Snape rolled his eyes at her insistence at using his water closet, but smirked as well, almost subconsciously enjoying the power he was able to exert over Tonks in this situation. As far as he had been concerned, he was going to be able to laugh at her whether she had given the sample or not. He was delighted that she choose to give it, however, because he was looking forward to seeing the look on her face when he told her the nature of her condition.
"Restrain your impatience, young lady. I can't be certain yet. Sit. Watch."
Placing the beaker of yellow liquid on the table, the wizard went to his shelf and selected a couple of jars of powder. Returning to the table, he started a fire in a small copper brazier and suspended the beaker above it. Adding a small amount of each powder in alternation with several deliberate turns of his stirring spoon, he appeared deep in concentration. Finally he put the spoon down and crossed his arms.