John Almon.

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' • • • •







The second of the above series of ratios (that of the differ-
ences in the arcs of the sun's zenith-distance) is based upon the
following estimate of the average monthly increase of solar alti-
tude at dil places in the temperate zones.

Mo8. from winter Mtetlce, 12 3 4 5 6

Increase of solar altitude, 8i* 12^ 28i'' 86<» 48f* 47^^
Ratio of increase, -069 ^266 -600 -746 •981 1*000

If we allow about 24 days for the cumulative effects of increas-
ing heat and cold, these ratios become properly comparable with
the monthlv ratios of temperature-variation, as in the following
table, which is compiled from the works of Dove and Guyot.

DiS. of time, mos.






Ratios of sines,






« " arcs.






K. Hemisphere,






S. Hemisphere,






Arctic region,
























S. America,


















Digitized by *



70 P. E. Chase on Brewiier^s Neutral Paint.

An extensive series of compiriflons* seems to warrant the
following inferences, all of which are confirmed by other oon-

1. Taking into view the entire land surface of the globe and
the entire range of the year, the direct heat of the sun and the
induced aerial currents appear to be about equally instrumental
in determining fluctuations of temperature.

2. The influence of the winds is most marked in the Northern
and Western hemispheres; that of solar obliquity, in the South-
em and Eastern hemispheres.

^. Where the sun's rays are least intense (as in the Polar Be-
gions) and where the winds are most variable, the ratios exhibit
the nearest parallelism to the increments of arc ; but where th»
winds are most uniform (in and near the region of monsoons),
they correspond more closely with the sinal increments.

4. The general changes of temperature at midwinter, and at
the equinoctial seasons (when the sun's declination is changing
most rapidly), are most dependent upon the local solar heat; the
midsummer changes are more subject to the influence of the

5. The greatest conflict of opposing forces occurs during the
sun's passage between the comparatively wind-governed North-
ern hemisphere and the sun-governed Southern hemisphere.
This conflict is manifested in the spring and autumn rains.

6. The closest and most general approximation of ratios is
shown in the monthly temperature change at midsummer, which
corresponds almost precisely with the change of arc.

2. On Brewster^s Neutral Point.

In the April number of the Philosophical Magazine, Sir David
Brewster says: "Dr. Eubenson has never been able to see, even
under the fine sky of Italy, the neutral point which I discovered
under the sun, and which, I believe, has never been seen by any
other observer than Mr. Babinet."

The point is question can be easily seen in Philadelphia on
any clear day, when the sun is more than 20^ above the horizon,
and I have reason to believe that it can be found with equal ease
at many other places in the United States, although I have not
been able to find any published observations except my own.f

As all the phenomena of skylight polarization are very inter-
esting, and as some of its laws are still imperfectly understood,
others may, perhaps, be induced to turn their attention in this
direction, so as to determine whether the difficulty experienced
by European observers is owing to a higher latitude, to a moister
atmosphere, or to some other cause.

* Sea PMOMdmn, Ao, loc dt

t Proft Attor. FKiL Socl, tqL z; this Journal, yoL zlii, ppu 111, 112.

Digitized by VjOOQIC


M. C. Lea on a Theory of PkoUhchemisiry. 71

A simple Savart polariaoope is saf&cient for making the obser-
yatioDS. In positing Brewster's neutral point, I have usually
raised the lower sash of an attic window so that the bottom of
the sash will screen the sun from the polarisoope. I have thus
been able, in every instance when the atmospheric conditions
seemed favorable, to see very distinctly the neutral pointy and
the oppositely polarized bands above and below.

Abt. IX. — OontrAutums toward a Theory of PhoUhchemistry ; by
M. Carsy Lea.

