Owing to the fact that they often flatly contradict one another, medical
experts do not stand very high in popular repute; nevertheless, it is a
positive fact that a single medical expert is worth half Scotland Yard
in the detection and prevention of crime. Thousands of rivals in love,
disagreeable husbands, dangerous political agitators, harsh masters and
mistresses, rich uncles, and people of that sort, would be popped off
with a few grains of arsenic, or a drop of prussic acid, only that it is
well known the doctor has the eyes of a hawk for poison. And, on the
other hand, many and many a family is saved from the suspicion attaching
to the sudden death of a member, and even many an innocent man from the
scaffold, by the proof of natural death which the doctor supplies.
Although great poisoning, shooting, stabbing, and other homicidal trials
have a wonderful fascination for all newspaper readers, very few fully
appreciate the medical evidence, which is usually the most important
link in the chain. The evidence is of three kinds--that of the ordinary
medical man, who sees the patient dying, perhaps, and performs the
post-mortem; that of the chemist, who, in his quiet laboratory, traces
the poison or identifies the blood stain; and that of the expert, who
gives his inference from the facts stated by the first two. It is these
experts who often differ from one another.
In a large number of cases the _post-mortem_ examination is the first
step in unravelling a mystery.
The man who performs it is not to be envied, for the smallest scratch on
his hand may admit a dose of deadly poison.
[Illustration: THE OLD STYLE DETECTIVE--EXAMINING SCENE OF MURDER.]
Many medical men, indeed, wear rubber gloves, and those less careful
generally cover their hands with a layer of sticky ointment. It takes
from two to four hours to do the job thoroughly.
But it is not all cutting up, as most people think. The first thing done
is to notice the position of the body, and whether there are any
weapons, bottles, or glasses near.
Then it is examined from head to toe for scratches, cuts, bruises,
moles, tattoo marks. Everything about the hair, eyes, teeth, nose, ears,
and other parts, is written down. The height, the age, the muscular
development, are all noted.
Of course, this inspection alone often reveals the cause of death.
Suppose, however, that no external injury is found and no organ is
diseased, the suspicion of poisoning naturally arises. In that case, the
doctor looks for certain marks that the commonest poisons make, and then
he places the stomach and other parts in glass jars, which are securely
covered, sealed, labelled, and handed to the analyst.
Poisoning is not much favoured by the Briton as a means of killing
either himself or anybody else. He generally does the deed in a more
open, if more brutal, way. But it is to be feared that a great many more
people get rid of undesirable contemporaries in this manner than is
popularly supposed.
[Illustration: THE DETECTIVE--NEW STYLE--IN THE LABORATORY.]
Probably, in most cases, the ordinary medical attendant is able to tell
whether a person is dying a natural death or is being carried off by
some deadly drug. His position, however, is not a pleasant one. It is
impossible to be certain; and, in order to make a full investigation, he
must suggest either that the victim is committing suicide, or that
someone else, perhaps his wife or son, is committing murder. And, after
all, the signs in the living are very obscure. Of course, if a person is
foolish enough (as many are) to drink sulphuric or nitric acid, his
mouth and throat are burned as if he swallowed coals of fire, the
former leaving black and the latter yellow stains; but when the poison
is arsenic, or opium, or strychnine, the symptoms are very like those of
certain diseases.
When the cholera was last in London, a father, mother, son, and daughter
dined together. Immediately after dinner, all, except the son, became
suddenly ill, and died in a few hours, with the symptoms of arsenic
poisoning.
The son, who was always quarrelling with the rest of the family, was
arrested on the doctor's report and charged with murder. But a
_post-mortem_ examination showed that cholera was the real cause of
death.
Apoplexy, in the same way, is very like opium poisoning; and
hydrophobia, lock-jaw, and even some cases of hysteria, closely resemble
poisoning by strychnine.
Still, when a healthy man grows suddenly ill soon after a meal, the
doctor keeps his eyes open, and if death follows he has a pretty shrewd
idea of what caused it.
