What the holes in a gas burner are for
Entraining air before combustion, and why the ratio matters.
The holes cut into a Bunsen burner or a gas hob, just downstream of the gas jet, exist to let ordinary room air get pulled into the gas stream before it ever reaches the flame, mixing fuel and air together in advance so the burner produces a clean, blue, premixed flame instead of a smoky, yellow diffusion one.
A gas jet that drags the room air in behind it
Gas leaving a small jet under pressure moves fast, and a fast-moving jet of any fluid drags the fluid around it along in the same direction through friction with whatever sits next to it. A burner fires its gas jet into the narrow throat of a tube, and the holes cut into that tube's side sit just past the jet, where the fast gas has already begun pulling surrounding air along. The jet's speed creates a region of lower pressure there, and air rushes in through the holes to fill it with nothing pumping or blowing it.
By the time gas and air reach the far end of the tube and the point of ignition, they have blended thoroughly, turning what would have been a slow diffusion flame into something much closer to a premixed one, ready to burn quickly and completely. The gas supply's own pressure is the only source of motion the system needs, which is part of why this arrangement has survived essentially unchanged in burner design for well over a century, with no motor, fan or separate air supply to control or fail.
The perfume atomiser works the same way
Squeezing the bulb on a perfume atomiser sends a fast jet of air across the open top of a small tube dipped into the scent. The speed of that jet, passing over the tube's opening, drops the pressure there enough to draw the liquid up and break it into a fine mist, with no second mechanism touching the perfume. In a burner the gas jet plays the squeezed air and the room air plays the perfume, and squeezing harder has its match too: raise the gas pressure and a faster jet entrains more air, for as long as the holes are big enough not to become the limit. Turn the gas right down and the weakened jet pulls in less, which is why a burner at its lowest setting often burns a little yellow at the tip.
Ten volumes of air for natural gas, twenty-four for propane
The air drawn in has to sit close to the amount the fuel needs to burn completely, since too little starves the flame of oxygen and too much dilutes and cools it. Natural gas, mostly methane, needs close to ten volumes of air for every volume of gas, while propane needs about twenty-four, so a burner converted from one to the other is fitted with a different jet size, changing how much fuel arrives to keep the entrainment ratio in range for the same throat and holes.
A yellow flame usually means blocked holes
Blocking or narrowing the holes, through a build-up of grease, a badly fitted burner cap, or a design that shrinks them to cut cost, reduces how much air is entrained and pushes the flame back toward the yellow, sooty behaviour they were cut to avoid. A burner that used to run clean and has turned yellow is very often showing a problem with its air, not with its fuel, and cleaning it is frequently the right fix. The change is more than cosmetic, since the same incomplete combustion that makes soot also makes carbon monoxide, which is why a yellowing burner is treated as a safety concern.