what happen in chapter 3?

The Professor is unable to unlock the secret of the Icelandic characters that make up the text of the book. He looks at different ways the letters could be encoded (assisted by his nephew), discovers a cryptogram, and realizes there are two different types of writing.

"These two writings are not by the same hand," he said; "the cipheris of later date than the book, an undoubted proof of which I see ina moment. The first letter is a double m, a letter which is not to befound in Turlleson's book, and which was only added to the alphabet in the fourteenth century. Therefore there are two hundred years between the manuscript and the document."

He then found some blots on the paper and miraculousy made out the name of another Icelander.

"Arne Saknussemm!" he cried in triumph. "Why that is the name of another Icelander, a savant of the sixteenth century, a celebrated alchemist!"

After this discovery, Uncle surmises the extra notes have been written in Latin, although a disordered Latin. For the duration of the chapter, Uncle and Axel spend their time trying to decode the text. They are not succesful.