At all events, he feels perfectly justified in assuming that the case is
not a normal one. He therefore hands over to the analyst the jars and
other receptacles containing the portions of the subject's body likely
to bear traces of the poison, knowing full well that if any poison is
there the analyst will infallibly detect it.
The analyst begins by making a series of what may be called "brews,"
mincing, pounding, boiling, cooling, filtering, decanting, and
distilling, over and over again. In these operations various solvents
are used in succession, plain water separating out one class of poisons,
alcohol dissolving out another group, benzol taking up a third, naphtha
a fourth, ammonia a fifth, and so on. This preliminary work takes, not
hours, but days to perform. At an early stage in it the operator
discovers such volatile poisons as prussic acid, chloroform, carbolic
acid, and phosphorus, if any of them be present. Later on he comes
across the alkaloids, such as strychnine, digitalin, cantharidin, and
other terrible poisons of that class.
Finally, the residue of the animal matter with which we have supposed
the medical detective to be experimenting is mixed with hydrochloric
acid, and distilled once again, after which it can contain no poison
except one of the metals.
Thus, in the course of his examination, the analyst has made a number of
decoctions, in one of which the poison is certain to be. In each
decoction there may be any one of several groups of poisons.
In which is it, and what is it? After all this patient labour the
solution is still far off. It may be a ptomaine from poisonous fish or
decayed meat, a deadly berry, or leaf, or root, a small quantity of
morphia, or phosphorus, or lead, or arsenic, or antimony.
Each brew is tested in turn. But, as illustrating the general procedure,
take the last, which contains whatever metal may have had the fatal
result. First, the chemist tests with "group reagents." He knows that if
he puts into the glass containing the last brew certain bodies in
succession, some metals, if they are there, cannot be kept from rushing
into the arms of one, others will as passionately embrace another,
others still will unite with a third, while some will always repudiate
any alliance. There are in all cases signs of the union, when it takes
place, such as a blue or white or red colour, or a powder falling to the
bottom, or a fizzing of escaping gas.
In practice the analyst puts a little of the brew in a small glass
test-tube, pours in some distilled water, and carefully drops in some
hydrochloric acid. Now, if there is either silver, mercury, or lead, in
the brew, down goes a white powder; if none of these things is there, no
change follows.
Next he adds some sulphuretted hydrogen water, a sort of aerated water
smelling of rotten eggs. If tin, platinum, bismuth, cadmium, arsenic, or
one of several other metals, is in the brew, a coloured powder falls to
the bottom. Should nothing occur, he adds other things, until he has
tested for five groups of metals.
When he finds a poison belonging to a certain group, he has still to
ascertain which of five or six bodies it is.
For instance, after adding the first two test-liquors, if he sees a
yellow coloration or precipitate, he knows that he has either arsenic or
tin or cadmium. He then adds some strong ammonia, after boiling the
liquid till the smell of rotten eggs has disappeared. If the powder
dissolves, and the colour goes, he is quite sure he has found arsenic.
In this business-like way the murderer is convicted.
But now arises the necessity for making doubly sure, and another kind of
test altogether is employed. Life and death hanging on the result, the
test must be beyond all doubt. But arsenic is one of those
self-assertive things about whose presence there cannot be the most
infinitesimal doubt. Give a man a particle the size of a mustard-seed,
and let him swallow it. When he dies bury him, and let him lie under the
earth for a quarter of a century. Then gather the few remnants, give
them to a chemist, and he will return you a considerable portion of the
poison in the same state as that in which it was administered.
[Illustration: ARSENIC CRYSTALS.]
Probably the most famous special test for arsenic is Marsh's, the
invention of a Woolwich chemist, and equally famous is Reinsch's, which
is performed as follows: The suspected liquid is put in a little glass
test-tube with some hydrochloric acid. Then a small bit of bright copper
is dropped in, and the test-tube is held over a flame.
