174 Dreyer and Douglas. Velocity of Beaction in [Nov. 16, 



Turning next to the " Tables " at the end of this paper, it should be 

 explained that when it is stated in any one of them that a given agglutinating 

 serum, or a solution of trypsin or of sulphuric acid, contains a certain number 

 of " units," these " units " may be taken as expressing absolute values, though 

 the "units" themselves are arbitrary. This is because control measurements 

 were carried out at the time in each set of experiments, and the number of 

 " units " present both before and after absorption has taken place for a given 

 time have been determined by the use of the same arbitrary standard. 



With experiments carried out with identical solutions or emulsions, but 

 made at different times, the resi;lts are obtained in values which are relative 

 and not absolute, and so cannot be directly compared with one another. 

 Accordingly in Tables III to XIV the experimental details and the absolute 

 quantities employed are recorded. In Table XV all the results are 

 summarised, but have been recalculated so as to give figures that are directly 

 comparable with one another. 



Tliese figures express relative and not absolute values ; in each case the 

 number of "units" originally present has been taken as 100, while the 

 quantities absorbed and the quantities left free in the fluid are expressed in 

 corresponding ratios or percentages. 



To the time of observation given in the tables must be added the interval 

 of time elapsing between the nrixing of the bacteria with the serum and the 

 .stopping of the centrifuge, less an unknown time X. This subtraction of the 

 time X is necessary, because it is impossible to say at what precise moment 

 -during the centrifugalisation the absorbing material was completely separated 

 from the supernatant fluid, and it is this moment whicli we regard as the end 

 of the reaction. Thus tlie time must be read as 



0 + (time — X), l + (tinie — X), ... 20+ (time — X), etc., minutes. 



Naturally the length of time X may be dif^Cerent according to the nature, 

 quantity, etc., of the aljsorbing matter used, i.e. bacteria, charcoal, etc, and 

 the concentration, nature, etc., of the fluid with which it is in contact, but is 

 constant throughout each individual experiment .since these conditions then 

 remain the same. 



In the following discussion, wlieu mentioning the various times, we speak 

 of 0 minutes instead of 0+(time — X) minutes, 3 minutes instead of 

 '.') + (time- X) minutes, etc., fur the sake of brevity. 



In Kxj)erini(!nt 1 (Tal)le III), where tliere were originally present 

 11)7-.^) units, the number of units left in the fluid decreases regularly as the 

 time increases, although the experiment was carried out at a temperature 

 higher than 0° C, i.e. circa 1° C. Thus it will be seen that after 0 minutes 

 10 units arc left, after 10 minutes 7 0, and aft(\r 80 minutes .0-4 units. 



