38 CHEMICAL CONSTITUENTS OE BOD Y AND FOOD. 
The active radicle cannot be one which contains phosphorus, since the 
synthesized colloids are free from that element. It may possibly be the 
amido-fatty radicle in a high state of condensation. 
Lilienfeld and Wolkowicz, 1 by the condensation of amido-acid compounds, 
have obtained substances which resemble proteoses in their reactions. 
Theories of proteid constitution. — Tbe views of Schiitzenberger 
on this subject will have been gathered from the preceding section. 
There now remain to be mentioned some other theories on the subject, 
which are in part deductions from the work of others, and partake 
more of the nature of speculation than of hypotheses that have been 
tested by experiment. 
Pfiugers theory. — The distinction between non-living proteids and 
living protoplasm was noted as early as 1821 by Rudolphi, 2 who 
wrote : " The components of the dead and living body do not exist under 
the same chemical conditions." A few years later the distinction 
between living and non-living proteids was emphasised by John 
Fletcher. 3 Pfliiger's theory 4 was, however, the first intelligible one to 
explain such differences. The non-living proteids, such as are contained 
in white of c<^, are stable and indifferent to neutral oxygen; but when 
such proteids are assimilated — that is, become part of a living cell — the 
molecules live by breathing oxygen. The assimilation of a proteid is 
probably due to the formation of ether-like combinations between 
the molecules of living proteid and the isomeric molecules of the food 
proteid, water being eliminated : this process of polymerisation produces 
large and heavy but still simple molecules ; and during its occurrence 
the nitrogen of the non-living proteid leaves the hydrogen with which it 
is combined in the form of amidogen (NH,), and enters into combination 
with carbon to form the much more unstable substance cyanogen (CN). 
We thus find uric acid, creatine, guanine, etc, as products of proteid 
metabolism, while none of such cyanogen-containing molecules are 
obtainable from nun-living proteid. 
Pfliiger's theory was put forward in 1876; but in the light of Drechsel's 
later work, tbe part involving exchange of nitrogen between cyanogen and 
amidogen is rendered unlikely, and with that the whole theory mustprobably fall. 
Loew's theory. — The researches of Loew and Bokorny 5 have taken 
the same direction as those of Pfliiger, in that they are attempts to 
explain the distinction between living and dead protoplasm. Living 
protoplasm or proteid (in the cells of various algse) has the property of 
reducing silver from a weak alkaline solution of silver nitrate; dead 
proteid has no such effect: animal protoplasm is so quickly killed by 
silver nitrate, that it does not give the reaction. The conclusion 
formed is, that something of the nature of an aldehyde occurs in living 
protoplasm. Formic aldehyde is probably formed in plants by the union 
of carbon and water ; if this is united to ammonia, aspartic aldehyde is 
formed, thus : — 
1 Arch./. Anal. u. Physiol., Leipzig, 1S94, Physiol. Abth. S. 383 and 555. 
2 "Grundriss dcr Physiologie," 1821. 
;; "Rudiments of Physiology," Edinburgh, 1837. 
4 Arcli.f. d.ges. Physiol., Bonn, Bd. x. S. 251. 
5 " Die chem. Kraftquelle im lebenden Protoplasms," Munich, 1882. Loew's most 
recent views on this subject will be found in a recently published pamphlet, " The Energy 
of Living Protoplasm," London, 1896. 