In a somewhat extended series of experiments published at
various times,* I endeavored to fix, as far as I was able, some
of the facts of photo-chemistry, and more especially the nature
of the action of light upon iodid of silver, at once the most im*
portant and the most aifficult of explanation of all the facts of
photo-chemistry which fall under our notice. The phenomena
exhibited by iodid of silver, in the point of view which they
assume to me, are the key to the whole matter, and based upon
them, I propose to offer some theoretical views upon the general

The study of light has always been largely aided by analogi-
cal reasoning from another source — that of sound, whose phe-
nomena probably afforded the first conception of the undulatory
theory, and in turn, discoveries made in light have aided our
knowledge of the phenomena of heat, many of which would
perhaps have been still unknown but for the aid so obtained,
it is therefore perfectly allowable to reason analogically back
from heat to light

The tendency of heat is always to equalize itself, by radiation
and conduction. The loss of heat in this way where the body
affected is much above the temperature of those that surround
it is enormously rapid, and this loss continues with diminishing
rapidity till an equilibrium is attained.

The same is the case with light, though the loss is there usu-
ally so much more rapid as to be almost simultaneous with the
reception, to our senses, and in the ordinary conditions of ob-
servation it is quite so. But the exceptions are perfectly well
marked. The phenomena of phosphoiisoence show that a body
may retain the impression of light for a considerable time. And
the phenomena of phosphorescence received an immense exten-
sion from the ingenious and b^utiful experiments of Becquerel,

* A brief r^umd of suuij' of tliese experimeots wu pablished in thif Journal
in ih« year 186S.

Digitized by


72 JIf . C. Lml on a Theory of Photo^hemiitry.

who showed that a very large nomber of bodies continued to
emit light for an appreciable time after the direct influence of
light ceased to operate upon them. Although the time might
be but a ver^ small fraction of a second, stUl it was rendered
brilliaDtlv evident to the sense, and the exact period oould be
measured. And when we consider the enormous rapidity with
which the phenomena of light take place, even the fraction of
a second is a long time, and it would oe exceedingly rash to at-
tempt to limit such phenomena to our powers of observation.

Just as with heat there exists in all probabilitv an absolute
zero at which heat- vibrations cease, so probably there is a light
zero at which the body ceases to vibrate luminously. }£xt
bodies ^to our perception) reach this zero immediately when
earned into darkness. Phosphorescent bodies^ form, however,
a striking exception.

Let us suppose that a body be surrounded bjr other bodies
equally illuminated, and that temporarily an additional quantity
of light falls upon it. On the cessation of this illumination,
the body will recover its condition of equilibrium with surround*
ing homes, by losing its excess of light in the following manner:
1. By reflection. 2. By transmission. 3. By conversion into
heat. 4. Bv chemical action. 5. By radiation.

That is, the body, if it be transparent, or have reflecting sur-
&oes, will part with a certain quantity of its light in those wajs.
If it is susceptible of chemical decomposition, a certain portion
of light will DC consumed in effecting that decomposition. And
what farther loss is necessary to take place in order to reach an
equilibrium, must take place by conversion into heat and by ra-
diation. As we have already seen, this radiation may be either
instantaneous, as in the case of most bodies, or it mav require
minutes, hours, or even days, as in the case of phosphorescent
bodies. This fact is of the utmost importance m the attempt
I here make to explain the phenomena of photochemistry.

In their influence upon combustion and decomposition, the
phenomena of light and heat exhibit a striking parallelism,
liach tends in some cases, to promote combination, but in the
vast majority of cases, to dissociate elements already combined.
Such especially is the action of light in the cases which I pro-
pose to consider.

I have shown elsewhere, that, contrary to long-established
opinion, perfectly pure iodid of silver, isolated from all other
substances, is sensitive to light, and this fact, now I believe uni-
versally admitted, must form the comer-stone of photo-chemis-
try. For iodid of silver is precisely the only substance fitted to
give us a clear view into the action of light upon matter in
general, by which I mean that this action is so much more evi-

Digitized by


M. C, Lea on a Theory of Photo-chemistry. 78

dent and appreciable in the case of iodid of silver, that we may
thence obtain views subsequently to receive a wide extension.