Now, arsenic has the wildest love for copper, and every trace of it in
the tube flies to the slip of copper and covers it with a grey coat.
Another metal does the same, certainly, but they can be distinguished
subsequently.
Presently the copper is removed, washed, dried, and placed in a tough
glass tube, very narrow at one end. This is held over a flame and
carefully heated, and then a phenomenon, not unknown, either, in the
loves of mortals, occurs. The arsenic abandons the copper, and clings in
crystals to the sides of the glass tube, where it can be recognised by
the aid of a magnifying-glass or microscope; and if the crystals are
heated with a bit of acetate of potash the odour drives the chemist from
the room.
To this curious fact, that arsenic loves copper when it is wet with warm
hydrochloric acid, and hates it when it is hot and dry, is due the
discovery of many a crime.
It is already plain to the reader that the analyst's task is not an easy
one. Sometimes the analytical examination is of vast extent; sometimes
it is greatly narrowed by hints from the family doctor. These hints are
interesting, and show that the doctor is, when he knows his business, a
real and a very skilful detective.
The doctor's eye is a wonderful one. When he enters a room, he not only
measures the patient from head to toe, notes the colour of his face, the
posture of his body, the signs of pain, stupor, or perhaps sham; but
observes the manner of the other people present, and sees every bottle,
glass, and cup in the place.
Now, although sudden death is usually from natural causes, when it
occurs soon after food there is always suspicion, as we have said. So,
if the doctor perceives great pain and nausea, he thinks of arsenic,
antimony, tinned meats, mushrooms, toadstools, and other things; if the
pupil of the eye is as small as a pin-head, and the sick man is drowsy,
he thinks of opium; if something seems to have caught hold of the
patient's heart, and to be squeezing it like a sponge, he thinks of
digitalis; if the poor victim is being worked like a puppet, and his
pupils are large with fear, he thinks of strychnine; if there is great
thirst, colic, and cramps in the legs, he thinks of lead.
[Illustration: IS IT ARSENIC, OR NOT?]
He knows that prussic acid kills like a bullet in the brain--a glass of
cold water taken while hot from exercise may do the same--and he smells
for it. He can also tell if it is phosphorus or carbolic acid, by the
smell.
He knows that relatives usually kill each other by means of particular
poisons; that other poisons are used for suicidal purposes; that the
photographer takes cyanide of potassium, the medical man and chemist
prussic acid or morphia, the poor man vermin-killer or oxalic acid, or
carbolic acid, or some such agonising destroyer of life. And thus,
though all poisons lead to the same end--stoppage of the breathing and
blood circulation--yet each has its own particular way of sending the
soul to eternity. He can therefore often tell the analyst detective how
to take a short cut.
[Illustration: THE SPECTROSCOPE--AN INSTRUMENT THAT HAS BEEN FATAL TO
MANY CRIMINALS.]
By the way, there is no such thing as a slow poison--that is, a poison
which, taken to-day, does not show its effects for weeks. This is a
fiction of the novelists. On the other hand, there is--except in the
case of prussic acid and nicotine--no death straight away after taking
poison, as one sees it on the stage, Shakespeare notwithstanding.
An actual case will show that the discovery of murder by the doctor and
analyst is not always plain sailing.
A good many years ago, a Mr. Sprague was tried for the murder of the
Walker family by means of the well-known poison of the deadly
nightshade. The medical evidence showed clearly that they all died from
belladonna poisoning, and belladonna was found in the rabbit-pie they
had for dinner. A common-sense jury, however, acquitted the prisoner;
and only recently have medical men solved the mystery by discovering
that rabbits can eat any quantity of this plant without suffering harm,
while their flesh becomes fatally poisonous.
A second case shows what wonders the chemists can work. A surgeon's wife
died from corrosive sublimate, given in a draught by her husband. He
said that, in making up the draught, he mistook a bottle of mixture,
which he had prepared for a sailor, for the water-bottle, and had poured
some of it into his wife's draught. The sailor's mixture was analysed,
and it certainly contained corrosive sublimate; but, not content with
finding the poison, the analyst measured the quantity present, and,
while the sailor's mixture contained only ten grains to an ounce of
liquid, the wife's draught contained fifteen grains, showing that the
surgeon's ingenious explanation was a lie!
