Silver iodine, liquid propane, dry ice, or salt in the case of producing rain or snow.
http://en.wikipedia.org/wiki/Cloud_seedingMind control?
Not much.
Biological pathogens is a different story but the altitude may present a problem as well as air currents. It would be dang difficult to accurately predict where the droplets would fall to earth at and freezing temps at even mid altitudes would have an effect on the viability of the pathogen of choice. But, living organisms can withstand very low temps if they are prefrozen in liq nitrogen. The hard part is getting them reconstituted and viable again. The transition from frozen to warm is usually what causes problems with (for instance) semen samples. Same with modified live virus vaccines. Tho not frozen, outside a host, they have to be kept within a certain narrow range of temperature in order to provide protection and most are a 2 part formula--they have to be mixed together just before use.
Non living chemicals, I don't know about.
[This message has been edited by maryjane (edited 02-11-2014).]
