A great and important question has arisen in medical jurisprudence,
whether any chemical proof of the presence of _white arsenic_, short of
its actual reduction to the state of metal, can be depended upon, or
ought to be received as evidence in the courts of criminal law. After a
full experimental investigation of the subject, and an impartial review
of all the facts which bear upon the question, I feel no hesitation in
declaring it to be my conviction, that _white arsenic may be detected
without any fear of fallacy_, _by a proper application of certain
tests_, and that the contrary opinion is entirely founded in error, and
unsupported by experiment, as will more fully appear in the sequel.[408]
(A) _Fused Nitrate of Silver_, or _Lunar Caustic_—For this test we are
indebted to Mr. Hume of London, who first gave it to the public in the
Philosophical Magazine for May 1809, vol. xxxiii. His method of applying
it is as follows: into a clean Florence flask introduce two or three
grains of the suspected powder, to which add about eight ounces of rain
or distilled water, and heat the solution until it begins to boil, then
while it boils frequently shake the flask, and add to the hot solution a
grain or two of sub-carbonate of potass, agitating the whole to make the
mixture uniform. Pour into a wine glass about two table spoonsful of the
solution, and touch the surface of the fluid with a stick of lunar
caustic. If arsenic be present, a beautiful yellow precipitate will
instantly proceed from the point of contact, and settle towards the
bottom of the glass as a flocculent and copious precipitate.
By this test the 60th part of a grain may be satisfactorily recognised
in two ounces of water. The presence of some alkali is essential to the
success of the experiment, since arsenious acid is unable, by the
operation of simple affinity, to decompose the nitrate of silver.[409]
The validity of this test has been questioned on the following grounds,
which shall be fairly examined in order.
OBJECTION 1. _The alkaline phosphates are found to produce precipitates
with silver, analogous in colour and appearance to the arsenite of
silver._ This is undoubtedly the case when the experiment is performed
in the manner just stated, but there are other reagents which will
immediately distinguish these bodies, as will be seen under the history
of the _Ammoniuret of silver_; I have also shewn that there is a mode of
so modifying the application of the silver test itself, that no error or
doubt can arise in the use of it from the presence of phosphoric
salts.[410] My method consists in conducting the trial on writing paper,
instead of in glasses, thus—drop the suspected fluid on a piece of white
paper, making with it a broad line; along this line a stick of lunar
caustic is to be slowly drawn several times successively, when a streak
is produced of a colour resembling that known by the name of _Indian
Yellow_; and this is equally produced by the presence of arsenic and
that of an alkaline phosphate, but the one from arsenic is rough, curdy,
and flocculent, as if effected by a crayon, that from a phosphate
homogeneous and uniform, resembling a water-colour laid smoothly on with
a brush; but a most important and distinctive peculiarity soon succeeds,
for in less than two minutes the phosphoric yellow fades into a _sad
green_, and becomes gradually darker, and ultimately quite black; while,
on the other hand, the arsenical yellow remains permanent, or nearly so,
for some time, when it becomes brown. In performing this experiment the
sun-shine should be avoided, or the transitions of the colour will take
place too rapidly. It would be prudent also for the inexperienced
operator to perform a similar experiment on a fluid known to contain
arsenic, and on another with a phosphoric salt, as a standard of
comparison. In this way the nitrate of silver, without the intervention
of any other test, is fully capable of removing every ambiguity, and of
furnishing a distinguishing mark of difference between the chemical
action of arsenic and that of the phosphates. Mr. Hume states that he
has repeated this experiment to his entire satisfaction,[411] and that,
in a late unfortunate case of poisoning, he derived considerable
information by its application. The laborious author of the London
Dispensatory accepts it as an excellent test, but observes that it is
rendered more luminous by brushing the streak lightly over with liquid
ammonia immediately after the application of the caustic, when, if the
arsenic be present, a bright queen’s yellow is produced which remains
permanent for nearly an hour; but that when the lunar caustic produces a
white-yellow before the ammonia is applied, we may infer the presence of
some alkaline phosphate, rather than that of arsenic. One of the great
advantages of this test is the very small quantity that is required for
examination; it would be well therefore for the operator to perform the
experiment in both ways on a separate paper.
OBJECTION 2. _The Muriates produce precipitates with silver so copious
and flocculent as to overcome every indication which the presence of
arsenic would otherwise afford._ Dr. Marcet proposes to obviate this
difficulty, by adding to the fluid to be examined dilute nitric acid,
and then cautiously applying the nitrate of silver until the
precipitation ceases; in this way the muriatic acid will be entirely
removed, whilst the arsenic, if it be present, will remain in solution,
and may be rendered evident by the affusion of ammonia, which will
instantly produce the yellow precipitate in its characteristic form.
