Why are formation and decay considered a cycle of nature?

They are considered so because are the indispensable parts of the nature cycle.Without formation,decay cannot be possible and similarly,formation cannot be possible without decay.In addition to this,THEY ARE IN A CHRONOLOGICAL MANNER SET BY NATURE and that's why cannot be seperated and so together constitute the cycle of nature.

The answer is joseph is observing the colour of the reaction mixture to see whether proteins are present in the given solution.