There was no error in entering a verdict for the defendant under leave reserved. Although there was evidence that the plaintiff, while preparing to light the heater, did not cause the presence of the gas responsible for the flashback explosion, there is no evidence that its presence was caused by the defendant's negligence. That gas company employees worked on the heater in early June, and subsequently at some other unspecified time in the summer, does not, by mere proximity of time, establish negligence by them as the cause of the explosion in September. Testimony by employees of the defendant, taken in conjunction with that of the plaintiff, could go no further than to permit an inference that there was a leak in the heater or its pilot safety valve. In the absence of expert or other testimony tending to establish a relationship between such a leak and something the defendant did or failed to do, there was no basis for a conclusion that it was caused by the defendant's negligence. Dolan v. Suffolk Franklin Sav. Bank, 355 Mass. 665 , 669-670. Triangle Dress, Inc. v. Bay State Serv. Inc. 356 Mass. 440 . See also Friese v. Boston Consol. Gas Co. 324 Mass. 623 , 628, 631.