Joni Mitchell has a cold. She had it for her Massey Hall concert last night, which was, as you've probably guessed, a sell-out, and it got in the way from time to time. Some notes didn't come out quite the way she must have intended them. On other passages, she sounded very nasally, and now and then her voice ran out of steam before the last bars.

But cold or no cold, Miss Mitchell put on a fine concert and there is this especially to say about her: I don't know how she does it, but somehow she is one folksinger who manages to sound genteel and spontaneous at the same time.

As she demonstrated last night, her voice is refined, polished and fashionable.

Yet, she can give most of her songs a feeling that they were being improvised on the spot, as if the thought expressed in the lyrics had just that minute come to her.

Her secret lies partly in her voice, partly in the melody lines she writes, partly in the way she brings the two together. She has an ingenious way of shifting a melody in mid-passage, or maybe it's mid-note, swooping upwards or dropping down a few notes as suddenly as someone might fall through a trap door.

It produces a strange effect, funny in a way, but invariably quite electrifying. That effect is underlined by the purity of her voice, that genteel quality. She suggests that she is always aiming for the perfect sound, pebbles dropping in a deep well and that sort of thing.

She gets the sound she wants most of the time too, though pure sound doesn't always mean clear words, and on many of her songs, unless you happened to have her records memorized, you couldn't always make out the lyrics. But then maybe that was her cold at work.

As for her repertoire, Miss Mitchell offered lots of her favorites, which are by today's terms already standards in the folk world. Carey "(You're a mean old daddy but I like you") came off beautifully, illustrating to perfection that genteel, spontaneous talent. Blue sounded fine in as torchy a vein as Miss Mitchell gets, Woodstock came off more like a lament than a celebration in her version, and the one about the Big Yellow Taxi sounded comfortable, like an old friend.

There were many others from her albums, as well as some newer pieces. Perhaps the best of the later stuff was one called, if I caught it right, The Balled of the Steel Blue City and the Swampfire, it was a serious song, not solemn, mind you, but serious in a good way.

And there was another good new one, Oh Honey You Turn Me On On The Radio. It was as you can guess, about disc jockeys and radio music, and it was a lot of fun. So was the concert. A good night of music.