In [2]: a = pd.Series([1,2,3,4,'.'])

In [3]: a
Out[3]:
0    1
1    2
2    3
3    4
4    .
dtype: object

In [8]: a.astype('float64', raise_on_error = False)
Out[8]:
0    1
1    2
2    3
3    4
4    .
dtype: object