
146 S.E.2d 7 (1966)
266 N.C. 119
Theresa McDuffie KEITH
v.
UNITED CITIES GAS COMPANY, a Corporation, and Duke Power Company, a Corporation.
No. 29.
Supreme Court of North Carolina.
January 14, 1966.
*13 Prince, Jackson, Youngblood & Massagee, Hendersonville, for defendant appellant United Cities Gas Co.
William I. Ward, Jr., Charlotte, Whitmire & Whitmire, Hendersonville, Carl Horn, Jr., and Harold D. Coley, Jr., Charlotte, for defendant appellant Duke Power Co.
Redden, Redden & Redden, Hendersonville, for plaintiff appellee.
LAKE, Justice.

The Appeal of the Gas Company
In Graham v. North Carolina Butane Gas Company, 231 N.C. 680, 684, 58 S.E. 2d 757, 17 A.L.R.2d 881, this Court, speaking through Ervin, J., said: "It is a scientific fact `that gas ordinarily used for fuel is so inflammable that the moment a flame is applied it will immediately ignite with an instant explosion, if it is present in any considerable volume.' * * * This being true, such gas is a dangerous substance when it is not under control." The original quotation is from Holmberg v. Jacobs, 77 Or. 246, 150 P. 284. It was again quoted with approval by this Court in Ashley v. Jones, 246 N.C. 442, 98 S.E.2d 667.
The plaintiff's evidence shows that before the explosion occurred the second fire had been burning in this one room building long enough for someone to discover it and turn in the fire alarm and for the Fire Department's truck to be en route to the fire. The plaintiff's evidence thus refutes her contention that gas was present in the building in substantial quantity when this fire began and its ignition by an electric spark started the fire. The plaintiff's evidence shows, furthermore, that natural gas, being lighter than air, rises when released into a room. Her evidence also shows that from the time of the first fire to the time of the second there was in the ceiling of this room an opening, three feet in diameter, leading immediately into an attic, at each end of which there was an unobstructed opening substantial in size.
The plaintiff's evidence is that following the first fire employees of the Gas Company, then present at the building, reported to her husband that they had cut the gas off, and a fireman stated to Mrs. Swann that this had been done. Her husband testified that when he left the building, after the inspection and conferences which followed the first fire, he was satisfied in his own mind that the gas was off. This was after the time when he and the plaintiff say they detected an odor of gas in the building.
Upon the defendants' motions for judgment of nonsuit, the plaintiff's evidence is to be interpreted in the light most favorable to her, all reasonable inferences favorable to her must be drawn therefrom, conflicts therein are to be resolved in her favor and evidence of the defendant establishing a different factual situation must be disregarded. Moss v. Tate, 264 N.C. 544, 142 S.E.2d 161; Sugg v. Baker, 261 N.C. 579, 135 S.E.2d 565; Coleman v. Colonial Stores, Inc., 259 N.C. 241, 130 S.E.2d 338; Ammons v. Britt, 259 N.C. 740, 131 S.E.2d 349.
There is nothing in the evidence to indicate that the gas was not completely shut off from the building following the first fire unless it be found in these circumstances: (1) Before leaving the premises, after the first fire, the plaintiff and her husband smelled an odor of gas in the building; (2) the next morning an explosion occurred in the building while it *14 was burning; (3) each of two witnesses, who were not present in the interval between the fires, testified that, in his opinion, a spark ignited gas in the building, this opinion resting upon the hypothesis that "an explosion occurred * * * that the building and its contents caught fire." We do not regard these circumstances as being inconsistent with the evidence offered by the plaintiff to the effect that the gas supply had been cut off.
The hypothesis upon which her expert witnesses based their opinion as to the presence of gas in the building, namely, that the explosion preceded the fire, is not supported by any evidence. It is contrary to the testimony of the only observer called as a witness for the plaintiff. Upon a motion for judgment of nonsuit all evidence favorable to the plaintiff, including evidence improperly admitted, must be considered. Langley v. Durham Life Insurance Co., 261 N.C. 459, 135 S.E.2d 38. Therefore, the opinions of these expert witnesses are, for this purpose, treated by us as if competent. So treated, they are merely opinions that an explosion, followed by a fire, indicates the presence of gas in the building.
