Yes, it's the code for Mustard Gas. Ciel and Sebastian's bath was bleach.
wow, i didn't know their bath was in bleach. hold how would that help them?
Personally the mustard gas or bleach were never mentioned in the manga, that would be an example of a 'speculation' or 'crack theory.' What if someone called you out for this and said you were wrong (in a rude way)? That wouldn't be nice, now would it?!
@dragonchildyuki I googled the code C4H8Cl2S and it seems to be the chemical formula for mustard gas. here is the link that has some detail on mustard gas if you want to know: http://cameochemicals.noaa.gov/chemical/5083
@dragonchildyuki: Yeah, their bath was bleach, Sebastian sent a sample of their bath to the palace, and they tested it and it's chemical code was shown on that slip of paper in the letter along with the mustard gas code as Marry just showed you.
The ultimate magic is so that they can make the mustard gas more pure and to do that they'll need is 2-chloroethanol with aqueous potassium sulfide. http://en.wikipedia.org/wiki/Mustard_gas#History
This is so exciting! I can't wait for more!!! <3