One drop of this black fluid will discolor the heart. (Drop some of the black gasoline into the jar which has gasoline.) It gets darker as I drop in more of the black fluid. This reminds us of sin in the heart. Saul was religious, but he had many sins, including hatred, as he journeyed toward Damascus to capture the followers of the Lord Jesus.
We shall let this jar with the red cross remind us of the cross on which Christ shed His precious blood. (The lid should be screwed on firmly, allowing the jar to be set upside down, so that the cross will be in the right position.) We now place the sinful heart at the foot of the cross, and we see what happened when Saul accepted Christ as his Saviour. (Unscrew the lid, placing the isinglass over the mouth of the jar to keep the water from spilling; turn the jar upside down, and place it on top of the jar with the black heart. Pull out the isinglass sufficiently to allow the gasoline to change positions with the water, A shallow pan can be used to catch that which spills.) When the red comes to the heart, the black leaves and goes to the cross. This is what happened when Saul took Christ as Saviour, and became Paul. His sins were on the cross, and Christ was in his heart.