EFFECT OF ARSENIC COMPOUNDS
I. PRESENCE OF ARSENIC IN PLANTS.
The occurrence of arsenic as an occasional constituent of plants has
been recognised for many years. Chatin (1845) found that if a plant were
supplied with arsenical compounds at the roots arsenic was absorbed, but
that it was distributed unequally to the various tissues. The greatest
accumulation of the element was in the floral receptacle and the leaves,
while it was scarce in the fruits, seeds, stems, roots and petals. E.
Davy (1859) commented on the presence of arsenic in plants cultivated
for food. He grew peas in pots and watered them for a short time with a
saturated aqueous solution of arsenious acid, the application being then
discontinued. The plants, apparently uninjured by the treatment, flowered
and formed seeds. On analysis arsenic was readily detected in all parts
of the plant, including the seeds. Other analyses revealed the presence
of the element in cabbage plants (from pots) and turnips (from field),
both of which had been manured with superphosphate containing some amount
of arsenic. This absorption of arsenic by the roots of plants was further
established by Phillips (1882).
Various physiological workers have pointed out that this element is
frequently or usually present in animal tissues. Cerný (1901) reached
the general conclusion that minimal traces of arsenic can occur in animal
organisms, but that these play no part in the organism and indeed are not
constant in their occurrence. Bertrand (1902) established its presence in
minute quantities in the thyroid glands of the ox and pig, hair and nails
of the dog, and the feathers of the goose. Gautier and Clausmann (1904)
realised the constant presence of arsenic in human tissues and recognised
that it must inevitably be introduced into the body with the food. This
led them to estimate the arsenic present in various animal and vegetable
foods, some of their results being given in the following table.
_Arsenic per 100 parts fresh substance in µ gr._ (= thousandth part of a
milligram)[8].
Wheat (Victoria--complete grain) ·7
„ (from Franche Comté) ·85
White bread ·71
Whole green cabbage ·2
Outside leaves of cabbage ·0 (absent)
Green haricots ·0 „
Turnip ·36
Potatoes 1·12
[8] 0^{µ gr.}, 1 = 0·0001 mg.
Arsenic was also found in wine and beer and in considerable quantities
in sea water and various kinds of salt. Since it cannot be found in some
things even in the least traces, the authors conclude that it is incorrect
to say that the element is always present or that it is essential to all
living cells.
S. H. Collins (1902) found that barley is able to absorb relatively
large quantities of arsenic. The plants were grown in pots on soil which
originally contained a certain amount of the substance, and various
combinations of arsenic acid, arsenious acid and superphosphate were
added. Particulars and details are not given by the author, except
that arsenic was detected by Reinsch’s test in the grains from all the
experimental pots, and in one case (not specified) in the upper and lower
halves of the straw and in the threshed ears. The analyses of the soil at
the close of the experiments showed the presence of 7–22 parts arsenious
acid per million.
Wehmer (1911) quotes references to the occurrence of arsenic in _Vitis
vinifera_. The element was detected in the ash of the must and its
presence was attributed to treatment of the plants with arsenical
compounds. In this connection it is interesting to note the observation
of Swain and Harkins (1908), who, while acknowledging the absorption of
arsenic from the soil by many plants, yet indicate that in the case of
those plants which are exposed to smelter smoke the arsenic is deposited
on the vegetation, and is not absorbed by the latter from the soil.
II. EFFECT OF ARSENIC ON THE GROWTH OF HIGHER PLANTS.
1. _Toxic effect._
(_a_) _Toxic action of arsenic compounds in water cultures in the presence
of nutrients._
The poisonous action of arsenic on plants has long been recognised. Chatin
(1845) gave accounts of tissues poisoned by strong arsenical solutions.
Nobbe, Baessler and Will (1884) carried on water culture experiments
with buckwheat, oats, maize and alder, and found that arsenic was a
particularly strong poison for these plants. When small quantities of
arsenious acid (As₂O₃) were added to the food solutions, growth was
measurably hindered by a concentration of 1/1,000,000 As (reckoned as
As). The element only appears in plants in very small quantity and can
never be detected in notable quantities. The aerial organs show the effect
of arsenical poisoning by intense withering, interrupted by periods
of recovery, but eventually followed by death. It was also found that
if plant roots were exposed to the action of arsenical solutions for a
short period, say ten minutes, and then were transferred to normal food
solutions, the action of the poison was delayed, but eventually hindering
of growth or death occurred, according to the strength of the poison used
in the first solution.