Blood is so characteristic a fluid that it might be supposed a skilful
analyst could never have any difficulty in recognising it. Of course, if
he were given, say, a cupful in its ordinary state, he could not make a
mistake. But he never gets a chance of earning his fee so easily.
When the police seek his assistance they give him, perhaps, a suit of
dirty clothes, which may be stained by two or three small dark spots
that might be anything.
Or perhaps he is given a rusty knife, or a perfectly clean hatchet, and
is asked to say if there is blood on it. And when he comes into court he
is expected to tell the jury whether the blood is human or animal, how
old it is, was it spilled from a living blood vessel, and in what part
of the body was this blood vessel.
Take an actual case. Years ago a celebrated murder was committed in
Eltham, and in the report of Dr. Letheby, the analyst, is the following
note:--
"On the evening of May 3rd I received from Mr. Mulvaney" (of the police)
"a brown paper parcel containing a pair of dark trousers, a man's shirt,
and a man's wide-awake hat. On the following evening I received from Mr.
Mulvaney a brown paper parcel containing a lock of hair, a pair of men's
boots, and a plasterer's hammer."
These were all very dirty, but that did not prevent the analyst from
finding a number of blood stains and hairs, and giving valuable and
decisive evidence at the trial.
What the analyst first does, when he receives such an article as a pair
of trousers, is to scrutinise every inch of its surface with a
magnifying glass. If he finds a little lump of dark-coloured stuff he
scrapes it off and puts it into a watch glass. If he discovers merely a
dark stain, he cuts out the piece of cloth and puts it into a small
quantity of distilled water.
[Illustration: HUMAN BLOOD MAGNIFIED 400 TIMES.]
Now he has to find out whether the suspicious-looking thing is really
blood, or whether it is merely red paint, or logwood, or cochineal, or
madder, or iron-mould. There are three ways of doing this, and he nearly
always utilises them all.
[Illustration: PIG'S BLOOD MAGNIFIED MANY TIMES.]
First, there is the marvellous spectroscope test. This test will reveal
the presence of the minutest trace of blood, and it is practically
infallible. It depends on the curious property, possessed by nearly all
bodies, of absorbing certain parts of the light that passes through
them. Sunlight passing through a prism is split up into the familiar
seven colours of the rainbow. But if a little blood dissolved in water
is placed in a glass tube, and if the light is made to pass through it
on its way to the prism, the blood takes something out of it; for now
among the seven bright colours are seen two dark bands near the middle
of the yellow ray. Nothing but blood gives these two bands in that
particular place, with the exception of two or three substances that are
not likely to be found on criminals' clothes. These are cochineal, mixed
with certain chemicals, hot purpurin sulphuric acid, and the red dye of
the banana-eater.
Blood, however, changes after it is shed. In stains a few weeks old the
colouring matter changes from what is technically called haemoglobin to
methaemoglobin, and, later still, to haematin. All of these give different
spectra. The analyst has standard spectra already mounted, and he
invariably looks at the mounted or standard specimen and the suspected
liquid at the same time, placing them side by side, so that a mistake is
impossible. All the red colours in the world, in fact, have been tried,
and, with the exceptions named above, none of them gives a spectrum like
the colouring matter of blood in any of its forms.
But though the spectroscope is a certain discoverer of blood, it can
draw no distinction between human and animal blood. That duty remains to
the microscope.
[Illustration: LITTLE LINKS IN THE CHAIN OF EVIDENCE.--THE CORPUSCLES IN
THE BLOOD OF DIFFERENT CREATURES.
Man. Mouse. Horse. Camel.
Toad. Pike. Pheasant. Pigeon.]