This mode however it must be confessed appears complicated, and requires
some chemical address for its accomplishment; it should be also known
that the yellow precipitate thus produced is not always permanent, for
it is soluble in the nitrate of ammonia formed during the process. Under
these circumstances, it is surely preferable to precipitate at once from
the suspected fluid all the substances which nitrate of silver can
effect, and then to expose the mixed and ambiguous precipitate so
obtained, to a low heat in a glass tube, when the arsenious acid will be
immediately separated by sublimation. In this way the presence of
muriates may even in certain cases be serviceable, especially if the
quantity of arsenic be minute; for by increasing the bulk of the
precipitate we shall decrease the difficulty of its examination. By this
process, also, I should propose to meet the embarrassments which arise
from the influence of various animal and vegetable substances, as milk,
broth, wine, &c. so frequently present in the suspected liquid, and
which are known to alter the character of the arsenical indications. In
this case, however, we must not rely upon any single precipitant; after
having thrown down all that is precipitable by the silver test, the
supernatant liquid should be decanted, slightly acidified by acetic
acid, and submitted to the action of Sulphuretted Hydrogen; when, should
any precipitate occur, it must be separated and added to the former. Dr.
Christison has demonstrated the importance of this proceeding by shewing
that the precipitates, occasioned by the Ammoniurets of silver and
copper, are soluble in certain vegetable infusions.[412] M. Orfila
proposes to remove the difficulties and embarrassments, occasioned by
the colouring matter of different media, by the application of
_Chlorine_, so as to change the colour to a shade that will not offer
any optical impediment to the characteristic indications of the
different tests. I am ready to admit that such a mode of proceeding may,
on certain occasions, assist the accomplished chemist in his analysis,
but in the hands of a person less accustomed to chemical manipulation, I
hesitate not to declare that it is subject to fatal fallacies;[413]
whereas, by precipitating the whole, and submitting the precipitate to
the process of sublimation, we shall avoid every source of error. Why
then should we attempt to pursue our game through the windings of a
labyrinth, when a direct road lies before us, by which we may at once
drive it into the open plain? Mr. Phillips has recently proposed the
addition of animal charcoal (_Ivory Black_) for the purpose of
destroying the colouring matter. He found that by mixing this substance
with the _Liquor Arsenicalis_, that the colouring matter was so
completely destroyed in a few minutes, that the test of nitrate of
silver, or any other might be readily applied. This experiment was
repeated with Port wine, gravy soup, and a strong infusion of onions,
and he succeeded in these cases in procuring a solution sufficiently
colourless for the application of the most delicate reagents. It might
be supposed, adds Mr. Phillips, that the Phosphoric acid which the
animal charcoal contains, might have some share in the production of the
yellow precipitate with silver; he found, however, that water, or wine,
which was merely digested on the animal charcoal, produced no effect
with the nitrate of silver, except a slight precipitate of chloride; and
this even, was prevented by lixiviation. I have, however, a serious
objection to offer to this proposal. Animal charcoal, by some mode of
operation not understood, possesses the property of removing certain
substances from their solution in water; I have already noticed this
effect with respect to Lime water, (see p. 247, _note_), and I have
lately found that it takes place with very dilute solutions of
Arsenic.[414] Hence charcoal, as we shall presently find, may be
employed for the purpose of detecting minute portions of arsenic.
OBJECTION 3. _Chromate of potass produces, with Nitrate of Silver, a
yellow precipitate which, when placed side by side with one produced by
Arsenious acid, cannot be distinguished by colour or appearance from
it_. This fact has lately been announced by Dr. Porter of the University
of South Carolina. (Silliman’s Journal, iii, 355.) But as the presence
of chromate of potass can never be suspected in any research after
arsenic, in cases of forensic interest, the fact is of no importance to
the physician.
Where the Arsenious acid is mixed with vegetable matter, and it becomes
difficult to separate it by filtration, the whole may be evaporated to
dryness, taking great care that the heat applied for such a purpose
never exceeds 250° _Fah:_ or we shall lose the arsenic by
volatilization. The residue thus obtained may then be submitted to a
higher temperature, in a subliming vessel, in order to procure the
arsenious acid in its pure state. Should the arsenious acid have, in the
first instance, been dissolved in oil, Dr. Ure proposes to boil the
solution in distilled water, and to separate the oil afterwards by the
capillary action of wick threads. If the arsenious acid be mixed with
resinous bodies, oil of turpentine may be employed as their solvent,
which will leave the arsenic untouched. Dr. Black directed the
application of alcohol for this purpose, but this is obviously improper,
since arsenious acid is soluble in that fluid.
