Collision of Two Highly Ionized Clouds of Gas

If two rarefied masses of gas collide at a sufficiently high speed, their relative motion will be stopped by a collectlve plasma instability rather than by collisions of individual particles. The possible influence of magnetic fields is ignored for the most part, but a criterion is found to decide uader what conditions the collective interaction remains domiuant even when they cannot be ignored. (W.D.M.)