With the microscope can be seen those red corpuscles which, in some
mysterious manner, seize on the oxygen of the air as it passes into the
lungs, shoulder it, so to speak, and rush away with it, like so many
ants, to the remotest parts of the body. Unfortunately, they can only be
seen in blood that has not been very long shed--that is to say, some
weeks or months. To see these, the analyst scrapes the little clot from
the piece of cloth, or wood, or iron, and places it on a slip of glass;
over this he carefully lays the little film called a cover-glass; and
then he gently places, at the edge of the latter, the tiniest possible
drop of water. This gradually insinuates itself, and soon dissolves the
blood clot; and, when the mixture is placed under a microscope
magnifying from 300 to 500 diameters, he sees one of several pictures.
The various shapes and arrangements taken by these little bodies are
illustrated on the following page. Small as they are--it would take
12-1/4 millions to cover a square inch--they have the most peculiar way
of behaving, and only the practised eye of the microscopist can
recognise them in all their disguises.
[Illustration: HUMAN BLOOD CORPUSCLES UNDER THE MICROSCOPE.]
Individually, the blood corpuscle is just like a tiny round biscuit, and
measures 1/3200 to 1/4000 of an inch across its face. It is these two
factors, the shape and measurement, which enable the medical man to say
whether the blood is human. The picture above shows how a corpuscle
looks under the microscope. Looking at its face, it is like a
thick-edged biscuit, with a dark depression in the centre. Some are
turned sideways in our illustration. These exist in blood and nothing
but blood, so that, when the spectroscope fails, the microscope
succeeds.
But it is not always that the analyst can get sufficient blood to place
under the microscope. Perhaps he gets a piece of cloth saturated with a
trifle of red fluid which he cannot scrape off, or perhaps he gets a
stain some months or years old (Dr. Tidy identified a blood stain one
hundred and one years old), in which the corpuscles are destroyed. Or
perhaps he gets a garment which has been carefully washed, on which
there is only the faintest trace of colouring matter. Even then the
microscope tells whether the stain is blood.
Our detective mixes the particle of blood-stained wood, or earth, or
dust, or cloth fibres, with water and caustic potash, and filters it.
Then he takes a drop of the liquid and places it in the useful
watch-glass. Into this he puts some glacial acetic acid and a crystal of
ordinary table salt. He heats the mixture and lets it cool. And, if it
is blood, he gets peculiar crystals visible under the microscope. These,
by the way, differ to some extent in different animals.
Another test is so new that it has not yet been given a fair trial. It
is as follows:--If a fairly large quantity of blood can be got, it is
burned, and the ash is analysed. Now, there are two salts always in
blood--sodium and potassium salts. But, while the quantity of the former
in human blood is usually twice that of the latter, it is six times as
great in the sheep's blood, eight times as great in the cow's blood, and
sixteen times as great in the blood of a fowl. Very important results
are expected from this principle.
Reliable as are the microscope and spectroscope, the analyst always uses
the third means at his disposal--the chemical test. For instance, he
gets a knife covered with dark red stains. Are they blood, or are they
only the rust formed by vinegar or the juice of a lemon that has
deceived so many people? Assuming that he has removed the stain, he
places the matter in any kind of tiny vessel, and drops in some tincture
of galls. If the thing is only rust, he has some excellent blue ink; if
it is blood, he finds that a reddish powder makes its appearance.
[Illustration: BURNING CLOTH IN THE LABORATORY.]
Perhaps he gets a handkerchief with a red stain. If the cloth is white
he can apply a test direct to it, but as a rule he prefers to dissolve
the stain out. Now, a handkerchief may be stained with a number of
different reddish things--Condy's fluid, jam, cochineal log-wood, or red
paint. He puts a drop of ordinary ammonia on the cloth. If the stain is
caused by currant, gooseberry, or other fruit juice it turns blue or
green; if it is Condy's fluid it becomes blue; if it is cochineal it
becomes crimson, and so on. But if it is blood, it does not change in
the least. Other tests might be described, but we have not the space.