Now when purd iodid of silver is exposed to light, it changes
slightly in color, and has acquired a new property, that of at-
tracting to itself a metallic precipitate in the act of forming, or
a metallic vapor already formed. Some have seen in this action
of light upon the iodid, a distinct reduction to sub-iodid. But
of this, proof is altogether wanting. An extension of the ex-
posure many million fold does not produce a reduction appre-
ciable by the most delicate reagents, and I have been enabled to
prove that such iodid perfectly recovers its sensibility in the
dark. That is, a film of such iodid, exposed for many hours to
a bright sun does not further darken beyond the change pro-
ducea by the first instants of diffuse light. And if then put
aside in the dark for a brief time it re-acquires the capacity by
exposure for a second, to receive an image.

But if, during this exposure, or subsequent to it, a bodj capa-
ble of combining with iodine be brought into cpntact with it, a
true chemical decomposition takes place. The silver loses half
of its iodine, is reduced to sub-iodid, and a vapor, or a precipi-
tate, under &vorable circumstances, is attractCKl to the parts so
acted upon.

What then is the nature of this change, this impression,
received in a second, and then slowly passing spontaneously
away ? Evidently a physical, not a chemical change, though
leading the way to a chemical change. But what is that physi-
cal change?

When light falls upon a compound body, its molecules are
made to vibrate, and if that vibration be carried to a certain
violence, the excursions of its atoms may exceed the limit which
their affinity permits, and the compound will be torn asunder.
We have an exact analogue of this in sound. A thin glass ves-
sel, if a certain note be sounded near it with sufficient force, will
be shattered, the excursion of its atoms exceeding the limits im-
posed by its attraction of cohesion. Vessels of other materials
would resist this and much greater strain. Just so with light.
Some bodies will be decomposed, others will resist So when
pure chhrid of silver is exposed to the light, it presently assumes
a violet color, losing at the same time one-half of its chlorine.
On the other hand, when pure iodid of silver isolated, is exposed
to light, no chemical change takes place. But the impression
of light is for a time persistent. Now the analogy which exists
between this efiect and phosphorescence has not been before per-
ceived. The " physical ^ impression of light is a persistence of the
invisible (or ^^chemicaV) rays exactly parallel to Ae persistence of
visible or luminoits rays, in phosphorescence. The vibrations ex-
cited by light ^re ija both cases not given out instantaneously

Ail Joub. Sol^eoond Sbbies, Vol. XLIV, No. 180.— Jult, 1887.

Digitized by VjOOQIC

74 M. C. Lea on a Theory of Phoio-chemistty.

as in mo6t light-phenomena that pais nnder oar notice, wheie
the retam to photo-eqailibriom is immediate, but this eqnilib-
rinm is only reached after the interval of hoars or daya

Consequently, so long as these vibrations of non-laminoos
light (so to speak) continae, the body under their infloence is
much more exposed to suffer decomposition than when under
normal influences. And if a film of such material has had parts
exposed to light while other parts have been protected, ana the
whole be then exposed to influences provocative of decomposi-
tion, it is evident that those influences may be so graduated that
they will tell only upon the parts predisposed by the impression
they have received.

This ftinction of light which I here endeavor to prove the ex-
istence of, may be conveniently termed Actineacence.

That this word does not express with entire accuracy what is
intended to be conveyed, is safficiently evident But it has the
advantage of connecting the phenomena with the parallel one
of phosphorescence, and is perhaps on the whole as well suited
as any other that could be found or made.

Where, through phosphorescence, a body temporarily retains,
and subsequently emits li^ht, that emission, in the gradual re-
turn to its photo-equilibnum, is rendered evident by the phe-
nomena that usually accompany the emission of light Sur-
rounding objects are illuminated, faintly but visibly. Should
we not therefore expect similar results in the case of aotines-

Not only should we find them, but they have actually been
described, though not understood. For in actinesoence we have
the explanation of the phenomena described by Niepce de St
Victor as the "storing up of light" (emmagasinement de la lu-
midreV St Victor showed that certain objects exposed to light
• and tnen carried into darkness had in some cases acquired the
power of acting chemically upon other bodies with which they
were placed in contact. His results were received at first with
sometning like derision, and the facts, which could not be denied,
were said to arise from some chemical decomposition which had
taken place in the body during its exposure to light, generat-
ing reducing agents which were subsequently given off in the