An explosion in a building to which gas pipes are connected is not, standing alone, evidence that gas escaped from such pipes into the building. There are many possible causes of an explosion in a building which has been burning for a substantial but undetermined interval of time. The doctrine of res ipsa loquitur does not apply so as to carry us, from proof of (1) natural gas service to a building plus (2) explosion, to the conclusion that gas had leaked from the pipes or fixtures. See Stansbury, North Carolina Evidence, § 227.
The testimony of the plaintiff and her husband that, following the first fire, they detected an odor of gas in the building is not inconsistent with her evidence showing that during the fire the supply of gas to the building was cut off. The odor may have continued for an interval after the closing of the valves prevented further escape of gas into the building. If, moreover, this could be regarded as evidence that gas was continuing to come into the building, it clearly establishes contributory negligence on the part of the plaintiff, for her evidence is that, knowing the representative of the Gas Company had left the premises, neither she nor her husband made any effort to inform the Gas Company of the continued presence of gas after its employee had supposedly shut off the supply, neither of them turned the cutoff valves on the individual units of equipment or examined the cutoff valves at the meter.
Although natural gas is so highly inflammable as to be an inherently dangerous commodity, so that the company supplying it must use a high degree of care to prevent its escape into a building, the company's liability for damage resulting from escaping gas is based upon its negligence. Ashley v. Jones, supra; Graham v. North Carolina Butane Gas Co., supra.
The plaintiff's evidence is that all of the gas burning equipment was in excellent condition before the first fire. It was maintained by her tenant's own service man. The Gas Company had no notice of any defect in any of the equipment prior to the first fire. When a gas company, engaged in supplying gas to a customer's building, becomes aware that gas is escaping from the fixtures into the building, it is the duty of the gas company to shut off the gas until further escape thereof can be prevented, even though the fixtures do not belong to the company and are not in its charge or custody. Graham v. North Carolina Butane Gas Co., supra. Interpreting the plaintiff's evidence in the light most favorable to her, it may be inferred that, at the time of the first fire, gas was escaping into the building from a water heater. Thus, the Gas Company, being advised of this fact, was under a duty to shut off the supply of gas to the building. However, *15 the plaintiff's evidence is that it did so and it will support no other inference.
The rule that, in passing upon a motion for judgment of nonsuit, the plaintiff's evidence must be taken to be true does not extend to an opinion by a witness, not present at the event, to the effect that a condition existed which is contrary to scientific truth so well established that the court will take judicial notice of it. As above noted, it is established scientific truth that natural gas present in quantity will explode immediately in the presence of fire. The statement by the plaintiff's expert witness, Cook, that notwithstanding a finding, supported by the greater weight of the evidence, that the second fire had been burning on the interior of the building for ten minutes before the explosion was heard, it would still be his opinion that the explosion was caused by an electric spark "getting in the natural gas" does not have to be taken, even upon motion for judgment of nonsuit, as establishing the fact that gas was in this building, in quantity, without exploding, in the presence of such fire. There is, furthermore, nothing in the record to show that the fire had not been burning more than ten minutes prior to the only explosion shown by the evidence.
There is, therefore, not sufficient evidence in the record to support a finding of negligence on the part of the Gas Company and its motion for judgment of nonsuit should have been granted.

The Appeal of the Power Company
Electricity is also inherently dangerous. Consequently, a company supplying it to a customer's building must use a high degree of foresight and must exercise the utmost diligence consistent with the practical operation of its business. Kiser v. Carolina Power & Light Co., 216 N.C. 698, 6 S.E.2d 713. Such company is not, however, liable for damages resulting from a fire, unless it be shown that the fire was proximately caused by the electricity supplied by the company to the building and that, in so supplying the electricity, the company was negligent. Fleming v. Carolina Power & Light Co., 232 N.C. 457, 61 S.E.2d 364.