At the same time that Nobbe, Baessler and Will were establishing the
great toxicity of the lower oxide of arsenic, Knop (1884) was carrying
the matter a step further by comparing the action of arsenious and arsenic
acid and their derivatives on plant growth. He established the fact that
while arsenious acid is a strong poison for maize plants, arsenic acid in
small quantities is not toxic to the roots and that the plants can produce
flowers and fruit in its presence. Arsenic acid applied as potassium
arsenate proved to be harmful to young maize seedlings if the solutions
contained ·05–·1 gm. arsenic acid per litre (= 1/–2/20,000 arsenic acid).
If however the plants were allowed to form 10–15 leaves in a pure food
solution and then when strongly rooted were transferred to a solution
of ·05 gm. arsenic acid per litre, they were found to grow strongly and
develope big healthy leaves. Careful measurements indicated that the
development is unchecked by the addition of the poison, though arsenic
was determined in the ash of the treated plants.
Stoklasa (1896, 1898) tested the effect of arsenic compounds on plant
growth with special attention to their comparative relation to phosphoric
acid. He corroborated Knop’s statement as to the greater toxicity
of arsenious acid and arsenites in comparison with arsenic acid and
arsenates, stating that 1/100,000 mol. wt. arsenious acid per litre causes
definite trouble in plants, while with arsenic acid 1/1000 mol. wt. per
litre first shows a noticeable toxicity. Water culture experiments were
made with and without phosphoric acid, in each case with and without the
addition of arsenic and arsenious acid. It was found that the arsenic
acid was unable to replace the phosphoric acid, the plants decaying in the
flower in the absence of the latter. In the complete absence of phosphoric
acid, arsenic acid causes a strong production of organic substances up to
the flowering time. The following figures were obtained with maize:--
·002 gm. As₂O₃ with P₂O₅ 2·84 gm. dry wt.
·005 gm. „ „ „ 2·37 „
·01 gm. As₂O₅ „ „ 67·32 „
·40 „ „ „ „ 64·13 „
·03 „ As₂O₅ without P₂O₅ 39·98 „
·07 „ „ „ „ 42·13 „
normal solution „ „ 12·93 „
„ „ with „ 65·84 „
Comparative experiments with the two arsenical oxides showed that varying
times were required to kill different plants. Young seedlings were brought
into solutions containing 1/10,000 mol. wt. arsenious acid (= ·019 gm.
As₂O₃ per litre) and the plants died in a very short time.
Hordeum distichum 46 _hours_
Polygonum Fagopyrum 84 „
„ Persecaria 90 „
With ten times the strength of arsenic acid (1/1000 mol. wt. = ·23 gm.
per litre) the plants took much longer to kill.
Hordeum distichum 24·5 _days_
Polygonum Fagopyrum 40 „
„ Persecaria 42 „
Various experiments have been carried on at Rothamsted with peas and
barley. With arsenious acid on barley a depressing influence is manifest
even at a concentration of 1/10,000,000, while no growth at all is
possible with 1/10,000 and upwards. Apparently the toxic action on the
root ceases at a higher strength than on the shoot, as with 1/1,000,000
and less the dry weight of the root remains practically constant. At this
same strength the shoots look better than the controls, but this is not
apparent in the dry weights (Figs. 9 and 10). With peas the depression
is again evident to 1/10,000,000, but the plants are more sensitive to
the higher concentrations, as no growth can take place in the presence
of 1/250,000 arsenious acid (Fig. 11). A striking difference is observed
with arsenic acid on barley, as apparently this does not act as a toxic
even with such comparatively great concentrations as 1/100,000, though
possibly the shoot is slightly depressed by this strength (Fig. 12).
[Illustration: Fig. 9. Photograph showing the action of arsenious acid on
barley in the presence of nutrient salts. (March 16th–May 9th, 1911.)
1. Control.
2*. 1/50,000 arsenious acid.
