Braley, J.
It may be assumed the jury could have found that the defendant’s servants, who were charged with the duty of making the changes, either knew or should have known of *575the very serious consequences which might follow if, after disconnecting the large gas main at the upper bridge, they failed to shut off completely the supply of gas coming through the smaller main at the lower bridge. Thompson v. Cambridge Gas Light Co. 201 Mass. 77, 80. In view of this peril to the health or lives of those whose houses were supplied by these mains, the act of one of the servants, who could have been found to have left the small main open after he had been ordered by the foreman to close it, would have warranted a finding of gross negligence within the meaning of R. L. c. 171, § 2, under'which the action is brought. Oulighan v. Butler, 189 Mass. 287. Mullins v. New York, New Haven, & Hartford Railroad, 201 Mass. 38. McNamara v. Boston & Maine Railroad, 202 Mass. 491.
But if the tenth request accordingly was properly refused, and the exceptions to the refusal to give the nineteenth request, not having been argued, must be treated as waived, the first and fourth requests should have been given.
By the provisions of the statute, before any recovery could be had, the burden of proof was on the plaintiff to introduce some affirmative evidence that her intestate exercised due care. If no positive testimony could be procured, proof of circumstances which excluded any act of contributory negligence would have been sufficient. Prince v. Lowell Electric Light Corp. 201 Mass. 276, 281, and cases cited. But if we turn to the testimony covering the period elapsing after the time she left the house of a friend shortly before midnight, and the next morning when between eight and nine o’clock she was found in her bedroom lying on the bed partially dressed, and although living and partially conscious, yet speechless from poisoning by illuminating gas, from the effects of which she steadily sank and died without recovering further consciousness, the most careful scrutiny reveals nothing as to her movements and conduct. If from the various circumstances of her life as they are disclosed, there may have been no motive for suicide, and the suggestion that she died by her own hand could have been negatived by the jury, all else rests upon pure conjecture. The plaintiff very urgently contends, that whenever the decedent may have sought repose, she must have left the gas burning, as the electric lighting arrangement with which the burner was equipped did not appear to have been defective, and *576the flow of gas from the jet, when the condition of affairs in the room was first discovered, continued unchecked. The fact, however, sought to be proved is, that neither by any careless act of commission or of omission did the decedent contribute to the accident which caused her death. But when all the facts which the witnesses observed are given their full probative effect they fail to establish a connection with the conclusion which must be reached. If upon retiring the gas remained lighted, but was subsequently extinguished by the insufficient pressure owing to the change of mains, or if in using the electric lighter by which the gas was not only ignited, but turned off, and which when set would not show from any observation of the fixture itself whether the gas was on or off, she in some way failed to stop the flow either wholly or partially, or if, having effectually done so, she then inadvertently switched the gas on again, are all mere assumptions. In this realm of speculation, where all theories necessarily end in supposition, neither hypothesis is exclusive, nor can it conclusively be inferred that one is more probable than another. French v. Sabin, 202 Mass. 240.
The action, therefore, cannot be maintained.
Exceptions sustained.