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1905] BARNES—THE THEORY OF RESPIRATION 89 
teids; second, the course of combustion, especially at low tempera- 
tures; third, the nature of anaerobic respiration, and its relation 
to aerobic respiration. Let me speak of these in order. 
CHEMISTRY OF THE PROTEIDS. 
A knowledge of the proteids, complex as they are, could be 
obtained only by a study of their decomposition products. Now 
there is a very remarkable uniformity in these decomposition prod- 
ucts. No matter what the organism from which they are derived, 
no matter how simple they are or how complex, when broken up by 
the process of digestion, or by boiling with acids, they yield invari- 
ably a series of products which have become in the last few years 
much better known. These are amino- or amido-acids; such sub- 
stances as leucin, tyrosin, arginin, glutamin, glycocoll, etc. Materials 
of this kind are invariably present, and certain ones are so invariably 
present that they can be used as the basis of distinctive tests for the 
occurrence of digestion or similar decompositions of proteids. This 
gave a clue to the nature of proteids which was followed by several 
observers, notably by Kosset, in the study of what are believed to 
be the very simplest proteids, because of the fewness and uniformity 
of the fractions into which they break up. These are the prota- 
mines. It has become clear from the study of these simple proteids 
that they are made up in somewhat the same way as the polysac- 
charides among the carbohydrates, that is by linking together a 
series of the amido-acids. This is possible because the amido- 
acids have a peculiar construction. They are, so to speak, different 
on different sides. On one side is-an acid group and on the other a 
basic group; and so the amido-acids can hang together in chains, 
or even be condensed or polymerized to make a simple proteid. 
Among the amido-acids, as in the carbohydrates, there are certain 
atomic groups, like CH,, CH,, CHOH, CH,OH, COOH, etc., 
which recur again and again, and in such groups the possibility of 
replacing a hydrogen atom or a hydroxyl radicle by some other 
atomic group is very great. 
Note, for instance, the comparatively simple acetic acid, CH,’ 
COOH. If we replace one of the three H atoms by the amido group, 
NH,, we have at once an amido-acid, glycocoll, CH 2(NH,)° COOH, 
