Badiant Heat. 209

examine the action of the rays impinging on them, and the
rays reflected by them.

Having prepared a second plate, B, and iodized it uniformly
to a yellow, I deposit it in the camera, and now placing the
first plate, A G, so that the rays coming on it through the
window from the sky shall be specularly reflected to the ob«
ject«glass of the camera, and the image of A G form upon B,
I allow the exposure to continue until the yellow of A G is
beginning to turn brown ; then I shut the camera and mer-
curialize both plates.

In consequence of what has been said {B%.)j it will be readily
understood, that of the bands on A G, the first one, which is
the bare metal, does not whiten in the mercury vapour ; the
second, which is yellow, mercurializes powerfully ; the third,
which is red, is less aSected ; the fourth, which is blue, still
less; and the fifth, which is lavender, hardly perceptible.

But the changes on B, which have been Brought about by
the rays reflected from A G, are precisely the converse ; the
band, which is the image of 6, is mercurialized powerfuHv;
that of c is untouched and absolutely black, d faintly
stained, e whitened, and f mercurialized, but little less
than b.

{65.) It follows from this,, that a white stripe on B corre-
sponds to a black one on A G, and the converse : and for the
depth of tint of the intermediate stripes those of the one are
perfecdy complementary to the corresponding ones of the

By the aid of these results, we are now able to ^ive an ac-
count of the variability of sensitiveness in photogenic prepara-
tions ; the yellow iodide of silver is excessively sensitive, be*
cause it absorbs all tlie chemical rays that can disturb it,
whilst the lavender is insensitive, because it reflects them.
Under this point of view, sensitiveness therefore is directly as
absorption and inversely as reflexion.

The superiority of Daguerre's preparation over common
sensitive paper may now be readily understood. It absorbs
all the rays that can afiect it, but the chloride of silver, spread
upon paper, reflects many of the active rays. The former,
when placed in the camera, eives rise to no reflexions that
can be imurious; the latter fills it with active light, and stains
the prooi all over. Hence the Daguerreotype has a sharp-
ness and mathematical accuracy about its lines, and a depth
in its shadows, which is unapproachable by the other. More-
over, the transiucency of the white chloride of silver, as well
as its high reflecting power, permits of particles lying out of

Phil Mag. & S, YoU 19. No. 123. Sept. 18*1. ?


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210 Professors Redtenbacher and Liebig

the lines of lifffat being affected, the luminous material be-
coming diffused in the paper.

The &C1, therefore, that a given compound remains un-
changed even in the direct rays of the sun, is nci proof that
light cannot decompose it ; it may reflect or transmit the ac-
tive rays as fast as it receives them. It results from this, that
optical forces can control and even check the play of chemical
affinities. Whilst thus it appears that there are points of
analogy between this chemical agent and radiant beat, we
must not too hastily infer that the Taws which regulate the one
obtain exclusively also with the other* As is well known,
there are striking analogies between radiant heat and light,
but there are also points of difference, the convertibility of
heat of one degree of reirangibility to another does not oc-
cur with light; there are also dissimilitudes in the plueno-
mena of radiation and its conse<]uences. I do not doubt, that
what has been communicated in this memoir, will, by the re-
searches of others, be greatly extended ; but it is not to be ex-
pected that a complete parallel can .be run between radiant
heat and the chemical ra3rs, any more than between radiant
hei^ and light.

From the phaenomena of the interference of these raya, of
^e sensitiveness or non-aensitiveneas of the utme chemical
compound being determined merely bv the fact of its thick*
ness or thinness, these, and many otner similar results, ob^
viousfy dependfM upon meehatiical principles, it seems to me
that very powerful evidence may be drawn against the nui-
teriality of light, and its entering into chemical union with
ponderable atoms. Those pbilosopiiers who have endea-
voured to prove the ondulatory theory, will probably find in
studying these subjects cogent evidence in fiivour of their

XXXI. On the Atomic Weight qf Cathoiu By Professors
Redtenbachee of Prague^ and Liebig ofCiessen^.

IN the analysis by oonbostion of organic stdistances wbidi
contain carbon and hydrogen, the obaervitfioa has (re-
qoentiy been made of late yean, that the weisht of the ele-
ments separately found by experiment, actually exceeds the

• TVaiwUi^ from ths Dfiginal GersMa by Dr. J. H. Gilbeii; aad coo-
imioJCBted by tbe CJi^mical twcieiVr hsviiig beeo read before the Society.
May 8th, 1841.

