Gold coins were discussed here a few days ago because advertisements offering them for sale frequently imply that the purchase of gold coins is an almost guaranteed way to make handsome profits.

I said there are two basic errors in that thesis. 1) There is no such thing as a sure investment.2) Buying gold coins is not the best way to bet that the price of gold bullion will continue to rise. The investor should decide whether he wants to invest in gold bullion or in rare coins.

A reader who signed herself "Josie" sent me an article by Q. David Bowers that appeared in Coin World on Jan. 29, 1975.

In that article, Bowers said that most collectors and investors confuse gold bullion with gold coins.He added that "the popular press" adds to the confusion when it uses the two terms interchangeably.

"Actually, gold bullion and gold coins are two different subjects and should be treated as such," he said.

He offered this example: Suppose the investor thinks that the price of copper will double in value. Does this mean he should buy 1877 Indian one-cent pieces, which sell for many hundreds of dollars each, on the theory that if copper doubles in price a rare $500 copper penny will be worth $1,000?

"Of course not," Bowers wrote. The value of the rare pennies "will be relatively unaffected by the metallic value of copper."

The same basic logic applies to gold coins, he said. One should not base a decision to buy a rare coin on the expectation that bullion will go up in price. To do that, he wrote, "is complete illogic."

One other comment on the coin column may be of interest to you. It came from Lew Hendrixson of Crofton and made reference to the Russian gold coins now being sold here -- the ones called chervonets. Lew wrote:

"Like the so often missued Greek word kudos, the Russian chervonets is also singular.

"The misleading 's' is really part of the 'ts' digraph used to transliterate a single Russian letter."

In Russian, the word is pronounced chair-VAWN-its, Lew says. The plural is chari-VAWN-tsy.

I found that most American dealers pronouce it shev-ron-ETS, transposing the 'r' and 'v' sounds. ANOTHER DANGER TO YOUR HEALTH

A few days ago, I mentioned an unusual "No Smoking" reminder posted in a filling station. Anthea M. DeVaughan responded with this report:

"While filling my tank at a station on Route 1 south of Alexandria, my children and I noticed a man with a cigrette in his mouth filling his tank.

"At one point, he even bent down to peer into the tank without removing the cigarette from his mouth. A female employee of the station carried a lighted cigarette as she walked around collecting payments, although 'No Smoking' signs were prominently displayed. These people apparently have no regard for their own lives, but they have no right to endanger the lives of others. To whom should violations of this kind be reported?"

I think the most effective action one could take would be to notify the station's insurance company.

But since we're not likely to know which company is involved, other possibilities come to mind. If the offense takes place in a jurisdiction that has a fire marshal, tell the marshal. If it happens in an area served by a volunteer fire department, tell the chief.

If the owner's name is posted, tell him. If the station is owned by a major oil company, write to the company.

Millions of people think gasoline won't ignite unless a spark actually touches the liquid. They don't realize that gasoline vapor is much more explosive than the liquid itself.

The fellow who peered into his gas tank with a cigarette in his mouth can thank his lucky stars that he must have picked a breezy day for that fool-hardly action. On a dead calm day, with no wind to scatter the gasoline vapor, he might have discovered that cigarette smoking is indeed hazardous to one's health. OUR DO-IT-YOURSELF PROJECT

Roy H. Millenson says there is one encouraging aspect to the anti-Khomeini sentiment that has been demonstrated all across our country. "It shows that Americans can demonstrate without being led by Jane Fonda or some other entertainment personality." JIMMY MAY KNOW SOMETHING

A lot of people assumed that the White House made a mistake when it sent letters of congratulation to Democratic candidates who were defeated at the polls. But maybe it wasn't a mistake at all.

By this time, winner Carter may have concluded that winning office isn't all it's cracked up to be.