38. Relevancy of statements as to any law contained in law- books.- When the Court has to form an opinion as to a law of any country, any statement of such law contained in a book purporting to be printed or published under the authority of the Government of such country and to contain any such law, and any report of a ruling of the Courts of such country contained in a book purporting to be a report of such rulings, is relevant. HOW MUCH OF A STATEMENT IS TO BE PROVED