Probably the most interesting of all his duties to the analyst is that
of judging from what animal the blood stains came. This can be done only
in some cases; that is, when the blood is not quite so old that the red
corpuscles have entirely lost their shape.
Of course this is a matter of the greatest importance when a man is on
his trial; for, in the first place, every spot of blood found on his
belongings is supposed to have come from his victim, although it may be
nothing more than the blood of a fish; and, in the second place, the
stock explanation of blood stains on his clothing offered by a prisoner
is that they came from some animal he killed. The plan is to ask him
what animal. Five times out of six he will say a domestic fowl or some
kind of bird especially if he is a poacher who has killed a
gamekeeper--and then he is done for.
Look at the pictures on page 149 and you have the whole thing in a
nutshell. It will be seen that the red corpuscles of the blood of birds,
reptiles, and fishes (with the exception of the cyclostomata) are oval,
while those of mammalian blood are round. Here is, at once, a sure way
of differentiating mammalian blood from that of the other three great
classes of animals. The only difficulty is that blood corpuscles get out
of shape, under certain circumstances, and are no longer either oval or
round. But there is another difference. A mammalian corpuscle is of
uniform substance throughout: that of a fish, bird, or reptile has a
small, dense spot near the centre, called a nucleus. Snails, slugs,
worms, and other low forms of animal life do not come into the question
at all, for their blood is generally colourless, and, if not, it is
blue-green, violet, brown, being scarcely ever red, and then not from
the presence of corpuscles.
All that remains for the analyst, therefore, supposing he finds a round
corpuscle, is to say to what mammalian animal it belongs. (The llama,
alpaca, camel, and their kin, by the way, have oval corpuscles.)
How are the corpuscles of different mammalia to be distinguished under
the microscope? Merely by their size. They have all been measured with
the greatest care, a specially small unit of length, called a micron,
having been invented for the purpose. It is only 1/25000 of an inch
long, and, expressed in tenths of a micron, the average diameter of a
human blood corpuscle is 77; of a dog, 73; of a rabbit, 69; of a cat,
65; of a sheep, 50; of a goat, 41; and of an elephant, 94. But these are
average measurements, and some corpuscles are smaller, some larger.
[Illustration: MORE TINY CLUES.--HUMAN HAIR CONTRASTED WITH ANIMALS'
HAIR, WOOL, AND FIBRE.
Cat's Hair. Bat's Hair. Berlin Wool. Reindeer's Hair. Woody Fibre.
Human Hair. Fox Hair. Hare's Hair. Squirrel's Hair. Human Hair Bulb.]
Therefore, when it is a question of whether the blood is that of a dog,
pig, hare, rabbit, or man, he would be a daring man that would give a
decided opinion. But it is certainly possible to come to a safe
conclusion as to whether it is that of a human being or a sheep, goat,
or elephant.
Owing to the influence of disease on the blood, however, it is never
really safe to say absolutely "This is human blood," and, in fact, all
that is generally stated in evidence is whether it is mammalian.
There is one other important piece of work the medical detective can
perform in his laboratory, in the way of tracking criminals; that is to
distinguish hairs from vegetable fibres, and human hairs from animals'.
Our illustrations show how it is done. He simply places the thing to be
tested under the microscope, and--as he is acquainted with every
description of hair, cotton, wool, silk and other fibre--he can tell at
a glance what it is.
Hair is more like wool than anything else, but wool is irregular and
hair is pretty regular in breadth. The hair of an adult, also, has a
streak in the middle.
[Illustration: AN IMPORTANT CLUE--MEASURING FOOTSTEPS.]
We append accurate illustrations, from microscopic photographs, of the
hairs of many animals. Obviously, there is no difficulty to the
practised eye in distinguishing them. In fact, most animals' hairs can
be known by the naked eye, or with a small magnifying glass; but that of
skye terriers and spaniels is wonderfully like human hair.
On all these little things hinges, very often, the terrible issue of
"guilty" or "not guilty"!
