Chemical weapons and poisonous gasses were forbidden in war as per the 1899 and 1907 Hague Conventions, which was *before* the First World War. Were chemical weapons used in considerable amounts before 1899?

It's from the 1899 convention section 2 article's 22 and 23:

22: The right of belligerents to adopt means of injuring the enemy is not unlimited.

23: Besides the prohibitions provided by special Conventions, it is especially prohibited:-- (a.) To employ poison or poisoned arms; (b.) To kill or wound treacherously individuals belonging to the hostile nation or army; (c.) To kill or wound an enemy who, having laid down arms, or having no longer means of defence, has surrendered at discretion.. etc.

Whilst not really widely deployed there were a variety of means you could use to kill your enemies all falling under the descriptor of 'poison' though it could be argued there the greater focus of the wording was more "treacherously". I.e. No poisoning enemy officers.

Side stepping the US civil war issue which I don't know much about older by far is the use of incendiaries like tar and it's associated fumes or smoke during siege warfare. (smoke/CO/CO2/SO2 etc v sappers in a mine for example, sulphur tars, naptha etc are all poisons)