But until now, the identity between the latent physical image
and the storing up of light has not even beeii guessed at, still
less the perfect parallelism between these phenomena and that
of phosphorescence. That a body may immediately regain its
equilibrium with respect to one sort of rays and slowly with
respect to another, involves no difficulty whatever; we contin-
ually see this in the facts of common phosphorescence. Differ-

Digitized by


M. C, Lea on a Thewry of Photo-cliemistry. 76

ent phosphorescent bodies emit lieht of different oolors. If one
phosphorescent body emits a red, another a green, another a
olue light, there is evidently no difficulty in conceiving that in
others, the tendency to a slow recovery of equilibrinm may be-
long to the still more refrangible and invisible rays of the spec-
trum. In fact this is so completely within the analogy of the
subject that it would be surprising if it were not the case.

Viewed in this manner, the physical impression of light, as
distinguished from the chemical, loses all its difficulty ana mys-
tery. A substance like iodid of silver is capable of decomposi-
tion by light, only when in contact with some substance having
an affinity for iodine. But if exposed in an isolated condition,
it becomes actinescent^ it retains the disturbance occasioned by
the light, and only gradually parts with that energy to surround-
ing objects. So long as any portion of this impression lasts, it
is capable of undergoing decomposition if brought into contact
with substances capable of taking up iodine from it

Iodid of silver when exposed to light in a state of perfect
purity and isolated from all other substances, undergoes no
chemical change. Carried into darkness, it continues to vibrate
in unison with the more highly refrangible rays, either those
entirely beyond the visible spectrum, or else those having a very
low illuminating power, and this in so faint a degree that no
phosphorescence is visible. If it now be brought into contact
with any substance which would have occasioned decomposi-
tion in presence of light, then, so long as this phosphorescence
of actinic rays, this admeacence continues, the same decomposi-
tion will take place. K simply left in darkness, this actmes-
cence will, as I have already shown, gradually expend itself,
precisely like ordinary phosphorescence. And also as in the
case of ordinary phosphorescence, a fresh exposure to light will
create a fresh impression, the iodid of silver having apparently
perfectly recovered its original condition. With perfectly pure
iodid of silver, twenty-four hours is sufficient to nearly oolit-
erate the action of li^ht, the actinescent power is exhausted,
or nearly so, and the iodid can be exposed again. If this sec-
ond exposure be made under a photographic negative, and an
ordinary photographic developer is applied, a clear sharp image
is obtained.

It seems worthy of remark that though no visible phospho-
rescence is noticeable in the case of iodid of silver, it is by no
means impossible that if examined by the ingenious instrument
constructed for Mr. Becquerel by Mr. Dubosc, a visible phos-

Shorescence might also be observed. However this may be, it
oes not affect the principles here laid down.

Digitized by


76 M, C, Lea on the Theory of Photo-cJiemistry.

The general views here expressed will be found to throw light
upon otber obscure photo-chemical phenomena besides the latent
physical image. Or these I shall briefly cite one.

If light fall upon a body decomposable bv light, its energy
will be expended in two directions. Part will be transmitted to
surrounding objects, part will be expended in dissociating one
or more of the elements of the body. As the intensity of the
light increases, the amplitude of the excursions also increases,
and a larger proportion will be expended in decomposition. It
therefore follows that the decomposition effected will not be in
the ratio of the intensity of the light, but will be greater in a
strong light and less in a weak ; that is, that a light of half
strength acting for a double time, will not effect an equal amount
of decomposition. Now this exactly accords with the univer-
sal experience of photographers who find that where media of
unequal thickness are interposed between a sensitive surface and
the fight, as in the case of a negative superimposed upon chlo-
rid of silver, the weaker the light the greater will be the con-
trast in the degrees of effect produced upon the different parts
of the film ; so much so that this is systematically adopted as a
base of operation.

To enlarge farther here on the application of these principles,
and the liffht they throw upon photographic phenomena, would,
however, be inconsistent with the limits I have imposed upon
myself Mv object here has been to prove the existence of a
well-marked but hitherto overlooked property of matter, and
further to show the identity of this property with what in pho-
to-chemistry has been vaguely described as the physical iniage.

