Engels explains a whole series of examples to illustrate "the negation of the negation" in his book Anti-Duhring.
What does "the negation of the negation" mean here ?
This is one of the famous three Laws of Dialectics. For a good understanding, you need to study the work and philosophy of Hegel and Karl Marx.
The Negation of the Negation
The law of the negation of the negation, which Hegel used as the fundamental law for the construction of his whole system of thought, has a far wider sphere of application in the system of nature. This law really expresses the fundamental form of development in nature.
The opposing forces at work in every single thing bring, about constant changes in its constitution. These changes accumulate in quantity until, at a certain determinate stage in the process of development, a distinct qualitative transformation or leap occurs. The thing loses its original identity and passes over into its opposite.
But the evolutionary process does not halt at the point of simple negation. The new form of material existence is no less self-contradictory than the old and subject to the same internal restlessness. The first negation in turn undergoes self-differentiation and division until it, too, passes into its own opposite and thereby becomes negated. The final result of this process is called the negation of the negation, a synthetic unity which has discarded the transitional forms but preserved within itself the essential content of both sides of the contradictory whole.
All the transformations of material motion studied by natural science exemplify the working of this law of the negation of the negation in physical reality. Engels employs the law to clarify the interconnections between mechanical and molecular motion, or heat.
Best wishes, Clive