eleven
o’clock
`` How do we know that ? '' replied Larsan . `` There was the dinner in the laboratory , the coming and going of the servants in attendance . There was a chemical experiment being carried on between ten and eleven o'clock , with Monsieur Stangerson , his daughter , and Daddy Jacques engaged at the furnace in a corner of the high chimney . Who can say that the murderer -- an intimate ! -- a friend ! -- did not take advantage of that moment to slip into `` The Yellow Room '' , after having taken off his boots in the lavatory ? ''