The doctrine of res ipsa loquitur, originating as an exception to the requirement that the plaintiff must prove the particular acts of negligence causing his injury, is applied at the discretion of the trial judge in cases where the acts of negligence are unknown to plaintiff or proof of them is not available to him. Since the doctrine permits of an inference of negligence from the circumstances of the case, these circumstances must be such as will warrant the inference, and various rules have been evolved to determine this.