A History of Science, vol 4 - Henry Smith Williams (epub e ink reader .txt) 📗

merely incidental, and

in no sense causal. This opinion of the great German chemist was

in a measure substantiated by experiments of his compatriot

Helmholtz, whose earlier experiments confirmed, but later ones

contradicted, the observations of Schwann, and this combined

authority gave the vitalistic conception a blow from which it had

not rallied at the time when Pasteur entered the field. Indeed,

it was currently regarded as settled that the early students of

the subject had vastly over-estimated the importance of

micro-organisms.

 

And so it came as a new revelation to the generality of

scientists of the time, when, in 1857 and the succeeding

half-decade, Pasteur published the results of his researches, in

which the question had been put to a series of altogether new

tests, and brought to unequivocal demonstration.

 

He proved that the micro-organisms do all that his most

imaginative predecessors had suspected, and more. Without them,

he proved, there would be no fermentation, no putrefaction—no

decay of any tissues, except by the slow process of oxidation. It

is the microscopic yeast-plant which, by seizing on certain atoms

of the molecule, liberates the remaining atoms in the form of

carbonic-acid and alcohol, thus effecting fermentation; it is

another microscopic plant—a bacterium, as Devaine had christened

it—which in a similar way effects the destruction of organic

molecules, producing the condition which we call putrefaction.

Pasteur showed, to the amazement of biologists, that there are

certain forms of these bacteria which secure the oxygen which all

organic life requires, not from the air, but by breaking up

unstable molecules in which oxygen is combined; that

putrefaction, in short, has its foundation in the activities of

these so-called anaerobic bacteria.

 

In a word, Pasteur showed that all the many familiar processes of

the decay of organic tissues are, in effect, forms of

fermentation, and would not take place at all except for the

presence of the living micro-organisms. A piece of meat, for

example, suspended in an atmosphere free from germs, will dry up

gradually, without the slightest sign of putrefaction, regardless

of the temperature or other conditions to which it may have been

subjected. Let us witness one or two series of these experiments

as presented by Pasteur himself in one of his numerous papers

before the Academy of Sciences.

EXPERIMENTS WITH GRAPE SUGAR

“In the course of the discussion which took place before the

Academy upon the subject of the generation of ferments properly

so-called, there was a good deal said about that of wine, the

oldest fermentation known. On this account I decided to disprove

the theory of M. Fremy by a decisive experiment bearing solely

upon the juice of grapes.

 

“I prepared forty flasks of a capacity of from two hundred and

fifty to three hundred cubic centimetres and filled them half

full with filtered grape-must, perfectly clear, and which, as is

the case of all acidulated liquids that have been boiled for a

few seconds, remains uncontaminated although the curved neck of

the flask containing them remain constantly open during several

months or years.

 

“In a small quantity of water I washed a part of a bunch of

grapes, the grapes and the stalks together, and the stalks

separately. This washing was easily done by means of a small

badger’s-hair brush. The washing-water collected the dust upon

the surface of the grapes and the stalks, and it was easily shown

under the microscope that this water held in suspension a

multitude of minute organisms closely resembling either fungoid

spores, or those of alcoholic Yeast, or those of Mycoderma vini,

etc. This being done, ten of the forty flasks were preserved for

reference; in ten of the remainder, through the straight tube

attached to each, some drops of the washing-water were

introduced; in a third series of ten flasks a few drops of the

same liquid were placed after it had been boiled; and, finally,

in the ten remaining flasks were placed some drops of grape-juice

taken from the inside of a perfect fruit. In order to carry out

this experiment, the straight tube of each flask was drawn out

into a fine and firm point in the lamp, and then curved. This

fine and closed point was filed round near the end and inserted

into the grape while resting upon some hard substance. When the

point was felt to touch the support of the grape it was by a

slight pressure broken off at the point file mark. Then, if care

had been taken to create a slight vacuum in the flask, a drop of

the juice of the grape got into it, the filed point was

withdrawn, and the aperture immediately closed in the alcohol

lamp. This decreased pressure of the atmosphere in the flask was

obtained by the following means: After warming the sides of the

flask either in the hands or in the lamp-flame, thus causing a

small quantity of air to be driven out of the end of the curved

neck, this end was closed in the lamp. After the flask was

cooled, there was a tendency to suck in the drop of grape-juice

in the manner just described.

 

“The drop of grape-juice which enters into the flask by this

suction ordinarily remains in the curved part of the tube, so

that to mix it with the must it was necessary to incline the

flask so as to bring the must into contact with the juice and

then replace the flask in its normal position. The four series of

comparative experiments produced the following results:

 

“The first ten flasks containing the grape-must boiled in pure

air did not show the production of any organism. The grape-must

could possibly remain in them for an indefinite number of years.

