During the final years of the Crimean War, the British, French, Ottoman, and Sardinian forces were entangled in the prolonged and bloody Siege of Sevastopol, having sought to pry the port and capital of Crimea from the hands of Russia. Before the Siege was over, the Allied forces and the Russians fought at Alma, Balaclava, Inkerman, Tchernaya, Redan, and finally the city of Sevastopol itself, covering little more than thirty-five miles in the span of a year, with over 230'000 causalities from both sides. The Battle would become a cultural staple of the period, as did the rest of the war, seeing both technological and hygienic advances, as well as radical changes in the thinking of many Russian and Russian subjected people. It's effects would also change the way wars were fought, many of the armies that fought undergoing large and sweeping reforms in the wake of the war. It was, in many senses, the first Modern War.
With the Siege of Sevastopol came innovative ideas on how to break the stalemate. One of these idea's came from the Scientist Lyon Playfair, who propose the use of artillery shells filled with Cacodyl Cyanide to break the stalemate. This was supported by Admiral Thomas Cochrane, and considered by Lord Palmerston, however was rejected by the Ordnance Department as being 'bad warfare'. Playfair would justify his proposal by noting the humane nature of gas as opposed to bullets.
The question is, of course, what if the Ordnance Department accepted Playfair's proposal, and fired Cyanide into Sevastopol? What would be the immediate and far reaching consequences?