Plaintiff's evidence, considered as it must be upon a motion for judgment of nonsuit, must be deemed to establish that the first fire burned insulation upon the wires carrying electric current through the building, so that the metal wires were thereafter exposed. In this situation, the meter was taken out, cutting off all current from the building. The plaintiff's husband, who was her spokesman at the fire, she being present, instructed the Power Company's employee, who took out the meter, not to replace it until the building was rewired. Nevertheless, the Power Company's service man subsequently reinstalled the meter. To turn electric current into the wiring system of a building, with notice that the wires therein are bare of insulation, is not consistent with that high degree of care which must be used by an electric power company in the handling of its product.
There is evidence in the record from which it might be found that, before this meter was reinstalled, all switches inside the building had been pulled to an off position so that no current could pass through the master switch, that the switches remained in this position until after the second fire and that the reinstallation of the meter was done at the request of an electrician employed by the plaintiff and was consented to by the plaintiff's husband and by Mrs. Swann. However, all of this evidence was introduced by the defendant Power Company and may not be considered in passing upon its motion for judgment of nonsuit. Considering the plaintiff's evidence alone and drawing from it all reasonable inferences favorable to her, the jury could infer that the reinstallation of the meter caused electricity to pass through the wiring system inside the building, parts of which were bare of insulation and that this *16 caused the second fire. Consequently, the Power Company's motion for judgment of nonsuit was properly overruled.
There was error, however, in permitting, over the Power Company's objection, plaintiff's witnesses Cook and Colb to testify, respectively: "My opinion is that a spark ignited gas in this building and this is what caused the fire"; and "There was gas present and an electrical spark ignited the gas."
Neither of these witnesses was present at the time of either fire. Each so testified in response to a question, to which the Power Company duly objected, which question was hypothetical in form and included, among its hypotheses, that "on the morning of the 9th of May, 1964, between 6:30 and 7 o'clock an explosion occurred in the Keith Building; * * * that the building and its contents caught fire and the blaze rose some 25 feet high and the building and contents were substantially damaged by the fire; * * *." The question was: "Under these circumstances, have you an opinion satisfactory to yourself as to the cause of the second fire on the morning of the 9th of May, 1964?"
The question is based on an hypothesis not supported by any evidence, namely, that the explosion preceded the fire and the building "caught fire" as a result of the explosion. The plaintiff's evidence is that the fire had been in progress for a substantial period of time before the explosion occurred. "To be competent, a hypothetical question may include only facts which are already in evidence or those which a jury might logically infer therefrom." Ingram v. McCuiston, 261 N.C. 392, 400, 134 S.E. 2d 705, 711; Jackson v. Stancil, 253 N.C. 291, 303, 116 S.E.2d 817; Dameron v. Rowland Lumber Co., 161 N.C. 495, 77 S.E. 694; State v. Holly, 155 N.C. 485, 71 S.E. 450; Burnett v. Wilmington, N. & N. Ry. Co., 120 N.C. 517, 26 S.E. 819; Stansbury, North Carolina Evidence, § 137.
Furthermore, the question was improper in form in that it calls for an opinion as to what was the cause of the second fire, rather than an opinion as to whether the situation, propounded as an hypothesis which might be found by the jury to be a fact, could have caused the fire. See Perfecting Service Co. v. Product Development & Sales Co., 259 N.C. 400, 414, 131 S.E.2d 9; Stansbury, North Carolina Evidence, § 137. The function of expert opinion is to assist the jury in evaluating and applying facts shown by other evidence. This question called for, and in answering it, each witness stated, as a fact, the existence of a condition which the witness, not having been present, could not know, but as to which he could only conjecture, namely, that there was gas in the building. As was said by Devin, J., later C. J., speaking for the Court, in Patrick v. Treadwell, 222 N.C. 1, 4, 21 S.E.2d 818, 821, the rule permitting an expert witness to express his opinion "should not be relaxed to the extent of opening the door to the statement of an evidential fact in issue beyond the knowledge of the witness under the guise of an expert opinion."
The admission of this testimony in response to this hypothetical question was prejudicial to the Power Company and requires a new trial as to it. Since there must, for this reason, be a new trial as to the Power Company, it is not necessary to consider its other assignments of error. They relate to matters which may not arise upon another trial.
Reversed as to United Cities Gas Company.
New Trial as to Duke Power Company.