2. 1/100,000 „ „
3. 1/150,000 „ „
4. 1/200,000 „ „
5. 1/250,000 „ „
6. 1/500,000 „ „
7. 1/1,000,000 „ „
8. 1/5,000,000 „ „
9. 1/10,000,000 „ „
10. 1/25,000,000 „ „
11. 1/50,000,000 „ „ ]
[Illustration: Fig. 10. Curve showing the mean value of the dry weights
of ten series of barley plants grown in the presence of arsenious acid
and nutrient salts. (March 16th–May 9th, 1911.)]
[Illustration: Fig. 11. Curve showing the mean value of the dry weights
of ten series of pea plants grown in the presence of arsenious acid and
nutrient salts. (June 8th–July 21st, 1910.)]
[Illustration: Fig. 12. Curve showing the mean value of the dry weights
of ten series of barley plants grown in the presence of arsenic acid and
nutrient salts. (Feb. 28th–April 24th, 1911.)]
With sodium arsenite the dilutions were carried further, to 1/250,000,000,
but this still depressed barley to some extent (Fig. 13). With peas
the results vary somewhat in the different tests, the depression with
1/2,500,000 and less being usually slight, though occasionally it is much
more strongly marked (Fig. 14). In a single series with sodium arsenate
barley was apparently unaffected by a concentration of 1/1,000,000, but
from this point down to 1/250,000,000 a constant _depression_ showed
itself, which was paralleled by a similar depression in the sodium
arsenite series from 1/25,000,000 to 1/250,000,000, the curves grading
downwards instead of up towards the normal. With peas sodium arsenate has
little or no action, though it is just possible that the rather irregular
curves indicate a very slight depression below the normal throughout.
[Illustration: Fig. 13. Curve showing the mean value of the dry weights
of ten series of barley plants grown in the presence of sodium arsenite
and nutrient salts. (Feb. 10th–April 18th, 1913.)]
[Illustration: Fig. 14. Curve showing the mean value of the dry weights
of ten series of pea plants grown in the presence of sodium arsenite and
nutrient salts. (June 27th–Aug. 10th, 1911.)]
(_b_) _Toxic effect of arsenic compounds in sand cultures._
Comparatively few tests seem to have been made as to the action of
arsenical solutions in sand cultures. Stoklasa (1898) repeated his water
culture work, using sand as a medium, and found analogous results by the
two methods, i.e. that arsenites are far more toxic than arsenates, and
also that the degree of toxicity of a salt varies with the plant to which
it is applied, as was shown by the fact that different plants lived for
varying times when treated with similar strengths of solution.
(_c_) _Toxic effect of arsenic when applied to soil cultures._
Daubeny (1862) watered barley plants with a solution of arsenious acid,
1 ounce in 10 gallons, five times in succession, and found that the crop
arrived at maturity about a fortnight earlier than the untreated part of
the crop, though the amount harvested was rather less. With turnips four
waterings had no effect upon the time of maturity, but again the crop was
slightly decreased. The analyses made indicated that no arsenic was taken
into the tissues, but that it merely adhered to the external surfaces.
Gorup-Besanez (1863) mixed 30 grams arsenious acid with 30·7 litres[9]
soil, growing two plants on this quantity of earth. Most of his
experimental plants (_Polygonum Fagopyrum_, _Pisum sativum_, and _Secale
cereale_) developed normally, but _Panicum italicum_ died soon after the
plants appeared above the surface, the leaves being very badly coloured.
Analyses by Marsh’s test showed no trace of arsenic in 20 grams dry matter
from _Secale cereale_, but in 148 grams _Polygonum Fagopyrum_ the presence
of arsenic was evident, though the mirror formed was weak. With such a
large proportion of arsenious acid in the soil it seems hardly conceivable
that the plants were not injured to some extent, and also it is probable
that with more careful analyses arsenic would have been detected in those
instances in which its presence was denied. Yet it must be remembered that
Davy (1859) had treated pea plants in pots with a _saturated_ solution
of arsenious acid for a short time and had stated that the plants were
uninjured. Thus both Gorup-Besanez and Davy concur in the opinion that
_Pisum sativum_ is indifferent to relatively large quantities of arsenious
acid when presented in the soil, whereas the Rothamsted experiments show
that in water cultures the plant is extremely sensitive even to minute
traces of the substance. It is possible that the arsenic in the solution
added to the soil enters into combination with other substances, forming
insoluble compounds, thus being removed from the sphere of action and
rendered unable to affect plant life. If this be so, the apparent immunity
of certain plants to arsenious acid is explained. F. C. Phillips (1882),
in his experiments on various flowering plants, such as geraniums, coleas
and pansies, found that compounds of arsenic in the soil exercised a
distinct poisoning influence, tending, when present in large amount, to
check the formation of roots, so that the vitality of the plant was so
far reduced as to interfere with nutrition and growth, or even to kill
it outright. He also stated that traces of arsenic were found in all the
plants grown upon the poisoned soil.
