294 Blackman . — Optima and Limiting Factors. 
So far we have spoken of conditions that affect the rapidity of vital 
chemical processes, whether these be (i) conditions of supply of material 
or of energy, or (2) tonic conditions that affect only the rate of metabolism. 
The former act in proportion to the quantity of matter or energy available, 
but the latter, of which temperature is the type, act by altering the velocity 
of chemical change. The further class of agents to be just mentioned here 
is that sometimes known as ‘chemical stimuli’ — substances of which small 
traces may produce large alterations of the rate of metabolism, noticeably 
of the rate of respiration. 
This susceptibility to the presence of small traces of unessential 
substances is so easily manifested that respiration is often justifiably spoken 
of as exhibiting stimulation effects, and as being controlled as to its 
magnitude by stimuli as well as by tonic conditions. 
Variations of this order would naturally be most disturbing in 
investigating the effect of tonic conditions, and it is therefore important to 
have as exact an idea as possible before one of their causation, significance, 
and possibilities. 
It is only within the last two or three years that our conception of the 
chemical organization of the cell has acquired sufficient solidarity to allow 
the investigator to face such facts without flinching. 
Regarding the cell, as we now may 1 * * , from the metabolic point of 
view, as a congeries of enzymes, a colloidal honeycomb of katalytic agents, 
as many in number as there are cell-functions, and each capable of being 
isolated and made to do its particular work alone in vitro , we look for light 
on the action of chemical stimuli in the cell to their effect on the action 
of isolated enzymes in vitro. Here, too, law and order is now known to 
reign, and while enzymes only ‘ accelerate 5 reactions without being incor- 
porated in their end products, yet the acceleration produced is proportional 
to the mass of the enzyme present, minute- as it is, and the effects of 
‘ activators ’ and ‘ paralyzators ’ of this action are also in proportion to their 
masses. 
Thus all these effects belong to the province of chemical dynamics, and 
the accelerating effects of ferments and activators upon respiration fall into 
the same category as the accelerating effect of increase of temperature. 
The analytical treatment of metabolic phenomena which is outlined 
here is then not made any the less certain in its procedure, though it is made 
more complex by the interaction of those metabolic effects which have been 
described by their investigators as stimulatory. 
These phenomena need not be considered further at present, and the 
essential quantitative laws of metabolic ‘ velocity of reaction ’ may no doubt 
1 For recent views on these points see Fr. Hofmeister, Die chemische Organisation der Zelle, 
Braunschweig, 1901 ; G. Bredig, Anorganische Fermente, Leipzig, 1901 ; F. Czapek, Biochemie 
der Pflanzen, Zweites Kapitel, 1904 ; E. F. Armstrong, Proc. Roy. Soc., vol. lxxiii, 1904. 
