It was so deadly, that anyone breathing the gas would be killed instantly. When the tripods launched this gas, it would leave behind a black powder. The gas is very dense, and could only kill those at ground level. People hiding on tall objects, such as trees or rooftops can escape the deadly effects of the gas.
The Black smoke is a currently unknown element which mixes with argon in our atmosphere (based on spectrometer readings)to reach it's poisonous state. However, this is very unlikely as argon is a very stable and inert gas which only bonds with other elements under extreme conditions.
The Martian Technology Report states: The Black Smoke, used to such terrible effect against the population of London, is more accurately described as an extremely fine dust or powder. It is quite substantially heavier than air, and when released tends to stay in a dense, hill-shaped cloud which slowly flattens and spreads across the ground.
The Smoke is insoluble in water; instead it forms a layer of scum on the surface which sinks slowly to the bottom, making way for more. It appears that the Smoke undergoes some chemical reaction upon contact with moisture which renders it inert, as it is quite possible to drink without harm water which has had the scum strained from it. Similarly, moisture in the air causes the microscopic particles of dust to slowly group together into larger particles, and sink gently to the ground.
Despite a degree of knowledge concerning the behaviour and effects of the Black Smoke, little is known of its chemical composition, and scientists are still not entirely certain of precisely how it has its effect. Spectrum analysis of the black powder points unmistakably to the presence of an unknown element with a brilliant group of three lines in the green, and it is possible that it combines with argon to form a compound which acts at once with deadly effect upon some constituent in the blood, causing it to near-instantly coagulate. Its behaviour when exposed to moisture only serves to enhance its lethal properties; as it is inhaled by its victims, it is believed to undergo a reaction with the natural moisture present in the lungs. This causes it to coat the internal surface of the lungs with a smothering layer of powder, preventing the victim from obtaining vital oxygen from the air. The only ways to avoid its effects are to move to a sufficiently elevated position that one is above the level of the cloud, or to filter the air one breathes through a moistened cloth of fine weave. Black smoke was stored in special canisters, and fired by a cylinder device, which is carried, as a separate object, in the tentacles of the fighting machine. As these launching cylinders are held separately, it has been suggested that some other device was intended to use the cylinders, and they were brought out in a makeshift fashion by the fighting machines. The cylinder launcher device launched the black smoke cylinders in an unknown manner, which produces no smoke. Compressed air, or electromagnets have been suggested as the propulsive device. The canisters also do not explode when they hit the ground, but rather shatter, and the smoke inside billows out. The described size of the clouds strongly suggests that the smoke within the canisters is highly compressed.
A few lines during the battle with the thunder child suggests that the flying machine also was capable of deploying black smoke, but whether this is done through cylinders, or by spraying it out is unknown.
The Martian fighting machines are described as normally dispersing the black smoke by spraying jets of heated water at them while wading through the cloud, so that we can be fairly certain the gas is either harmless to Martians, or the clouds are lower than 100 feet. Earlier notes by H.G. Wells indicate that he had originally intended the cylinder launching devices to be capable of firing cylinders that would disperse water to clear away black smoke clouds.
The Black smoke is rarely depicted in other media, and then only on unusual occasions. Black smoke was mentioned in the Orson Welles radio broadcast, where two soldiers were firing at a fighting machine. They donned gas masks, but continued to cough, and died after some time. This would be in keeping with a blood agent type of chemical gas, but not with nerve agents, or skin irritants. Black smoke also features in a number of comic book versions, where it often dissolves the flesh off of human bodies. Often roleplaying games will mention black smoke, but often suggest that it can be filtered through a gas mask made of a cloth soaked in water, or one may hold their breath, and outrun the cloud.