[9] 30 grams arsenious acid to 30·7 “cubik Decimeter” soil = about
·1%.
In this connection it is interesting to note that a certain proportion
of arsenic is frequently present in the superphosphate used as manure.
In view of the known toxicity of arsenical compounds to plant life the
question arose as to whether superphosphate manuring would exercise a
detrimental influence on account of its arsenic content. Experiments
carried out by Stoklasa (1898), however, indicate that there is not
sufficient arsenic in maximum doses of superphosphate to exercise a
toxic action in the field.
(_d_) _Physiological considerations._
The physiological action of arsenic compounds on plant life early
attracted the attention of investigators. Chatin (1845) put forward some
rather curious and unexpected considerations with regard to this action.
He stated that the effect of arsenic on plant growth is determined more by
the constitution and temperament of individual plants than by their age,
and that apparently difference in the sex of plants is of no significance.
The chief determining agent, however, is the species, and Chatin found
that as a general rule Cryptogams are more sensitive than Phanerogams,
and Monocotyledons than Dicotyledons, as is shown by the fact that under
treatment the former perish first. Some extreme exceptions exist, though,
as _Mucor mucedo_ and _Penicillium glaucum_ will grow on moist arsenious
acid, whereas leguminous plants are killed by an arsenical solution in a
few hours. Chatin held the view that elimination of the poison succeeded
its absorption, and that this elimination is complete if the plant lives
long enough. Here again the species exerts a great influence on the
excretory functions of the plants. Lupins and Phaseolus are presumably
able to eliminate in six weeks all the arsenious acid they can absorb
without dying. Most Dicotyledons need 3–5 months, while Monocotyledons
retain traces of poison for six months after its absorption. Lichens
are said to eliminate it more slowly still. Again, woody species are
longer in freeing themselves than herbaceous, and young plants carry out
the elimination more easily than old plants. The excretory function is
influenced by other physiological factors such as dryness and season. The
toxic effects and elimination are supposed to act inversely and parallel,
the absorbed arsenious acid combining with alkaline bases, making a very
soluble salt which is excreted by the roots. Calcium chloride is given as
the antidote to arsenious acid, all soluble acid being “neutralised” by
it. This view of the elimination of arsenic apparently did not gain much
support, as no further references to the matter have so far come to light.
In view of the work of some modern investigators (Wilfarth, Römer and
Wimmer) on the excretion of salts by plant roots, the idea may prove of
fresh interest. Chatin also found that moving or still air influenced the
working of the poison, indicating that the external physical conditions
affect the toxic action considerably. Nearly forty years later Nobbe,
Baessler and Will found that, if transpiration were hindered by placing
plants in a dark or moist room, it was possible to keep the plants
turgescent in arsenic solutions for a long time without thereby increasing
the toxic effect later on. The poisonous action proceeds from the roots,
of which the protoplasm is disorganised and the osmotic action hindered.
Finally, in the presence of sufficient of the poison, the root dies
without growth.
Stoklasa (1896, 1898) again found that phanerogamic plants can withstand
arsenic poisoning for some time in the dark or in CO₂-free air, provided
that glucose is given in the food solution. The arsenic poisoning is
at its maximum during carbon assimilation by means of chlorophyll. The
toxic action of arsenious and arsenic acids, especially in phanerogams,
is due to injury to the chlorophyll activity. The destruction of the
living molecule is far more rapid in the chlorophyll apparatus than in
the protoplasm of the plant cell.
Thus it seems that the physiological cause of the toxicity of arsenic is
partly a direct action on the root protoplasm, whereby its osmotic action
is hindered, and partly a detrimental action upon those functions which
are directly concerned with the elaboration processes of nutrition.