Some years ago, a woman was found dead with a knife lying loosely in her
hand. This fact might mislead people into thinking it was a case of
suicide; but the fact that the knife was not held tight made the doctor
suspicious. He examined the blood on the knife, and found woollen fibres
which resembled those of the husband's clothes. This discovery so acted
on the husband that he confessed his guilt.
On another occasion a Taunton man was seen last in company with a man
subsequently found dead. In the Taunton man's possession was a knife
with a slight film of blood on the blade. He said he had been cutting
raw beef. The analyst easily showed, however, that the blood on the
knife came from a living animal; and, further, he found on it some
little scales from the lining of the human gullet. The Taunton man was
convicted.
A remarkable instance of the analyst's power was given in a Cornwall
murder case. A man was found with his head broken. On a hammer belonging
to a suspect were a couple of grey hairs. This hammer, however, had been
used for beating goat-skins, and, in fact, it was found in a hedge on
which a goat-skin was spread out to dry.
But the medical witness swore that the two hairs came from somebody's
eyebrow, and, on comparing them with the dead man's eyebrow, they
corresponded!
In one case a man was very near being hanged--and in the old days,
doubtless, he would have been hanged--mainly because a knife with red
stains was found in his possession. The medical witness found that they
were rust caused by an acid fruit; and then it was found that the
prisoner had actually used a knife for cutting a lemon. But, curiously,
this stain is so very like blood that the naked eye of even the most
skilful medical jurist would be deceived by it.
[Illustration: FOOTPRINTS.--(1) WHEN RUNNING. (2) STANDING. (3) WALKING.]
Footprints are usually left to the police to interpret. But, very
probably, the result is often a miscarriage of justice. When the police
are working up a case they would not be human if they did not view
evidence with a certain amount of bias. The scientific witness, on the
other hand, has no personal interest one way or the other. And,
moreover, the comparison of a naked foot with its supposed print on the
ground, or the fitting of a boot to a boot-mark, is a process requiring
not only the most exact measurements, but consideration of the kind of
mark made on different kinds of soil, and in the various positions taken
by the foot in standing, walking, and running. In running we press
mainly on the toes, and in walking the greater part of the foot comes
down, and the longer the foot rests on the ground the deeper is the
impress. In fact, an expert can make a pretty shrewd guess as to the
rate at which the owner of the foot was travelling, by considering the
size and depth of the footprint.
In order to make a comparison a cast has to be taken, if the mark is on
soft ground. This is done by heating the footprint with a hot iron, and
filling it in with paraffin. From this a plaster cast is taken, and it
can be preserved for comparison until someone is arrested.
When the footprint is found in snow, gelatine is used to take the form
of it, and from this also a plaster cast is made.
Of course, these operations have to be carried out with the greatest
care, for footprints are frequently the strongest pillars of an
indictment. In order to compare the foot of the suspected person, he is
made to walk, stand, and run, over a surface similar to that on which
the incriminating print has been found. There is one case in which the
scientific detective is certain--when the person has stood still on
soft, but firm and tenacious, soil.
The footprints represented in our sketch are those of course of naked
feet, which give the clearest impression. But a corresponding variation
occurs in all footprints made by persons wearing boots, so that the
attitude or action of the wearer is easily told.
Now and again some deformity, such as the possession of six or of only
four toes, leaves no room for doubt. When the mark has been made by
boots, rather than with the naked foot, it is frequently easy to
identify it by the arrangement and number of the nails, by a missing
nail, or a patch, or a hole, or a heel worn on one side.
Nevertheless, footprints are, to the medical man, exceedingly doubtful
evidence, although from this view the police, and probably the jury,
differ.
Taking him altogether, the medical detective does his work with a skill,
certainty, and absence of prejudice, worthy of emulation by all engaged
in hunting down the criminal. The story of modern medical detective work
is one of the most romantic of our times.
[Illustration: MURDER OR SUICIDE--WHICH?]