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THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. [September 10 , 1570 . 
this little flask I put some of the same oxidized baryta 
which I used just now, and I will fit up the flask in 
such a manner that the gas, which will come off in a 
tolerably large quantity, can be collected for examina¬ 
tion. I will then put in contact with it a substance 
called silver oxide, first driving out of the flask all the 
air which it at present contains. Having driven out the 
air, I put in a few drops of the nitrate which I employed 
in the first instance, and then I will put in a solution of 
silver oxide, which is, in some respects, a good deal like 
this chromic acid, at all events in one important respect, 
for it has oxygen, which it can give up under sufficiently 
strong pressure. You now see there is a great deal of 
effervescence going on, and the gas which is coming off' 
from the little flask is rising into this jar, where we 
shall very easily be able to ascertain whether it is oxygen 
by the ordinary test. I should have been glad, if it had 
been convenient to do so, to give you one other instance 
in which a remarkable fact was discovered by Professor 
Brodie, viz. a case of an action of this kind, where the 
oxygen taken from the peroxide is in quantity exactly 
equal to the quantity of oxygen from the other body. 
Whilst that gas is collecting, I must enter shortly upon 
a theoretical question, apologizing for doing so, not that 
I am ashamed of it, for it is one of the most important 
theories we possess, but on account of the brevity with 
which I am compelled to treat it. Oxygen, in the 
free state, is admitted by chemists to consist of two 
little atoms linked together. In each of the com¬ 
pounds which I used there was one little atom of the 
kind. One atom leaves each of them, and when I get 
free oxygen, I affirm that there has been a process of 
combination, that the oxygen from the one substance 
actually combined chemically with the oxygen from the 
other. This is a theoretical result which has been, in 
great part, established by Sir Benjamin Brodie, with 
the help of materials from various sources. What I 
mentioned in the other case holds good equally in 
regard to chromic acid and the other cases in which 
there was apparently no definite proportion of the 
kind. There is an actual chemical combination 
between the oxygen of one substance and that of 
the other, it is not merely that the one substance is 
compelled to decompose because the other is decom¬ 
posing; there is between the one substance and the 
other an interchange, so that a constituent from each one 
combines with a constituent from the other. To do 
justice to the importance of this fact I should need to 
describe a great number of chemical reactions, which at 
present would be impracticable, but you may take my 
word for it, that the kind of process which I have de¬ 
scribed is now known to be one of the commonest in 
chemistry. The other day, when I mixed two of the 
commonest substances, there were interchanges between 
the constituents by a process perfectly analogous to that 
which takes place here. Here it happens, by an excep¬ 
tional circumstance, that the element which from the 
one body combines with the element of the other is of a 
like kind,, whereas, as a rule, you find that unlike ele¬ 
ments unite together in these processes. Thus it is that 
the anomaly which Liebig noticed ceases to be an ano¬ 
malyand is brought back to a case of common regular 
chemical action by the aid of that theoiy to which I 
have just alluded. 
To return to our experiment. This glass vessel is 
now full of the gas, and by applying a taper which has 
been lighted and blown out, but is still glowing, we 
shall find, on putting it into the jar, that it immediately 
ignites, which is the ordinary test of oxygen gas. By 
the aid of that theory, which has been discovered since 
the time of Liebig’s suggestion, this one case of appa¬ 
rently anomalous action has been proved to be a per¬ 
fectly normal and regular case of combination, and the 
same kind of thing hais been done with regard to other 
cases of the same description. A number of other pro¬ 
cesses which he classes with these may be shown to be 
due, not to any exceptional force that is at work in these 
cases, not to the force of any particular contagious action 
among chemical substances, but to the ordinary forces 
which induce chemical combination in the cases best 
known to us. Liebig’s theory of contagious action has 
been alluded to, by a high authority in this country 
upon philosophical matters, as being a law of chemical 
action of a generality comparable to the law of gravita¬ 
tion in astronomy, and for that reason, if for no other, it 
must be of considerable importance to know what bear¬ 
ing our most advanced knowledge has upon that law. 
I dare say you see the connection between it and the 
case of fermentation. I will not go into particulars, 
further than is necessary in order to show you the gene- 
ral analogy. 
First, I will take the case of alcoholic fermentation, as 
being the case best known. The ferment consists of 
little cells—which I hope I shall be able to show you at 
our next meeting—each one containing several chemical 
compounds, but itself a little living being. I will not 
say at present whether they are animals or plants. 
When you have these little organisms in water, or 
sugar, or in any moist substance, they are constantly, 
and of necessity, undergoing decomposition. You may 
arrest the decomposition by various agents, but if you 
do so, you kill them, or suspend their activity as yeast. 
No case is known to us of their acting like yeast with¬ 
out undergoing at the same time a process of chemical 
decomposition,—being broken up into simpler substances 
than those which were contained in them. I pointed 
out, last week, that the sugar which is being decomposed 
by the yeast is by that process being broken up into 
substances which were contained in it, and that was what 
Liebig noticed. He said that this yeast is a substance 
which tends to decompose,—it is breaking up into simpler 
substances, and it induces in these particles of sugar 
which are in contact with it a decomposition similar to 
its own. The action which it is undergoing is contagious, 
and passes over to the contiguous particles of sugar; and 
he adduced cases like that of oxygen, as affording ana¬ 
logies among simple well-known bodies. I think what 
I have said with regard to the case of oxygen will be 
sufficient to show you that in those simple cases the idea 
of contagion is certainly not applicable. 
A foreigner, who was describing some time ago the 
luxuriance of the crops in America, spoke of a bushel of 
mice being sown in a field, and a hundred bushels of 
mice being reaped. Of course, what he meant to say 
was maize , or Indian corn ; but I am reminded of that 
anecdote by the necessity I am under for a moment of 
asking you to consider for a while some living beings 
under their general functions only. Suppose you had a 
bushel of actual English mice, and you put them into a 
granary full of corn. There clearly would soon be a 
great change. You are supposed to know nothing more 
about the particular organization of these little beings 
than you know about the particular organization of the 
little yeast-cells. You know that these little things eat 
grain, and that in place of the grain which they eat there 
appear various products of decomposition, which can be 
easily collected and examined. They give off carbonic 
acid, and so forth, and if you examined the state of that 
granary after a time, you would find a chemical change, 
or rather a set of chemical changes, going on in the 
organisms of these mice. The substance of which they 
consist would be actually wasting away; they would be 
giving off carbonic acid, and nitrogenous and other 
products. And if you also examined the state of the coni 
which was there at first, you would find that it finally 
passed over into these same products; and I say that the 
theory of contagious action is as much applicable to the 
action of the bushel of mice in the granary full of wheat, 
as to the action of the yeast cells upon a solution of sugar. 
There is in the one case, as in the other, an assimilation 
by the living organism of the material upon which it 
acts. The materials undergo certain changes, of which 
