Hello, tanks for your answer. That was the problem. I tried several ways to change the speed, but in fact none of them was really working (as I'm using a Due it is different)... So I had to modify a define in the Wire library in the folder sam of hardware. So the problem is now solved,thanks!
But is there a solution I could put in the code, instead of modifying the library? And this would be useful to update the examples in the library to work properly with an arduino Due.