2. _Effect of arsenic compounds on germination._
In view of the great toxicity of arsenic to plants in their various stages
of development, one would naturally expect to find a similar action with
regard to the germination of the seeds. Davy (1859) casually mentioned
cases in which watering with arsenical solutions or dipping seeds in
arseniated water prevented germination. Heckel (1875) found that arsenious
acid checks germination and kills the embryo at relatively feeble doses,
·25 gm. to 90 gm. water.[10] Guthrie and Helms (1903–4–5) carried out a
systematic series of experiments to test the effect of arsenic compounds
upon different farm crops. Various amounts of arsenious acid were added
to soil in pot experiments, and the seeds of the several crops were
then sown. With barley, wheat and rye 0·10% arsenious acid had little
or no effect on germination, while an increase in the poison exercised
a retarding action. Maize could withstand 0·40% arsenious acid without
retardation being perceptible. The aftergrowth with the different crops
varied considerably. The wheat plants with 0·10% arsenious acid grew all
right at first, but later on they developed weakly. The toxic action
increased rapidly as the strength of the poison rose in the different
pots. Barley proved even more sensitive than wheat, for even 0·05%
arsenious acid affected the growth adversely. After a time the plants
with 0·05–0·06% recovered and grew strongly, though not so well as the
controls, but those with 0·10% practically died off. Rye behaved in the
reverse way from wheat. The plants with 0·10% were slightly checked at
first but later recovered and made growth quite equal to the check plants.
Growth was stunted with 0·20% arsenious acid, and the plants were killed
with 0·30%, so that rye is far less sensitive than barley. With maize the
growth was slightly affected with 0·05% As₂O₃, and increasingly so with
greater quantities. It was also found that the action of 0·8% As₂O₃ was
strongly adverse to the germination of all plants, and that above this
strength germination was altogether prevented.
[10] In the present state of our knowledge such a concentration
seems relatively strong!
The results show very clearly how impossible it is to draw any general
conclusions with regard to the action of arsenic compounds on plants, as
they emphasise the strong individuality of the species in their reaction.
3. _Do arsenic compounds stimulate higher plants?_
The question of stimulation due to arsenic does not seem to have engaged
the attention of investigators to any extent. Water culture experiments at
Rothamsted have so far yielded negative results, and no stimulation has
yet been obtained with any plant, with the possible exception of white
lupin with sodium arsenite. In a single series a stimulus was suggested,
beginning to make itself felt at 1/500,000, rising to an optimum at
1/10,000,000. No stress can be laid on this result, as it is never safe
to draw any certain conclusions without several repetitions of the same
experiment. With arsenic acid on barley a possible stimulus is sometimes
indicated to the eye, the plants being fine and of a particularly healthy
dark colour, but this is not corroborated by the dry weights. Additional
tests were made with peas and barley, treated with sodium arsenite and
arsenate, the dilutions being carried down to 1/250,000,000, but no
evidence of stimulus was obtained, so that it hardly seems possible that
arsenic can act as a stimulative agent for these two plants when grown in
water cultures. It had been thought that the failure to find a stimulation
point hitherto might be due to the too great concentration of the toxic
substance rather than to the actual inability of the poison to stimulate,
but this hypothesis must now be dismissed so far as these plants are
concerned.
III. EFFECT OF ARSENIC COMPOUNDS ON CERTAIN OF THE LOWER PLANTS.
1. _Algae._
Loew (1883) was sceptical concerning the specific toxicity of arsenic for
plant protoplasm. He was convinced that arsenic and arsenious acid were
poisonous to algae, not because of their specific character as arsenical
compounds, but because of their acid nature, algae being peculiarly
sensitive to any acid, and he maintained that these substances were not
more poisonous than vinegar or citric acid. He placed various species
of _Spirogyra_ in solutions of ·2 gm. potassium arsenate per litre water
(1/5000), and found that the algae grew well without making any abnormal
growth in a fortnight, showing hardly one dead thread. Some of this
alga was then transferred to a 1/1000 solution of potassium arsenate.
This suited it excellently and it increased and the appearance under
the microscope was very fresh and strong, which was attributed more to
the potash than to the arsenic acid. Loew maintained that for the lower
animals and for many of the lower plants arsenic in the form of neutral
salts is not a poison. When the differentiation of the protoplasm into
certain organs reaches a specific degree in the higher plants, then the
poisonous action of the arsenic compounds comes into play.