It has been stated that, in consequence of the inability of arsenious
acid to decompose nitrate of silver by simple elective attraction, the
presence of an alkali becomes indispensable in the examination, for
which purpose Dr. Marcet has suggested the superior advantages which
will attend the use of ammonia, in cases where the arsenic has not been
previously combined with a fixed alkali, since it does not, when added
singly, decompose nitrate of silver; a circumstance which in using the
fixed alkalies is very liable to occasion fallacy. This led Mr. Hume to
improve his original plan, by forming at once a compound, which he calls
the _Ammoniaco-nitrate of silver_, but which may with more propriety be
designated as an _Ammoniuret_.[415] This is a triumph in the art of
analysis; for whilst it obviates the necessity of ascertaining the exact
proportion of alkali required in each experiment,[416] it possesses the
valuable property of not in the least disturbing the phosphate of soda.
(B.) _Sulphate of Copper._ Like the preceding test, this also requires,
for its success, that the arsenious acid should be combined with some
alkali, in which case, by the operation of double elective attraction,
an arsenite of copper is thrown down of a very striking and
characteristic colour, being that of the well-known pigment called
_Scheele’s green_; if arsenic be not present in the liquid so assayed,
and a fixed alkali has been employed, the result will be a delicate
_sky-blue_, instead of the _grass-green_ precipitate.
Mr. Hume avails himself also of the peculiar property of ammonia to form
a metallic salt, and has employed it with copper: he takes the sulphate
or acetate of that metal, and by the same process as that described for
the preparation of an ammoniuret of silver, forms another test. In using
this, however, care must be taken that it be not too highly
concentrated, for in that state it will not produce precipitation. Much
controversy has taken place on the subject of sulphate of copper as a
test for arsenic, and it has been stated, with more confidence than
truth, that a _decoction of onions_ has the property of imparting to the
copper precipitate, which is produced by a fixed alkali, a colour and
appearance analogous to that which is occasioned by arsenic. This
opinion was boldly advanced and supported on a most important[417] trial
at the Lent assizes for Cornwall in 1817. Since this event an
opportunity[418] has occurred which has enabled me to examine this
alleged fact, by a fair and appropriate series of experiments, the
result of which satisfactorily proved that the opinion was grounded on
an optical fallacy, arising from the _blue_ precipitate assuming a
_green_ colour, in consequence of having been viewed through a yellow
medium.[419] The phosphoric salts may also, under similar circumstances,
be mistaken for arsenic, for the intense blue colour of the phosphate of
copper will thus necessarily appear green. This instance of optical
fallacy is not solitary, for _corrosive sublimate_ has been said to
possess the character of an alkali, because it turns the syrup of
violets green, whereas this change is to be attributed solely to the
combination of the yellow hue of the sublimate with the blue colour of
the violet.
Whenever therefore such a source of fallacy can be suspected, the
operator would do well to repeat his experiment on white paper, in the
manner I have before proposed, and the results which are obtained in
glasses should always be examined by day-light, and viewed by reflected
and not by transmitted light. The presence of Peroxide of Iron in the
Cupreous salt will also impart a green colour to the precipitate
produced by an alkali. To obviate any fallacy which might arise from
this circumstance, Mr. Phillips proposes to add some pure Potass to the
sulphate of copper; if pure, a fine blue precipitate will be thus
obtained; to this may be then added the suspected solution, and if
Arsenious acid be present, it will then convert this blue precipitate to
a green one.
(C.) _Sulphuretted Hydrogen._ This is a very delicate test for arsenic,
producing with its solution a beautiful golden coloured liquor, which
after a short time lets fall a precipitate. Mr. Phillips,[420] in
reviewing the third edition of the present work, has stated, that no
such precipitate occurs, but I find that in close, as well as in vessels
exposed to the air, it takes place by repose. By this reagent, so small
a quantity as 1⁄000,000 may be detected in solution; and it may be also
stated in farther proof of the utility of this test, that it is less
affected than any other by the presence of animal or vegetable matter.
The method of preparing a solution of sulphuretted hydrogen gas is
extremely simple. Put into an oil flask about two ounces of undiluted
muriatic acid and an ounce and a half of powdered Sulphuret of Antimony;
fit a cork to the flask and pass through it the short leg of a small
glass tube twice bent at right angles; pass the longer leg of the tube
into a phial containing distilled water, and then by the heat of a
spirit lamp applied to the flask, sulphuretted hydrogen gas will be
abundantly liberated, and though much of it will escape, yet a
sufficient quantity will be dissolved by the water. The annexed sketch
represents the apparatus proposed by Mr. Phillips, and which will
require but little practical skill, either for its construction or use.