An abstract of Dr. March&nd's paper on the same tubject will be tofigitA
la the Proceedings of tbeSedetj, to be gt^ea fo ovr oest Nambsr,


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m the Mcmk Weight qf Carbon. SI 1

original weighi of the malUr «iibipitled to combustion. In
the analyses we po^^ew of napbtbalin by Mit««ber]icb» by
DomB«, and by Woskre^en^kyf this is particularly remarkable.
One hundred parte of napbtb^UD gave lo Mitocberlieh,^

1. «.

Carbon ............. 9^9^ 94-440

Hydrogen •••••!••• 0*M 0*225

loo-eo loo'sea

One hundred parts of naphthalin gave to Dumofff-*

J. 9. 3t 4, 6,

Carbon ,M 94^2 84*22 94*27 94*9 94*9
Hydroflan 6*3 6*9Q 6*26 6'2 6*1

JOo7 100*52 100*63 mi lOVO

And to Woskresensky; 100 parts of the same substance

h 2. 3, 4. 6. 6r

Carbon 04*623 94*598 950398 93*668 84*395 94-494
Hydrogen 6*529 6-389 5-3830 6'142 6306 6-526

101-158 100-897 100*4098 99810 lOpeOl 101-080

This contta^t ofi^rrmca in »o many carefully conducted
wperiment^f indicatiM a common «wrce of error upon which
it ia dependant I it can only be attributed to two causes*
One of tbeae may be «ough( m the d^fbct^ of H^ nMbod of
Analv9ii^ t|ie other in the fuppo^Hion that the prodnctv of the
6ombu9tion (wAter and carbooip Acid) have d)&rent campor
eitione from those usually a^^igned tg them. If indeed either
wAter er eerbonie aeid cpntaioi iom<9wbat lass pf bydr(Mien
or pf oarbon iban we at present pyppose, th#n as we i^aknlate
ihm the qnantities found of the fprm$r bodies, tb# «xpess in
ibe analyses is diminj»bed in (be sam^ proportion.

Let U8 suppose, for example, that carbonic acid contains

only 76 oarbm i&staad of 76'*37 to 300 4^ oKygaut and wAter
only IS bydrnpn instead of Vi'^196 U> \0Q OKygePr and
then we sbaU beve no access in any of the analyses quoted^
whilst the wperimmtal come to agree perfectly with the icaU
eolaled results- Are we then entitled to make such changes
in tbe atomie weii^tis, pro<:eeding as we do upon the asisiuned
•cevraqr of experiment^ whiph« from the complex nature of
tliQ apfNuratuSi can mik^ no eiaim to absoluto precisioni or



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212 Professors tledtenbacher and Liebig

ought we not first to compare these with other experiments^
in which this source of error is entirely avoided ?

A question also arises, whether napluhalin^ a substance the
atomic weight of which cannot be determined with certainlyi
as it enters undecomposed into no combination, is a proper
substance to select as the means of determining the atomic
weight of carbon or of hydrogen ? That body must indeed
be rejected on this account, for it is not in our power to con-
trol our analytical results from a knowledee of the weight of
its atom, that is, the sum of the atomic weignts of the elements
composing it.

When we also consider, that the naphthalin, which in the
above experiments was submitted to combustion in a glass
tube with oxide of copper, is a volatile body, that it cannot
be introduced into the combustion tube, with oxide of copper
that is absolutely free from moisture; and bear in mind also,
that, owing to the volatilitv of the substance, this mobture
cannot previously to combustion be removed by means of
exhaustion, we cannot doubt the existence of a source of error,
which must increase the per-centage of hydrogen beyond that
which actually existed in the substance ; for, however small
the quantity of this hygroscopic moisture may be^ it is never-
theless always present; it is weighed with the chloride of
calcium tube, and its hydrogen added to that contained in
the substance.

In all analyses hitherto conducted^ even those in which
the whole of the hygroscopic water had been removed as
nearly as possible before combustion, by means of exhaustion,
it is observed that the experiment invariably gives rather
more hydrogen than is indicated by calculation. This excess
amounts in good analyses to from 0*1 to 0*2 per cent. It is
found, however^ that this, in reference to the quantity of sub-
stance employed in analysis, is not sufScient to affect the pro-
portion of the elements, to the extent observed in the analyses
of naphthalin; the excess in those analyses is however dimi-

Online LibraryW. Franklin (Wallace Franklin) JonesPhilosophical magazine → online text (page 86 of 131)