1999 Ford Taurus Have picture what is it ?

Have coolant leakage, this photo is of the left side firewall just a few inches down and to the left of the 2 heater hoses. Coolant is inside this "thing" can anyone tell me what this "thing" is and for? See picture item is circled in red.

Thanks


btw: anyone with heating problems please check your water pump. Had the same crappy heat situation found out that the water pumps in the 99 Taurus's are famous for the fins inside to deteriorate causing low circulation of coolant. Water pump never leaked would have never guessed.[img:96c5d6c60a]http://www.2carpros.com/forum/automotive_pictures/281772_HPIM0365_2.jpg[/img:96c5d6c60a]
[img:96c5d6c60a]http://www.2carpros.com/forum/automotive_pictures/281772_HPIM0365_3.jpg[/img:96c5d6c60a]