Knop (1884) found that certain unicellular green algae grew luxuriantly
in a neutral solution supplied with potassium arsenate. Bouilhac (1894)
concerned himself chiefly with the possibility of the replacement of
phosphates by arsenates. He recognised that the influence of arsenic
is not the same on all species of plants, so he confined his attention
to certain of the algae. _Stichococcus bacillaris_ Naegeli was found
to live and reproduce itself in a mineral solution containing arsenic
acid. Even in the presence of phosphoric acid the arsenic acid favours
growth, the best dose being about 1/1000. The arsenic acid is capable of
partly replacing phosphoric acid. Other species of algae, _Protococcus
infusionum_, _Ulothrix tenerrima_, and _Phormidium Valderianum_ invaded
the original culture of Stichococcus from the atmosphere, but with no
arsenic or phosphoric acid their development was poor. The jars with
arsenic compounds were invaded by still more species which grew strongly.
Under these conditions it is evident that these algae are capable of
assimilating arsenic, and the addition of arsenic acid to a solution
free from phosphoric acid is sufficient to enable these algae to live
satisfactorily, the arsenates in this case replacing the phosphates.
Ono (1900) found that algae are favourably influenced by small doses of
poisons, the optimal quantity for algae being lower than that for fungi.
Protococcus showed a possible stimulus when grown in concentrations of
potassium arsenate varying from ·00002–·0005%. This possible stimulus is
interesting in view of the failure to observe stimulation in higher plants
by minute traces of arsenic.
2. _Fungi._
The effect of arsenic on fungi is of special interest in that it has a
direct bearing upon hygienic and commercial interests. Gosio (1892, 1897,
1901) found that certain of the fungi, _Mucor mucedo_ and _Aspergillus
glaucum_, will grow on various arsenic compounds and exercise a reducing
influence on them. These moulds attack all oxygen compounds of arsenic
including copper arsenite, and develope arsenical gases. Sulphur compounds
of arsenic are not influenced by these fungi. The same moulds would,
if cultivated in soil containing arsenic, develope hydrogen arsenide.
_Penicillium glaucum_ has such a strong and definite action on arsenic
compounds that he states that there is no doubt of the possibility of
poisoning by arsenical gas in a room hung with paper containing arsenic.
The compounds are so extraordinarily potent that if a mouse is placed
in a vessel in which the mould is strongly developed in the presence
of arsenic, it dies in a few seconds. _Penicillium brevicaule_ uses the
element in its development as a food substance. If material containing
arsenic is placed in contact with dead fungi no reaction occurs. The life
activity of the mould is evidently necessary for the reaction by which
the arsenic-containing gases are liberated. Csapodi (1894) put forward the
earlier results of Gosio and noted that the so-called arsenical fungicides
do not only fail to kill the mould fungi but actually favour their
development. This action explains why wallpaper containing arsenic is so
disadvantageous in a room. Abba (1898) severely tested Gosio’s method
of detecting arsenic by means of growths of _Penicillium brevicaule_,
whereby arsenic gases are liberated, vindicating the method completely,
and establishing the test as an exceptionally delicate one. Segale (1904)
applied the same method to the detection of the presence of arsenic in
animal tissues.
Ono (1900) grew Penicillium cultures with solutions of potassium arsenate
and found no important differences either of depression or stimulation.
Orlowski (1902–3) stated that small doses of arsenic (1/1000–1/100% Sodium
arsen--[11]) stimulate the growth of _Aspergillus niger_, larger doses up
to ⅛% retard growth, while ⅙% kills. Spores of the fungus taken from soil
containing arsenic are said to possess an immunity against arsenic, in
that they germinate in the presence of an arsenic content which rapidly
kills control fungi. This immunity is not specific for arsenic, but
extends also to other poisons. The chemical composition and water content
are not altered.
[11] The exact compound is not specified in the abstracted paper,
1/1000–1/100% Natr. Ars. being given.
_Conclusion._
The toxic effect of arsenic upon higher plants is much more marked
with arsenious acid and its compounds than with arsenic acid and its
derivatives. No definite evidence of stimulation has yet been obtained
with any arsenic compound, however great the dilution at which it is
applied. With certain algae a stimulus may occur, and it is possible that
arsenic acid is capable of replacing phosphoric acid to some extent under
certain conditions. With fungi the toxic effect of great concentrations
is marked with certain species, but there are others which are capable of
living happily on arsenical compounds and of liberating highly poisonous
arsenic gas.