128 MESSES. A. Y. HARCOTTRT AND W. ESSON ON THE LAWS OE CONNEXION 
quantity, 604 millionths of a gramme of potassic iodide or 9 times that quantity, or 
whether hydric chloride or hydro-sodic carbonate be substituted for hydric sulphate, 
whether the temperature be 0° or 50° C., and whether the portion of change require for 
its accomplishment intervals of one or two minutes, or intervals of half an hour or an 
hour, this reaction still conforms to the law that the amount of change is at each moment 
proportional to the amount of changing substance. 
In these experiments the actual observation has been of the rate of production of 
iodine. But the production of this substance is only one part of the chemical change 
which occurs. The whole change is represented in its simplest form by the equation 
H 2 0 2 +2H I=2H 2 0+I 2 , 
and we are able, knowing the quantity of iodine that has been formed, to infer from it 
the quantity of water formed and the quantities of hydric peroxide and iodide that have 
disappeared. Now if we assign a particular weight to the molecule of iodine, the equa- 
tion will represent that change by which this amount of iodine is formed, together with 
the proportional quantity of water, while corresponding quantities of hydric peroxide 
and iodide disappear. We thus obtain an expression for a particular amount of change. 
The unit change may be defined to be that in which 254 millionths of a gramme of 
iodine are formed, and when the equation written above is used to express this unit 
change it will be written in italics. That is to say, the expression 
H 2 0 2 + 2HI=2E 2 0+I 2 
represents the disappearance of 34 millionths of a gramme of hydric peroxide, and of 
256 millionths of a gramme of hydric iodide, and the formation of 36 millionths of a 
gramme of water, and 254 millionths of a gramme of iodine. The expression 
n{E 2 0 2 +2EI=2E 2 0+I 2 ) 
represents the occurrence of n units of change. Further, since in these experiments the 
liquid system is homogeneous, the total change which occurs during any interval of time 
depends upon the quantity of change occurring in each unit of volume and the number 
of such units. For unit of volume we may conveniently adopt the cubic centimetre. In 
stating the amounts of other reagents than those which appear in the equation of the 
reaction, it will sometimes be convenient to express these amounts in units corresponding 
to those proposed above, being their molecular weights taken not as relative numbers, 
but as so many millionths of a gramme. For example, in enumerating the conditions of 
a particular experiment we shall mean by H 2 S 0 4 , or a unit of hydric sulphate, 98 
millionths of a gramme of that substance; by KI, or a unit of potassic iodide, 266 
millionths of a gramme of that salt. 
In each set of experiments we commence with a system which contains elements 
capable of undergoing a certain quantity of change. We may express this by saying 
that there exists at starting a certain amount of potential change. As time elapses this 
potential change gradually becomes actual. From this point of view the change 
occurring in the system is analogous to the motion of a heavy body falling freely, which 
