Examples of the wetting agent include polyhydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol monoethyl ether, 1,3-butylene glycol and glycerol; polyhydric alcohol polymers such as polyethylene glycol, polypropylene glycol and polyglycerol; sugar alcohols such as xylitol, sorbitol and maltitol; and methylcellulose and carboxymethylcellulose.