376 Analyses of Books. \ [May, 
nied by a suppression of forces, but the compounds pass into 
another class, and exercise another series of affinities. Some of 
these compounds become alkalies ; while others become acids. 
Now the alkalies and acids are capable of neutralizing each 
other ; and, therefore, possess opposite forces. It may, at first 
sight, appear unaccountable that the same operation should pro- 
duce two kinds of forces quite opposite to each other. M. CErsted 
is led to what he considers as the true explanation by the follow- 
ing facts: 
1. All those bodies that become strong alkalies by combustion 
have the property of decomposing water and depriving it of its 
oxygen. Such bodies must of course possess a great degree of 
combustibility. But all the bodies that become acid by com- 
bustion have little action on water, unless favoured by peculiar 
circumstances. They are, however, oxidized in the air with the 
aa facility, and this oxidizement is singularly promoted by 
eat. 
2. Those bodies that become alkaline unite with only a small 
quantity of oxygen, while those that become acid unite with a 
great quantity of that substance. 
3. Those oxides which possess alkalinity in the greatest per- 
fection are not saturated with oxygen. Those saturated oxides 
that combine with acids are capable of being separated from 
acids by much weaker forces than the non-saturated oxides, In 
the oxides of bodies moderately combustible, and which are not 
combined with much oxygen, we see acidity and alkalinity 
existing at once. Very combustible bodies saturated with 
oxygen form compounds (water for example) neither acid nor 
alkaline. 
From the consideration of these facts, M. CErsted concludes, 
that those products of combustion which still possess an excess: 
of the force of combustibility are alkaline ; while those in which 
that force is perfectly destroyed, and in which the burning force, 
on the contrary, is in excess, are acid. In_a certain state of « 
equilibrium of these forces there is an equilibrium of acidity and 
alkalinity. But our author is of opinion that we must not merely 
attend to this state of the forces, but take into consideration 
that the forces by the effect of combustion are brought into a 
state of activity quite new; for the force of combustibility no 
longer acts as such in the alkalies, nor the burning force in the 
acids. Sometimes indeed we see both kinds of forces in the 
same substance. Thus in ammonia we find both combustibility 
and alkalinity existing together, and in nitric acid we have an 
example of the burning force and acidity in the same substance, 
In some saturated oxides where the burning force of the oxygen 
is but little restrained by the contrary attraction, we see it exhi- 
biting almost all its force, and yet the oxide exhibits no signs 
of acidity. We have an example of this in the peroxides of lead 
and manganese. One of the forces ought then to be limited, 