Those in the second series, containing the water in which the

grapes had been washed separately and together, showed without

exception an alcoholic fermentation which in several cases began

to appear at the end of forty-eight hours when the experiment

took place at ordinary summer temperature. At the same time that

the yeast appeared, in the form of white traces, which little by

little united themselves in the form of a deposit on the sides of

all the flasks, there were seen to form little flakes of

Mycellium, often as a single fungoid growth or in combination,

these fungoid growths being quite independent of the must or of

any alcoholic yeast. Often, also, the Mycoderma vini appeared

after some days upon the surface of the liquid. The Vibria and

the lactic ferments properly so called did not appear on account

of the nature of the liquid.

 

“The third series of flasks, the washing-water in which had been

previously boiled, remained unchanged, as in the first series.

Those of the fourth series, in which was the juice of the

interior of the grapes, remained equally free from change,

although I was not always able, on account of the delicacy of the

experiment, to eliminate every chance of error. These experiments

cannot leave the least doubt in the mind as to the following

facts:

 

Grape-must, after heating, never ferments on contact with the

air, when the air has been deprived of the germs which it

ordinarily holds in a state of suspension.

 

“The boiled grape-must ferments when there is introduced into it

a very small quantity of water in which the surface of the grapes

or their stalks have been washed.

 

“The grape-must does not ferment when this washing-water has been

boiled and afterwards cooled.

 

“The grape-must does not ferment when there is added to it a

small quantity of the juice of the inside of the grape.

 

“The yeast, therefore, which causes the fermentation of the

grapes in the vintage-tub comes from the outside and not from the

inside of the grapes. Thus is destroyed the hypothesis of MM.

Trecol and Fremy, who surmised that the albuminous matter

transformed itself into yeast on account of the vital germs which

were natural to it. With greater reason, therefore, there is no

longer any question of the theory of Liebig of the transformation

of albuminoid matter into ferments on account of the oxidation.”

 

FOREIGN ORGANISMS AND THE WORT OF BEER

 

“The method which I have just followed,” Pasteur continues, “in

order to show that there exists a correlation between the

diseases of beer and certain microscopic organisms leaves no room

for doubt, it seems to me, in regard to the principles I am

expounding.

 

“Every time that the microscope reveals in the leaven, and

especially in the active yeast, the production of organisms

foreign to the alcoholic yeast properly so called, the flavor of

the beer leaves something to be desired, much or little,

according to the abundance and the character of these little

germs. Moreover, when a finished beer of good quality loses after

a time its agreeable flavor and becomes sour, it can be easily

shown that the alcoholic yeast deposited in the bottles or the

casks, although originally pure, at least in appearance, is found

to be contaminated gradually with these filiform or other

ferments. All this can be deduced from the facts already given,

but some critics may perhaps declare that these foreign ferments

are the consequences of the diseased condition, itself produced

by unknown causes.

 

“Although this gratuitous hypothesis may be difficult to uphold,

I will endeavor to corroborate the preceding observations by a

clearer method of investigation. This consists in showing that

the beer never has any unpleasant taste in all cases when the

alcoholic ferment properly so called is not mixed with foreign

ferments; that it is the same in the case of wort, and that wort,

liable to changes as it is, can be preserved unaltered if it is

kept from those microscopic parasites which find in it a suitable

nourishment and a field for growth.

 

“The employment of this second method has, moreover, the

advantage of proving with certainty the proposition that I

advanced at first—namely, that the germs of these organisms are

derived from the dust of the atmosphere, carried about and

deposited upon all objects, or scattered over the utensils and

the materials used in a brewery-materials naturally charged with

microscopic germs, and which the various operations in the

store-rooms and the malt-house may multiply indefinitely.

 

“Let us take a glass flask with a long neck of from two hundred

and fifty to three hundred cubic centimetres capacity, and place

in it some wort, with or without hops, and then in the flame of a

lamp draw out the neck of the flask to a fine point, afterwards

heating the liquid until the steam comes out of the end of the

neck. It can then be allowed to cool without any other

precautions; but for additional safety there can be introduced

into the little point a small wad of asbestos at the moment that

the flame is withdrawn from beneath the flask. Before thus

placing the asbestos it also can be passed through the flame, as

well as after it has been put into the end of the tube. The air

which then first re-enters the flask will thus come into contact

with the heated glass and the heated liquid, so as to destroy the

vitality of any dust germs that may exist in the air. The air

itself will re-enter very gradually, and slowly enough to enable

any dust to be taken up by the drop of water which the air forces

up the curvature of the tube. Ultimately the tube will be dry,

but the re-entering of the air will be so slow that the particles

of dust will fall upon the sides of the tube. The experiments

show that with this kind of vessel, allowing free communication

with the air, and the dust not being allowed to enter, the dust

will not enter at all events for a period of ten or twelve