This explanation of these obscure phenomena seems so simple
and sufficient as scarcely to require proof. Nevertheless that
proof is afforded by the reactions of iodid of silver, perhaps the
most remarkable substance with which chemistry makes us ac-
quainted. This theory rests upon two properties for whose ex-
istence I have lonff contended, and which I believe I have suc-
ceeded in establishing, namely, the sensUivenees to Ught of todid
of Sliver even when perfectly isoiated, and its spontaneous recovery </
that sensitiveness after obliteration through powerful action of
light, by simply remaining in darkness. From these facts I
deduce the conclusion that the latent image is simply due to a
phosphorescence of chemical or actinic rays to which property I
give the njime of actinescence.

Digitized by


C. A. Goessmann an the Chemistry of Brines. 77

Abt. IX. — Qmtribtdidn to the Chemistry of Brines; by Chables

All nataral solutions of chlorid of sodium, which are used
for the manu&cture of salt, are more or less contaminated by
Tarious saline admixtures. The peculiar influence exerted by
certain of these foreign compounds on the chemical composition,
the form, and the general external appearance of the chlorid of
sodium, during its separation as a commercial article, even under
the same system of manufacture and with the same precautions,
has occupied the attention of chemists fix)m the earliest times in
the history of this branch of chemical industry.

A classification of brines, for the purpose of aiding their in-
vestigation, had in the course of events become advisable ; and
among others, Karsten* at an early date, made some advances
in this direction. In his valuable discussions upon brines he
has adopted the following principle; the brines are divided into
two classes; the first class induoes all brines containing chlorid
cf magnesium and sulphates; and the second class those contain-
ingchlorid of caicium beside chlorid of magnesium.

He fikvored at that time, the volcanic theory, in regard to the
origin of the rock salt (primitive deposits), a view long since
discarded as a general rule, in &vor of its marine origin. He
considered it self-evident, that the brines, in every well inves-
tigated case could be proved to originate from the dissolving ac-
tion of an underground fresh water current upon rock salt.
Their differences in composition were ascribed, — leaving the con-
centration as to amount of salt dissolved entirely out of the
question, — in regard both to quantity and to quality, to the pe-
culiar nature and condition of the strata, which had intercepted
their passage to the surface. According to his view, decaying
pyrit4, sulphates and chlorids of the metals and earths were
the main cause of the contamination of these solutions of rock
fialt. The extent to which these compounds happened to be met
with, deeided ultimately the amount of foreign admixtures thus
imparted, while their final qualitjr and relative proportion was
-determined by the order of succession in which the contaminated
brines chanced to traverse limestone rocks or dolomites. The
gypsum present was considered in most cases to be the result of
the reaction of soluble sulphates— particularly of the alkalies^
of magnesia or of iron, upon chlorid of calcium ; moreover, pri-
mary and secondary deposits of chlorid of sodium were admitted.
The presence of chlorid of calcium consequently was looked up-
on as merely accidental, no certain relation as to its connection
with a particular geological age, being presumed. Subsequent

• a J. Kanten, Salinenkunde, vol. i, p. 280. Berlin, 1841

Digitized by


78 C A. Goessmann on the Chemistry of Brines.

investigations gave more importance to the presence or absence
of this compound ; and its presence has since been recognized
as especially characteristic of the salt deposits of ante-tertiary
date. Consequently these have been considered as a product of
the constant admixture of the oceanic waters of preceding geo-
logical periods ; while on the other hand its absence in our pres-
ent ocean and in most salt deposits of a more recent date, is an
established fact Although the results obtained by numerous
investigations are such as scarcely to admit of a doubt, that
changes in re^rd to the chemical composition of the oceanic
waters have taken place in the course of time, and are still in
progress, we must acknowledge, that our ideas concerning the
main features of the primitive or silurian oceans are still vague,
and especially so upon this one point.

As mineral waters and brines issuing from Silurian rocks are

Online LibraryJohn AlmonThe American journal of science and arts → online text (page 59 of 102)