[Illustration]
But it is not always necessary to prepare a watery solution of
Sulphuretted Hydrogen, a stream of the gas, introduced into the
suspected liquor by means of the above apparatus, will act with equal
delicacy, and possesses, as Dr. Christison has observed, the advantage
of not diluting it. Before applying this test, it is necessary to add an
acid, if any alkali should exist along with the Arsenic, otherwise no
precipitate will take place.
(D.) _Alkaline Hydro-sulphurets._ These bodies do not affect the
arsenious solution, unless a few drops of acetic acid be added. To the
_Hydro-sulphuret_, or perhaps more properly, _Hydroguretted Sulphuret_,
of Ammonia there is an insuperable objection, since this fluid, when
diluted, possesses the colour which we expect to produce by the action
of Sulphuretted Hydrogen upon Arsenious Acid.
(E.) _Charcoal Powder._ This test was proposed by Mr. A. Thomson,
(London Dispensatory, 2nd edition, p. 53.) Into the suspected solution
stir a moderate quantity of charcoal powder, allow it to settle, then
pour off the supernatant liquor, and when the powder which remains is
dry, sprinkle some of it on a red hot poker, when, if the solution
should contain Arsenic, the odour of Garlic will be rendered sensible. I
have already offered some remarks upon the cause of this phenomenon
(_page 304_.)
There are several other tests by which arsenic may be identified. The
process described in the Dublin Pharmacopœia for the preparation of
_Arsenias Kali_, the arseniate, or rather super-arseniate of potass,
which has been long known under the name of “the arsenical salt of
Macquer,” has been strongly advised as a collateral proof; it consists
in decomposing the nitrate of potass[421] by the arsenious acid, but
since this problem requires that the suspected poison should be in a
solid and palpable form, it is impossible to examine its pretensions to
our confidence, without being reminded of the story so often told to us
in our infancy, of catching a bird by laying salt upon its tail.
It is necessary to observe in this place, that the _arseniate_, like the
_arsenite of potass_, or that of _ammonia_, is obedient to the silver
test, but that instead of the yellow precipitate which is produced by
the latter salt, we obtain, by the former, a red or brick-coloured one.
If arsenious acid and quick-lime be heated together in a glass tube, a
sudden ignition is occasioned at a certain temperature, when metallic
arsenic will sublime, and an arseniate of lime be formed. In this case
one portion of the arsenious acid is robbed of its oxygen to complete
the acidification of the remainder.
In taking an impartial review of all the evidence which the
investigation of this subject can furnish, it must appear to the most
fastidious, that the silver and copper test above described are capable,
under proper management, of furnishing striking and infallible
indications, and that in most cases they will be equally conclusive, and
in some even more satisfactory in their results, than the metallic
reproduction upon which such stress has been laid, and for this obvious
reason, that unless the quantity of metal be considerable,[422] its
metallic splendour and appearance is often very ambiguous and
questionable. It has to my knowledge happened to a medical person, by no
means deficient in chemical address, to ascribe to the presence of
arsenic that which was no other than a film of very finely divided
charcoal: in this state of doubt the last resource was to ascertain
whether it yielded, or not, upon being heated, an alliaceous odour.
Surely an unprejudiced judge would prefer the evidence of sight, as
furnished by the arsenical tests, to that of smell, as afforded in the
last experiment. No one will attempt to deny that it is the duty of the
medical practitioner who is called upon to decide so important a
question as the presence of arsenic, to prosecute by experiment every
point which admits the least doubt; he should also remember that in a
criminal case, he has not only to satisfy his own conscience, but that
he is bound, as far as he is able, to convince the public mind of the
accuracy and truth of his researches; and he fails in his duty if he
omits, through any false principle of humanity, to express the strong
conviction which the success of his experiments must necessarily have
produced in his mind. Let it however be remembered, that the application
of chemical reagents on solutions suspected to contain arsenic, so far
from throwing any obstacle in the way of the metallic reduction of that
body, are the very steps which should be adopted as preparatory to the
“_experimentum crucis_,” since the precipitates which are thus produced
may be collected, and easily decomposed, as before stated. Those who for
judicial purposes may require farther information upon these subjects
are referred to the second volume of our work on “_Medical
Jurisprudence_.” _Tit: Poisons._