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Natural gas is a naturally occurring fuel extracted from deep within the earth. It is not one gas but a mix of various gases. The types of gasses vary from well to well. Natural gas is primarily methane (alias "cow" flatulence), but also contains other flammable gases such as propane, butane, carbon monoxide and hydrogen. The main uses for natural gas in homes are for heating, hot water, clothes drying and cooking.
Propane is extracted from natural gas, and is one of the so-called LP gases. 'LP' is an acronym for "liquefied petroleum". Many years ago, it was found that propane could be turned into a liquid, or "liquefied", under fairly low pressure. This quality allows shipping and storage of large volumes of propane gas in relatively small containers. Other gases can also be liquefied, but propane is by far the most common LP gas. Because of this the terms LP gas and propane are often used interchangeably.
Propane is the fuel commonly used for portable appliances such as gas grills, stoves, lanterns, soldering guns and heaters. However, homes without access to natural gas can install large LP tanks outside and pipe the gas in for use with stationary appliances. Because of the consistency of the product, LP gas appliances can be designed to be more efficient than natural gas appliances. The portability of the fuel makes LP gas accessible to a wider number of people.
Because all homes do not have access to natural gas, many manufacturers design their products to use LP gas instead. This makes them more widely useful and, of course, salable!! Depending on the product and its design, it may also be compatible with natural gas. This, of course, is something the company must determine so that their product is used safely. In the case of your fireplace logs, for example, if you cannot legally use the fuel gas they were designed for within your fireplace, contact the manufacturer. They may have a conversion kit available to allow the logs to use the other gas.
State safety departments have written laws concerning the use of gas appliances. These laws are not consistent across the country. The best source of information concerning your local regulations would be the state regulatory agency involved.